Add Splitter VFD to library.

* "Simultaneous and equivalent" Read-Write and Write-Only channels for
  file I/O.
* Only supports drivers with the H5FD_FEAT_DEFAULT_VFD_COMPATIBLE flag for
  now, preventing issues with multi-file drivers.

Add Mirror VFD to library.

* Write-only operations over a network.
* Uses TCP/IP sockets.
* Server and auxiliary server-shutdown programs provided in a new directory,
  `utils/mirror_vfd`.
* Automated testing via loopback ("remote" of localhost).

10 files changed, 2335 insertions, 0 deletions

diff --git a/utils/CMakeLists.txt b/utils/CMakeLists.txt
new file mode 100644
index 0000000..2d5626e
--- /dev/null
+++ b/utils/CMakeLists.txt
@@ -0,0 +1,4 @@
+cmake_minimum_required (VERSION 3.10)
+project (HDF5_UTILS C)
+
+add_subdirectory (mirror_vfd)
diff --git a/utils/COPYING b/utils/COPYING
new file mode 100644
index 0000000..4a23112
--- /dev/null
+++ b/utils/COPYING
@@ -0,0 +1,12 @@
+
+  Copyright by The HDF Group.
+  All rights reserved.
+
+  The files and subdirectories in this directory are part of HDF5.
+  The full HDF5 copyright notice, including terms governing use,
+  modification, and redistribution, is contained in the COPYING file
+  which can be found at the root of the source code distribution tree
+  or in https://support.hdfgroup.org/ftp/HDF5/releases.  If you do
+  not have access to either file, you may request a copy from
+  help@hdfgroup.org.
+
diff --git a/utils/Makefile.am b/utils/Makefile.am
new file mode 100644
index 0000000..b4409ac
--- /dev/null
+++ b/utils/Makefile.am
@@ -0,0 +1,26 @@
+#
+# Copyright by The HDF Group.
+# All rights reserved.
+#
+# This file is part of HDF5.  The full HDF5 copyright notice, including
+# terms governing use, modification, and redistribution, is contained in
+# the COPYING file, which can be found at the root of the source code
+# distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.
+# If you do not have access to either file, you may request a copy from
+# help@hdfgroup.org.
+##
+## Makefile.am
+## Run automake to generate a Makefile.in from this file.
+##
+#
+# Utils HDF5 Makefile(.in)
+#
+
+include $(top_srcdir)/config/commence.am
+
+CONFIG=ordered
+
+# All subdirectories
+SUBDIRS=mirror_vfd
+
+include $(top_srcdir)/config/conclude.am
diff --git a/utils/mirror_vfd/CMakeLists.txt b/utils/mirror_vfd/CMakeLists.txt
new file mode 100644
index 0000000..405c420
--- /dev/null
+++ b/utils/mirror_vfd/CMakeLists.txt
@@ -0,0 +1,64 @@
+cmake_minimum_required (VERSION 3.10)
+project (HDF5_UTILS_MIRRORVFD C)
+
+#-----------------------------------------------------------------------------
+# Add the mirror_server executable
+#-----------------------------------------------------------------------------
+
+set (mirror_server_SOURCES
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_remote.c
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_server.c
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_writer.c)
+add_executable (mirror_server ${mirror_server_SOURCES})
+target_include_directories (mirror_server PRIVATE "${HDF5_UITLS_DIR};${HDF5_SRC_DIR};${HDF5_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (mirror_server STATIC)
+  target_link_libraries (mirror_server PRIVATE ${HDF5_TOOLS_LIB_TARGET} ${HDF5_LIB_TARGET})
+else ()
+  TARGET_C_PROPERTIES (mirror_server SHARED)
+  target_link_libraries (mirror_server PRIVATE ${HDF5_TOOLS_LIBSH_TARGET} ${HDF5_LIBSH_TARGET})
+endif ()
+set_target_properties (mirror_server PROPERTIES FOLDER utils)
+set_global_variable (HDF5_UTILS_TO_EXPORT "${HDF5_UTILS_TO_EXPORT};mirror_server")
+set (H5_DEP_EXECUTABLES ${H5_DEP_EXECUTABLES} mirror_server)
+
+#-----------------------------------------------------------------------------
+# Add the mirror_server_halten_sie executable
+#-----------------------------------------------------------------------------
+
+set (mirror_server_halt_SOURCES ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_server_halten_sie.c)
+add_executable (mirror_server_halt ${mirror_server_halt_SOURCES})
+target_include_directories (mirror_server_halt PRIVATE "${HDF5_UITLS_DIR};${HDF5_SRC_DIR};${HDF5_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (mirror_server_halt STATIC)
+  target_link_libraries (mirror_server_halt PRIVATE ${HDF5_TOOLS_LIB_TARGET} ${HDF5_LIB_TARGET})
+else ()
+  TARGET_C_PROPERTIES (mirror_server_halt SHARED)
+  target_link_libraries (mirror_server_halt PRIVATE ${HDF5_TOOLS_LIBSH_TARGET} ${HDF5_LIBSH_TARGET})
+endif ()
+set_target_properties (mirror_server_halt PROPERTIES FOLDER utils)
+set_global_variable (HDF5_UTILS_TO_EXPORT "${HDF5_UTILS_TO_EXPORT};mirror_server_halt")
+set (H5_DEP_EXECUTABLES ${H5_DEP_EXECUTABLES} mirror_server_halt)
+
+##############################################################################
+##############################################################################
+###           I N S T A L L A T I O N                                      ###
+##############################################################################
+##############################################################################
+
+#-----------------------------------------------------------------------------
+# Rules for Installation of tools using make Install target
+#-----------------------------------------------------------------------------
+if (HDF5_EXPORTED_TARGETS)
+  foreach (exec ${H5_DEP_EXECUTABLES})
+    INSTALL_PROGRAM_PDB (${exec} ${HDF5_INSTALL_BIN_DIR} utilsapplications)
+  endforeach ()
+
+  install (
+      TARGETS
+          ${H5_DEP_EXECUTABLES}
+      EXPORT
+          ${HDF5_EXPORTED_TARGETS}
+      RUNTIME DESTINATION ${HDF5_INSTALL_BIN_DIR} COMPONENT utilsapplications
+  )
+endif ()
diff --git a/utils/mirror_vfd/Makefile.am b/utils/mirror_vfd/Makefile.am
new file mode 100644
index 0000000..448e4cd
--- /dev/null
+++ b/utils/mirror_vfd/Makefile.am
@@ -0,0 +1,30 @@
+#
+# Copyright by The HDF Group.
+# All rights reserved.
+#
+# This file is part of HDF5.  The full HDF5 copyright notice, including
+# terms governing use, modification, and redistribution, is contained in
+# the COPYING file, which can be found at the root of the source code
+# distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.
+# If you do not have access to either file, you may request a copy from
+# help@hdfgroup.org.
+##
+## Makefile.am
+## Run automake to generate a Makefile.in from this file.
+#
+# HDF5 Library Makefile(.in)
+#
+
+include $(top_srcdir)/config/commence.am
+
+AM_CPPFLAGS+=-I$(top_srcdir)/src
+
+bin_PROGRAMS = mirror_server mirror_server_halten_sie
+
+mirror_server_SOURCES = mirror_server.c mirror_writer.c mirror_remote.c
+#mirror_writer_SOURCES = mirror_writer.c mirror_remote.c
+
+# All programs depend on the hdf5 library
+LDADD=$(LIBHDF5)
+
+include $(top_srcdir)/config/conclude.am
diff --git a/utils/mirror_vfd/mirror_remote.c b/utils/mirror_vfd/mirror_remote.c
new file mode 100644
index 0000000..81a3625
--- /dev/null
+++ b/utils/mirror_vfd/mirror_remote.c
@@ -0,0 +1,225 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* Common operations for "remote" processes for the Mirror VFD.
+ *
+ * Jacob Smith, 2020-03-06
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log
+ *
+ * Purpose:     Write message to the logging stream/file.
+ *              If logging info pointer is NULL, uses logging defaults.
+ * ----------------------------------------------------------------------------
+ */
+void
+mirror_log(struct mirror_log_info *info,
+           unsigned int            level,
+           const char             *format,
+           ...)
+{
+    FILE *stream = MIRROR_LOG_DEFAULT_STREAM;
+    unsigned int verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+    hbool_t      custom = FALSE;
+
+    if (info != NULL && info->magic == MIRROR_LOG_INFO_MAGIC) {
+        stream = info->stream;
+        verbosity = info->verbosity;
+        custom = TRUE;
+    }
+
+    if (level == V_NONE) {
+        return;
+    }
+    else
+    if (level <= verbosity) {
+        if (custom == TRUE && info->prefix[0] != '\0') {
+            HDfprintf(stream, "%s", info->prefix);
+        }
+
+        switch (level) {
+        case (V_ERR) :
+            HDfprintf(stream, "ERROR ");
+            break;
+        case (V_WARN) :
+            HDfprintf(stream, "WARNING ");
+            break;
+        default:
+            break;
+        }
+
+        if (format != NULL) {
+            va_list args;
+            HDva_start(args, format);
+            HDvfprintf(stream, format, args);
+            HDva_end(args);
+        }
+
+        HDfprintf(stream, "\n");
+        HDfflush(stream);
+    } /* end if sufficiently verbose to print */
+} /* end mirror_log() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_log_bytes
+ *
+ * Purpose:     "Pretty-print" raw binary data to logging stream/file.
+ *              If info pointer is NULL, uses logging defaults.
+ * ----------------------------------------------------------------------------
+ */
+void
+mirror_log_bytes(struct mirror_log_info *info,
+                 unsigned int            level,
+                 size_t                  n_bytes,
+                 const unsigned char    *buf)
+{
+    FILE *stream = MIRROR_LOG_DEFAULT_STREAM;
+    unsigned int verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+
+    if (buf == NULL) {
+        return;
+    }
+
+    if (info != NULL && info->magic == MIRROR_LOG_INFO_MAGIC) {
+        stream = info->stream;
+        verbosity = info->verbosity;
+    }
+
+    if (level <= verbosity) {
+        size_t bytes_written = 0;
+        const unsigned char *b = NULL;
+
+        /* print whole lines */
+        while ((n_bytes - bytes_written) >= 32) {
+            b = buf + bytes_written; /* point to region in buffer */
+            HDfprintf(stream,
+                    "%04zX  %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X\n",
+                    bytes_written,
+                    b[0], b[1], b[2], b[3],   b[4], b[5], b[6], b[7],
+                    b[8], b[9], b[10],b[11],  b[12],b[13],b[14],b[15],
+                    b[16],b[17],b[18],b[19],  b[20],b[21],b[22],b[23],
+                    b[24],b[25],b[26],b[27],  b[28],b[29],b[30],b[31]);
+            bytes_written += 32;
+        }
+
+        /* start partial line */
+        if (n_bytes > bytes_written) {
+            HDfprintf(stream, "%04zX ", bytes_written);
+        }
+
+        /* partial line blocks */
+        while ((n_bytes - bytes_written) >= 4) {
+            HDfprintf(stream, " %02X%02X%02X%02X",
+                    buf[bytes_written],   buf[bytes_written+1],
+                    buf[bytes_written+2], buf[bytes_written+3]);
+            bytes_written += 4;
+        }
+
+        /* block separator before partial block */
+        if (n_bytes > bytes_written) {
+            HDfprintf(stream, " ");
+        }
+
+        /* partial block individual bytes */
+        while (n_bytes > bytes_written) {
+            HDfprintf(stream, "%02X", buf[bytes_written++]);
+        }
+
+        /* end partial line */
+        HDfprintf(stream, "\n");
+    } /* end if suitably verbose to log */
+} /* end mirror_log_bytes() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log_init
+ *
+ * Purpose:     Prepare a loginfo_t structure for use.
+ *
+ * Return:      Success: Pointer to newly-ceated info.
+ *              Failure: NULL. Either unable to allocate or cannot open file.
+ * ----------------------------------------------------------------------------
+ */
+loginfo_t *
+mirror_log_init(char *path, char *prefix, unsigned int verbosity)
+{
+    loginfo_t *info = NULL;
+
+    info = (loginfo_t *)HDmalloc(sizeof(loginfo_t));
+    if (info != NULL) {
+        info->magic = MIRROR_LOG_INFO_MAGIC;
+        info->verbosity = verbosity;
+        info->stream = MIRROR_LOG_DEFAULT_STREAM;
+        info->prefix[0] = '\0';
+
+        if (prefix && *prefix) {
+            HDstrncpy(info->prefix, prefix, MIRROR_LOG_PREFIX_MAX);
+        }
+
+        if (path && *path) {
+            FILE *f = NULL;
+            f = HDfopen(path, "w");
+            if (NULL == f) {
+                HDfprintf(MIRROR_LOG_DEFAULT_STREAM,
+                        "WARN custom logging path could not be opened: %s\n",
+                        path);
+                info->magic += 1;
+                HDfree(info);
+            }
+            else {
+                info->stream = f;
+            }
+        }
+
+    } /* end if able to allocate */
+
+    return info;
+} /* end mirror_log_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log_term
+ *
+ * Purpose:     Shut down and clean up a loginfo_t structure.
+ *
+ * Return:      Success: SUCCEED. Resources released.
+ *              Failure: FAIL.    Indeterminite state.
+ * ----------------------------------------------------------------------------
+ */
+herr_t
+mirror_log_term(loginfo_t *info)
+{
+    if (info == NULL || info->magic != MIRROR_LOG_INFO_MAGIC) {
+        return FAIL;
+    }
+    if (info->stream != stderr || info->stream != stdout) {
+        if (HDfclose(info->stream) < 0) {
+            return FAIL;
+        }
+    }
+    info->magic += 1;
+    HDfree(info);
+    return SUCCEED;
+} /* end mirror_log_term() */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_remote.h b/utils/mirror_vfd/mirror_remote.h
new file mode 100644
index 0000000..9132d51
--- /dev/null
+++ b/utils/mirror_vfd/mirror_remote.h
@@ -0,0 +1,51 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* Common definitions for "remote" processes for the Mirror VFD.
+ *
+ * Jacob Smith, 2020-03-06
+ */
+
+#include "hdf5.h"
+#include "H5private.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define V_NONE 0
+#define V_ERR  1
+#define V_WARN 2
+#define V_INFO 3
+#define V_ALL  4
+
+#define MIRROR_LOG_DEFAULT_STREAM stdout
+#define MIRROR_LOG_DEFAULT_VERBOSITY V_WARN
+#define MIRROR_LOG_PREFIX_MAX 79
+#define MIRROR_LOG_INFO_MAGIC 0x569D589A
+
+typedef struct mirror_log_info {
+    uint32_t magic;
+    FILE     *stream;
+    unsigned int verbosity;
+    char     prefix[MIRROR_LOG_PREFIX_MAX+1];
+} loginfo_t;
+
+void mirror_log(loginfo_t *info, unsigned int level,
+        const char *format, ...);
+void mirror_log_bytes(loginfo_t *info, unsigned int level,
+        size_t n_bytes, const unsigned char *buf);
+loginfo_t *mirror_log_init(char *path, char *prefix, unsigned int verbosity);
+int mirror_log_term(loginfo_t *loginfo);
+
+herr_t run_writer(int socketfd, H5FD_mirror_xmit_open_t *xmit_open);
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_server.c b/utils/mirror_vfd/mirror_server.c
new file mode 100644
index 0000000..314f067
--- /dev/null
+++ b/utils/mirror_vfd/mirror_server.c
@@ -0,0 +1,645 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * "Server" application to associate a Mirror VFD Driver with a Writer.
+ *
+ * Server waits on a dedicated port for a Driver to attempt to connect.
+ * When connection is made, reads a message from the Driver.
+ * If message is "SHUTDOWN", Server closes connection and terminates.
+ * Else, if it receives an encoded OPEN xmit (from Driver), the Server forks
+ * itself; the child becomes a dedicated Writer and maintains connection with
+ * the Driver instance, and the parent process remains a Server and returns
+ * to listening for incoming requests.
+ * Else, the message is not recognized and is ignored.
+ *
+ *
+ *
+ * mirror_server [args]
+ *
+ * Primary server for coordinating mirror VFD connections with the remote
+ * process.
+ *
+ * args:
+ * --help, -h     Print help message and exit.
+ * --port=N       Positive integer for primary listening port.
+ * --verbosity=N  Debugging verbosity
+ *                0: none
+ *                1: errors
+ *                2: details
+ *                3: all
+ * --logpath=S    File path to direct debugging output, if any.
+ *                Default of none prints output to stdout.
+ *
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define MAXBUF 2048             /* max buffer length.                        */
+#define LISTENQ 80              /* max pending mirrorS requests              */
+#define DEFAULT_PORT 3000       /* default listening port                    */
+#define MAX_PORT_LOOPS 20       /* max iteratations through port range       */
+#define PORT_LOOP_RETRY_DELAY 1 /* seconds to wait between port scans        */
+
+/* semi-unique "magic" numbers to sanity-check structure pointers */
+#define OP_ARGS_MAGIC    0xCF074379u
+#define SERVER_RUN_MAGIC 0x741B459Au
+
+
+/* ---------------------------------------------------------------------------
+ * Structure:   struct op_args
+ *
+ * Purpose:     Convenience structure for holding arguments from command-line.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to help validate a pointer to this struct type.
+ *      Must be OP_ARGS_MAGIC to be considered valid.
+ *
+ * `help` (int)
+ *      Flag that the help argument was present in the command line.
+ *
+ * `main_port` (int)
+ *      Flag that the help argument was present in the command line.
+ *
+ * `verbosity` (int)
+ *      Number between 0 (none) and 4 (all) that controls how much detail
+ *      the program prints as a course of logging.
+ *
+ * `log_prepend_serv` (int)
+ *      Flag that the logging messages should have 'S- ' at the start of each
+ *      line.
+ *
+ * `log_prepend_type` (int)
+ *      Flag that the logging messages should have the assocaited verbosity
+ *      level present in the line (e.g., "WARN", "ERROR", or "INFO").
+ *
+ * `log_path` (char *)
+ *      Path string from the command line, giving the absolute path
+ *      for the file for logging output. Can be empty.
+ *
+ * `writer_log_path` (char *)
+ *      Path string from the command line, giving the absolute path
+ *      for the file for writer's logging output. Can be empty.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct op_args {
+    uint32_t    magic;
+    int         help;
+    int         main_port;
+    int         verbosity;
+    int         log_prepend_serv;
+    int         log_prepend_type;
+    char        log_path[PATH_MAX+1];
+    char        writer_log_path[PATH_MAX+1];
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure:   struct server_run
+ *
+ * Purpose:     Convenience structure for holding information about a server
+ *              in operation.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to help validate a pointer to this struct type.
+ *      Must be SERVER_RUN_MAGIC to be considered valid.
+ *
+ * `log_stream` (FILE *)
+ *      File handle where logging output is directed.
+ *      By default, is stdout.
+ *
+ * `opts` (struct opt_args)
+ *      Contained structure, holds the server's configuration.
+ *
+ * `listenfd` (int)
+ *      File descriptor of the listening socket.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct server_run {
+    uint32_t            magic;
+    struct op_args      opts;
+    struct mirror_log_info *loginfo;
+    int                 listenfd;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    usage
+ *
+ * Purpose:     Print the usage message to stdout.
+ * ---------------------------------------------------------------------------
+ */
+static void
+usage(void)
+{
+    HDfprintf(stdout,
+              "mirror_server [options]\n"                                     \
+              "\n"                                                            \
+              "Application for providing Mirror Writer process to "           \
+              " Mirror VFD on file-open.\n"                                   \
+              "Listens on a dedicated socket; forks as a Writer upon receipt" \
+              " of a valid OPEN xmit.\n"                                      \
+              "\n"                                                            \
+              "Options:\n"                                                    \
+              "--help [-h]         : Print this help message and quit.\n"     \
+              "--logpath=PATH      : File path for logging output "           \
+                  "(default none, to stdout).\n"                              \
+              "--port=PORT         : Primary port (default %d).\n"            \
+              "--verbosity=NUM     : Debug printing level "                   \
+                  "0..4, (default %d).\n",
+              DEFAULT_PORT,
+              MIRROR_LOG_DEFAULT_VERBOSITY);
+} /* end usage() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    parse_args
+ *
+ * Purpose:     Read command line options and store results in args_out
+ *              structure. Fails in event of unrecognized option.
+ *
+ * Return:      0 on success, -1 on failure.
+ * ---------------------------------------------------------------------------
+ */
+static int
+parse_args(int argc, char **argv, struct op_args *args_out)
+{
+    int i;
+
+    /* preset default values
+     */
+    args_out->main_port = DEFAULT_PORT;
+    args_out->help = 0;
+    args_out->log_prepend_serv = 1;
+    args_out->log_prepend_type = 1;
+    args_out->verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+    /* preset empty strings */
+    HDbzero(args_out->log_path,        PATH_MAX+1);
+    HDbzero(args_out->writer_log_path, PATH_MAX+1);
+
+    if (argv == NULL || *argv == NULL) {
+        mirror_log(NULL, V_ERR, "invalid argv pointer");
+        return -1;
+    }
+
+    /* Loop over arguments after program name and writer_path */
+    for (i=2; i < argc; i++) {
+        if (!HDstrncmp(argv[i], "-h", 3) || !HDstrncmp(argv[i], "--help", 7)) {
+            mirror_log(NULL, V_INFO, "found help argument");
+            args_out->help = 1;
+            return 0;
+        } /* end if help */
+        else
+        if (!HDstrncmp(argv[i], "--port=", 7)) {
+            mirror_log(NULL, V_INFO, "parsing 'main_port' (%s)", argv[i]+7);
+            args_out->main_port = HDatoi(argv[i]+7);
+        } /* end if port */
+        else
+        if (!HDstrncmp(argv[i], "--verbosity=", 12)) {
+            mirror_log(NULL, V_INFO, "parsing 'verbosity' (%s)", argv[i]+12);
+            args_out->verbosity = HDatoi(argv[i]+12);
+        } /* end if verbosity */
+        else
+        if (!HDstrncmp(argv[i], "--logpath=", 10)) {
+            mirror_log(NULL, V_INFO, "parsing 'logpath' (%s)", argv[i]+10);
+            HDstrncpy(args_out->log_path, argv[i]+10, PATH_MAX);
+        } /* end if logpath */
+        else {
+            mirror_log(NULL, V_ERR, "unrecognized argument: %s", argv[i]);
+            return -1;
+        } /* end if unrecognized argument */
+    } /* end for each arg after the path to writer "receiver process" */
+
+    mirror_log(NULL, V_INFO, "all args parsed");
+
+    return 0;
+} /* end parse_args() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    prepare_listening_socket
+ *
+ * Purpose:     Configure and open a socket.
+ *              In event of error, attempts to undo its processes.
+ *
+ * Return:      Success: non-negative (the file descriptor of the socket)
+ *              Failure: -1
+ * ---------------------------------------------------------------------------
+ */
+static int
+prepare_listening_socket(struct server_run *run)
+{
+    struct sockaddr_in server_addr;
+    int _true = 1; /* needed for setsockopt() */
+    int ret_value = -1;
+    int ret = 0; /* for checking return value of function calls */
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "preparing socket");
+
+    server_addr.sin_family      = AF_INET;
+    server_addr.sin_addr.s_addr = HDhtonl(INADDR_ANY);
+    server_addr.sin_port        = HDhtons((uint16_t)run->opts.main_port);
+
+    mirror_log(run->loginfo, V_INFO, "socket()");
+    ret_value = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (ret_value < 0) {
+        mirror_log(run->loginfo, V_ERR, "listening socket:%d", ret_value);
+        goto error;
+    }
+
+    mirror_log(run->loginfo, V_ALL, "setsockopt()");
+    HDsetsockopt(ret_value, SOL_SOCKET, SO_REUSEADDR, &_true, sizeof(int));
+
+    mirror_log(run->loginfo, V_INFO, "bind()");
+    ret = HDbind(ret_value, (struct sockaddr *)&server_addr,
+            sizeof(server_addr));
+    if (ret < 0) {
+        mirror_log(run->loginfo, V_ERR, "bind() %s", HDstrerror(errno));
+        goto error;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "listen()");
+    ret = HDlisten(ret_value, LISTENQ);
+    if (ret < 0) {
+        mirror_log(run->loginfo, V_ERR, "H5FD server listen:%d", ret);
+        goto error;
+    }
+
+    return ret_value;
+
+error:
+    if (ret_value >= 0) {
+        HDshutdown(ret_value, SHUT_RDWR);
+        HDclose(ret_value);
+    }
+    return -1;
+} /* end prepare_listening_socket() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    init_server_run
+ *
+ * Purpose:     Set up server_run struct with default and specified values.
+ *
+ * Return:      Zero (0) if successful, -1 if an error occurred.
+ * ---------------------------------------------------------------------------
+ */
+static struct server_run *
+init_server_run(int argc, char **argv)
+{
+    struct server_run *run;
+
+    run = (struct server_run *)HDmalloc(sizeof(struct server_run));
+    if (run == NULL) {
+        mirror_log(NULL, V_ERR, "can't allocate server_run struct");
+        return NULL;
+    }
+
+    run->magic = (uint32_t)SERVER_RUN_MAGIC;
+    run->opts.magic = (uint32_t)OP_ARGS_MAGIC;
+    run->listenfd = -1;
+
+    if (parse_args(argc, argv, &(run->opts)) < 0) {
+        mirror_log(NULL, V_ERR, "can't parse arguments");
+        usage();
+        goto error;
+    }
+
+    if (run->opts.help) {
+        usage();
+        return run; /* early exit */
+    }
+
+    run->loginfo = mirror_log_init(run->opts.log_path, "s- ",
+            run->opts.verbosity);
+
+    run->listenfd = prepare_listening_socket(run);
+    if (run->listenfd < 0) {
+        mirror_log(NULL, V_ERR, "can't prepare listening socket");
+        goto error;
+    }
+
+    return run;
+
+error:
+    if (run != NULL) {
+        HDfree(run);
+    }
+    return NULL;
+
+} /* end init_server_run() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    term_server_run
+ *
+ * Purpose:     Close opened items in a sever_run and release the pointer.
+ *
+ * Return:      Zero (0) if successful, -1 if an error occurred.
+ * ---------------------------------------------------------------------------
+ */
+static int
+term_server_run(struct server_run *run)
+{
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "shutting down");
+
+    if (run->listenfd >= 0) {
+        HDshutdown(run->listenfd, SHUT_RDWR); /* TODO: error-checking? */
+        HDclose(run->listenfd); /* TODO: error-checking? */
+        run->listenfd = -1;
+    }
+
+    if (mirror_log_term(run->loginfo) < 0) {
+        mirror_log(NULL, V_ERR, "can't close logging stream");
+        return -1; /* doesn't solve the problem, but informs of error */
+    }
+    run->loginfo = NULL;
+
+    (run->magic)++;
+    (run->opts.magic)++;
+    HDfree(run);
+    return 0;
+} /* end term_server_run() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    accept_connection
+ *
+ * Purpose:     Main working loop; process requests as they are received.
+ *              Does nothing if the run option help is set.
+ *
+ * Return:      -1 on error, else a non-negative file descriptor of the socket.
+ * ---------------------------------------------------------------------------
+ */
+static int
+accept_connection(struct server_run *run)
+{
+    struct sockaddr_in client_addr;        /**/
+    socklen_t          clilen;             /**/
+    struct hostent    *host_port = NULL;   /**/
+    char              *hostaddrp;          /**/
+    int                connfd = -1; /* connection file descriptor */
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    /*------------------------------*/
+    /* accept a connection on a socket */
+    clilen = sizeof(client_addr);
+    connfd = HDaccept(run->listenfd, (struct sockaddr *)&client_addr, &clilen);
+    if (connfd < 0) {
+        mirror_log(run->loginfo, V_ERR, "accept:%d", connfd);
+        goto error;
+    }
+    mirror_log(run->loginfo, V_INFO, "connection achieved");
+
+    /*------------------------------*/
+    /* get client address information */
+    host_port = HDgethostbyaddr(
+            (const char *)&client_addr.sin_addr.s_addr,
+            sizeof(client_addr.sin_addr.s_addr),
+            AF_INET);
+    if (host_port == NULL) {
+        mirror_log(run->loginfo, V_ERR, "gethostbyaddr()");
+        goto error;
+    }
+
+    /* function has the string space statically scoped -- OK until next call */
+    hostaddrp = HDinet_ntoa(client_addr.sin_addr);
+    /* TODO? proper error-checking */
+
+    mirror_log(run->loginfo, V_INFO,
+            "server connected with %s (%s)",
+            host_port->h_name,
+            hostaddrp);
+
+    return connfd;
+
+error:
+    if (connfd >= 0) {
+        close(connfd);
+    }
+    return -1;
+} /* end accept_connection() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    wait_for_child
+ *
+ * Purpose:     Signal handler to reap zombie processes.
+ * ---------------------------------------------------------------------------
+ */
+static void
+wait_for_child(int sig)
+{
+    while (HDwaitpid(-1, NULL, WNOHANG) > 0);
+} /* end wait_for_child() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    handle_requests
+ *
+ * Purpose:     Main working loop; process requests as they are received.
+ *              Does nothing if the run option `help` is set.
+ *
+ * Return:      -1 on error, else 0 for successful operation.
+ * ---------------------------------------------------------------------------
+ */
+static int
+handle_requests(struct server_run *run)
+{
+    int         connfd = -1;        /**/
+    char        mybuf[H5FD_MIRROR_XMIT_OPEN_SIZE];      /**/
+    int         ret;                /* general-purpose error-checking */
+    int         pid;                /* process ID of fork */
+    struct sigaction sa;
+    int         ret_value = 0;
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    if (run->opts.help) {
+        return 0;
+    }
+
+    if (run->listenfd < 0) {
+        mirror_log(NULL, V_ERR, "invalid listening socket");
+        return -1;
+    }
+
+    /* Set up the signal handler */
+    sa.sa_handler = wait_for_child;
+    HDsigemptyset(&sa.sa_mask);
+    sa.sa_flags = SA_RESTART;
+    if (HDsigaction(SIGCHLD, &sa, NULL) == -1) {
+        perror("sigaction");
+        return 1;
+    }
+
+    /* Keep listening for attempts to connect.
+     */
+
+    while (1) { /* infinite loop, exited via break or goto */
+        mirror_log(run->loginfo, V_INFO, "server waiting for connections...");
+
+        connfd = -1;
+
+        connfd = accept_connection(run);
+        if (connfd < 0) {
+            mirror_log(run->loginfo, V_ERR, "unable to receive connection");
+            goto error;
+        }
+
+        /* Read handshake from port connection.
+         */
+
+        ret = (int)HDread(connfd, &mybuf, H5FD_MIRROR_XMIT_OPEN_SIZE);
+        if (-1 == ret) {
+            mirror_log(run->loginfo, V_ERR, "read:%d", ret);
+            goto error;
+        }
+        mirror_log(run->loginfo, V_INFO, "received %d bytes", ret);
+        mirror_log(run->loginfo, V_ALL, "```");
+        mirror_log_bytes(run->loginfo, V_ALL, ret,
+                (const unsigned char *)mybuf);
+        mirror_log(run->loginfo, V_ALL, "```");
+
+        /* Respond to handshake message.
+         */
+
+        if (!HDstrncmp("SHUTDOWN", mybuf, 8)) {
+            /* Stop operation if told to stop */
+            mirror_log(run->loginfo, V_INFO, "received SHUTDOWN!", ret);
+            HDclose(connfd);
+            connfd = -1;
+            goto done;
+        } /* end if explicit "SHUTDOWN" directive */
+        else
+        if (H5FD_MIRROR_XMIT_OPEN_SIZE == ret) {
+            H5FD_mirror_xmit_open_t xopen;
+
+            mirror_log(run->loginfo, V_INFO,
+                     "probable OPEN xmit received");
+
+            H5FD_mirror_xmit_decode_open(&xopen, (const unsigned char *)mybuf);
+            if (FALSE == H5FD_mirror_xmit_is_open(&xopen)) {
+                mirror_log(run->loginfo, V_WARN,
+                         "expected OPEN xmit was malformed");
+                HDclose(connfd);
+                continue;
+            }
+
+            mirror_log(run->loginfo, V_INFO,
+                     "probable OPEN xmit confirmed");
+
+            pid = HDfork();
+            if (pid < 0) { /* fork error */
+                mirror_log(run->loginfo, V_ERR, "cannot fork");
+                goto error;
+            } /* end if fork error */
+            else
+            if (pid == 0) { /* child process (writer side of fork) */
+                mirror_log(run->loginfo, V_INFO,
+                         "executing writer");
+                if (run_writer(connfd, &xopen) < 0) {
+                    HDprintf("can't run writer\n");
+                }
+                else {
+                    HDprintf("writer OK\n");
+                }
+                HDclose(connfd);
+
+                HDexit(EXIT_SUCCESS);
+            } /* end if writer side of fork */
+            else { /* parent process (server side of fork) */
+                mirror_log(run->loginfo, V_INFO, "tidying up from handshake");
+                HDclose(connfd);
+            } /* end if server side of fork */
+
+        } /* end else-if valid request for service */
+        else {
+            /* Ignore unrecognized messages */
+            HDclose(connfd);
+            continue;
+        } /* end else (not a valid message, to be ignored) */
+
+    } /* end while listening for new connections */
+
+done:
+    if (connfd >= 0) {
+        mirror_log(run->loginfo, V_WARN, "connfd still open upon cleanup");
+        HDclose(connfd);
+    }
+
+    return ret_value;
+
+error:
+    if (connfd >= 0) {
+        HDclose(connfd);
+    }
+    return -1;
+} /* end handle_requests() */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    struct server_run  *run;
+
+    run = init_server_run(argc, argv);
+    if (NULL == run) {
+        mirror_log(NULL, V_ERR, "can't initialize run");
+        HDexit(EXIT_FAILURE);
+    }
+
+    if (handle_requests(run) < 0) {
+        mirror_log(run->loginfo, V_ERR, "problem handling requests");
+    }
+
+    if (term_server_run(run) < 0) {
+        mirror_log(NULL, V_ERR, "problem closing server run");
+        HDexit(EXIT_FAILURE);
+    }
+
+    HDexit(EXIT_SUCCESS);
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+int
+main(int argc, char **argv)
+{
+    HDprintf("Mirror VFD was not built -- cannot launch server.\n");
+    HDexit(EXIT_FAILURE);
+}
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_server_halten_sie.c b/utils/mirror_vfd/mirror_server_halten_sie.c
new file mode 100644
index 0000000..ccd5824
--- /dev/null
+++ b/utils/mirror_vfd/mirror_server_halten_sie.c
@@ -0,0 +1,214 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:     Stop the mirror server
+ *              Exists for cross-platform, optionally remote shutdown.
+ */
+
+#include "H5private.h" /* System compatability call-wrapper macros */
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define MSHS_OPTS_MAGIC 0x613B1C15u /* sanity-checking constant */
+#define MSHS_IP_STR_SIZE 20
+#define MSHS_DEFAULT_IP "127.0.0.1"
+#define MSHS_DEFAULT_PORTNO 3000
+
+
+/* ----------------------------------------------------------------------------
+ * Structure:   struct mshs_opts
+ *
+ * Purpose:     Convenience structure to hold options as parsed from the
+ *              command line.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique constant to help verify pointer integrity.
+ *
+ * `help` (int)
+ *      Flag that the help argument was present.
+ *
+ * `portno` (int)
+ *      Port number, as received from arguments.
+ *
+ * `ip` (char *)
+ *      IP address string as received from arguments.
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mshs_opts {
+    uint32_t    magic;
+    int         help;
+    int         portno;
+    char        ip[MSHS_IP_STR_SIZE + 1];
+};
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    usage
+ *
+ * Purpose:     Print usage message to stdout.
+ * ----------------------------------------------------------------------------
+ */
+static void
+usage(void)
+{
+    HDprintf("mirror_server_halten_sie [options]\n"                           \
+             "System-independent Mirror Server shutdown program.\n"           \
+             "Sends shutdown message to Mirror Server at given IP:port\n"     \
+             "\n"                                                             \
+             "Options:\n"                                                     \
+             "    -h | --help Print this usage message and exit.\n"           \
+             "    --ip=ADDR   IP Address of remote server (defaut %s)\n"      \
+             "    --port=PORT Handshake port of remote server (default %d)\n",
+             MSHS_DEFAULT_IP,
+             MSHS_DEFAULT_PORTNO);
+} /* end usage() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    parse_args
+ *
+ * Purpose:     Parse command-line arguments, populating the options struct
+ *              pointer as appropriate.
+ *              Default values will be set for unspecified options.
+ *
+ * Return:      0 on success, negative (-1) if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+parse_args(int argc, char **argv, struct mshs_opts *opts)
+{
+    int i = 0;
+
+    opts->magic = MSHS_OPTS_MAGIC;
+    opts->help = 0;
+    opts->portno = MSHS_DEFAULT_PORTNO;
+    HDstrncpy(opts->ip, MSHS_DEFAULT_IP, MSHS_IP_STR_SIZE);
+
+    for (i=1; i < argc; i++) { /* start with first possible option argument */
+        if (!HDstrncmp(argv[i], "-h", 3) || !HDstrncmp(argv[i], "--help", 7)) {
+            opts->help = 1;
+        }
+        else
+        if (!HDstrncmp(argv[i], "--ip=", 5)) {
+            HDstrncpy(opts->ip, argv[i]+5, MSHS_IP_STR_SIZE);
+        }
+        else
+        if (!HDstrncmp(argv[i], "--port=", 7)) {
+            opts->portno = HDatoi(argv[i]+7);
+        }
+        else {
+            HDprintf("Unrecognized option: '%s'\n", argv[i]);
+            usage();
+            opts->magic++; /* invalidate for sanity */
+            return -1;
+        }
+    } /* end for each argument from command line */
+
+    /* auto-replace 'localhost' with numeric IP */
+    if (!HDstrncmp(opts->ip, "localhost", 10)) { /* include null terminator */
+        HDstrncpy(opts->ip, "127.0.0.1", MSHS_IP_STR_SIZE);
+    }
+
+    return 0;
+} /* end parse_args() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    send_shutdown
+ *
+ * Purpose:     Create socket and send shutdown signal to remote server.
+ *
+ * Return:      0 on success, negative (-1) if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+send_shutdown(struct mshs_opts *opts)
+{
+    int                live_socket;
+    struct sockaddr_in target_addr;
+
+    if (opts->magic != MSHS_OPTS_MAGIC) {
+       HDprintf("invalid options structure\n");
+       return -1;
+    }
+
+    live_socket = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (live_socket < 0) {
+        HDprintf("ERROR socket()\n");
+        return -1;
+    }
+
+    target_addr.sin_family = AF_INET;
+    target_addr.sin_port = HDhtons((uint16_t)opts->portno);
+    target_addr.sin_addr.s_addr = HDinet_addr(opts->ip);
+    HDmemset(target_addr.sin_zero, '\0', sizeof(target_addr.sin_zero));
+
+    if (HDconnect(live_socket, (struct sockaddr *)&target_addr,
+            (socklen_t)sizeof(target_addr))
+        < 0)
+    {
+        HDprintf("ERROR connect() (%d)\n%s\n", errno, HDstrerror(errno));
+        return -1;
+    }
+
+    if (HDwrite(live_socket, "SHUTDOWN", 9) == -1) {
+        HDprintf("ERROR write() (%d)\n%s\n", errno, HDstrerror(errno));
+        return -1;
+    }
+
+    if (HDclose(live_socket) < 0) {
+        HDprintf("ERROR close() can't close socket\n");
+        return -1;
+    }
+
+    return 0;
+} /* end send_shutdown() */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    struct mshs_opts opts;
+
+    if (parse_args(argc, argv, &opts) < 0) {
+        HDprintf("Unable to parse arguments\n");
+        return EXIT_FAILURE;
+    }
+
+    if (opts.help) {
+        usage();
+        return EXIT_SUCCESS;
+    }
+
+    if (send_shutdown(&opts) < 0) {
+        HDprintf("Unable to send shutdown command\n");
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    HDprintf("Mirror VFD not built -- unable to perform shutdown.\n");
+    return EXIT_FAILURE;
+}
+
+#endif /* H5_HAVE_MIRROR_VFD */
diff --git a/utils/mirror_vfd/mirror_writer.c b/utils/mirror_vfd/mirror_writer.c
new file mode 100644
index 0000000..9cd9b4c
--- /dev/null
+++ b/utils/mirror_vfd/mirror_writer.c
@@ -0,0 +1,1064 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Remote writer process for the mirror (socket) VFD.
+ *
+ * Writer is started with arguments for slaved port.
+ * Awaits a connection on the socket.
+ * Handles instructions from the master 'Driver' process.
+ *
+ * Current implementation uses Sec2 as the underlying driver when opening a
+ * file. This is reflected in the source (H5FDmirror.c) of the Mirror driver.
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define HEXDUMP_XMITS 1           /* Toggle whether to print xmit bytes-blob */
+                                  /* in detailed logging  */
+#define HEXDUMP_WRITEDATA 0       /* Toggle whether to print bytes to write */
+                                  /* in detailed logging  */
+#define LISTENQ 80                /* max pending Driver requests */
+
+#define MW_SESSION_MAGIC        0x88F36B32u
+#define MW_SOCK_COMM_MAGIC      0xDF10A157u
+#define MW_OPTS_MAGIC   0x3BA8B462u
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct mirror_session
+ *
+ * Bundle of information used to manage the operation of this remote Writer
+ * in a "session" with the Driver process.
+ *
+ * magic (uint32_t)
+ *       Semi-unique "magic" number used to sanity-check a structure for
+ *       validity. MUST equal MW_SESSION_MAGIC to be valid.
+ *
+ * sockfd (int)
+ *       File descriptor to the socket.
+ *       Used for receiving bytes from and writing bytes to the Driver
+ *       across the network.
+ *       If not NULL, should be a valid descriptor.
+ *
+ * token (uint32t)
+ *      Number used to help sanity-check received transmission from the Writer.
+ *      Each Driver/Writer pairing should have a semi-unique "token" to help
+ *      guard against commands from the wrong entity.
+ *
+ * xmit_count (uint32_t)
+ *      Record of trasmissions received from the Driver. While the transmission
+ *      protocol should be trustworthy, this serves as an additional guard.
+ *      Starts a 0 and should be incremented for each one-way transmission.
+ *
+ * file (H5FD_t *)
+ *      Virtual File handle for the hdf5 file.
+ *      Set on file open if H5Fopen() is successful. If NULL, it is invalid.
+ *
+ * log_verbosity (unsigned int)
+ *      The verbosity level for logging. Should be set to one of the values
+ *      defined at the top of this file.
+ *
+ * log_stream (FILE *)
+ *      File pointer to which logging output is written. Starts (and ends)
+ *      with a default stream, such as stdout, but can be overridden at
+ *      runtime.
+ *
+ * reply (H5FD_mirror_xmit_reply_t)
+ *      Structure space for persistent reply data.
+ *      Should be initialized with basic header info (magic, version, op),
+ *      then with session info (token, xmit count), and finally with specific
+ *      reply info (update xmit_count, status code, and message) before
+ *      transmission.
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mirror_session {
+    uint32_t                  magic;
+    int                       sockfd;
+    uint32_t                  token;
+    uint32_t                  xmit_count;
+    H5FD_t                   *file;
+    loginfo_t                *loginfo;
+    H5FD_mirror_xmit_reply_t  reply;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct sock_comm
+ *
+ * Structure for placing the data read and pre-processed from Driver in an
+ * organized fashion. Useful for pre-processing a received xmit.
+ *
+ * magic (uint32_t)
+ *      Semi-unique number to sanity-check structure pointer and validity.
+ *      Must equal MW_SOCK_COMM_MAGIC to be valid.
+ *
+ * recd_die (int)
+ *      "Boolean" flag indicating that an explicit shutdown/kill/die command
+ *      was received. Potentially useful for debugging and or "manual"
+ *      operation of the program.
+ *      0 indicates normal operation, non-0 (1) indicates to die.
+ *
+ * xmit_recd (H5FD_mirror_xmit_t *)
+ *      Structure pointer for the "xmit header" as decoded from the raw
+ *      binary stream read from the socket.
+ *
+ * raw (char *)
+ *      Pointer to a raw byte array, for storing data as read from the
+ *      socket. Bytes buffer is decoded into xmit_t header and derivative
+ *      structures.
+ *
+ * raw_size (size_t)
+ *      Give the size of the `raw` buffer.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct sock_comm {
+    uint32_t            magic;
+    int                 recd_die;
+    H5FD_mirror_xmit_t *xmit_recd;
+    char               *raw;
+    size_t              raw_size;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct mirror_writer_opts
+ *
+ * Container for default values and options as parsed from the command line.
+ * Currently rather vestigal, but may be expanded and/or moved to be set by
+ * Server and passed around as an argument.
+ *
+ * magic (uint32_t)
+ *      Semi-unique number to sanity-check structure pointer and validity.
+ *      Must equal MW_OPTS_MAGIC to be valid.
+ *
+ * logpath (char *)
+ *      String pointer. Allocated at runtime.
+ *      Specifies file location for logging output.
+ *      May be NULL -- uses default output (e.g., stdout).
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mirror_writer_opts {
+    uint32_t    magic;
+    char       *logpath;
+};
+
+static int do_open(struct mirror_session *session,
+        const H5FD_mirror_xmit_open_t *xmit_open);
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_init
+ *
+ * Purpose:     Populate mirror_session structure with default and
+ *              options-drived values.
+ *
+ * Return:      An allocated mirror_session structure pointer on success,
+ *              else NULL.
+ * ----------------------------------------------------------------------------
+ */
+static struct mirror_session *
+session_init(struct mirror_writer_opts *opts)
+{
+    struct mirror_session *session = NULL;
+
+    mirror_log(NULL, V_INFO, "session_init()");
+
+    if (NULL == opts || opts->magic != MW_OPTS_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid opts pointer");
+        goto error;
+    }
+
+    session = (struct mirror_session *)HDmalloc(sizeof(struct mirror_session));
+    if (session == NULL) {
+        mirror_log(NULL, V_ERR, "can't allocate session structure");
+        goto error;
+    }
+
+    session->magic         = MW_SESSION_MAGIC;
+    session->sockfd        = -1;
+    session->xmit_count    = 0;
+    session->token         = 0;
+    session->file          = NULL;
+
+    session->reply.pub.magic         = H5FD_MIRROR_XMIT_MAGIC;
+    session->reply.pub.version       = H5FD_MIRROR_XMIT_CURR_VERSION;
+    session->reply.pub.op            = H5FD_MIRROR_OP_REPLY;
+    session->reply.pub.session_token = 0;
+    HDbzero(session->reply.message, H5FD_MIRROR_STATUS_MESSAGE_MAX);
+
+    /* Options-derived population
+     */
+
+    session->loginfo = mirror_log_init(opts->logpath, "W- ",
+            MIRROR_LOG_DEFAULT_VERBOSITY);
+
+    return session;
+
+error:
+    if (session) {
+        HDfree(session);
+    }
+    return NULL;
+} /* end session_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_stop
+ *
+ * Purpose:     Stop and clean up a session.
+ *              Only do this as part of program termination or aborting startup.
+ *
+ * Return:      0 on success, or negative sum of errors encountered.
+ * ----------------------------------------------------------------------------
+ */
+static int
+session_stop(struct mirror_session *session)
+{
+    int ret_value = 0;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "session_stop()");
+
+    /* Close HDF5 file if it is still open (probably in error) */
+    if (session->file) {
+        mirror_log(session->loginfo, V_WARN, "HDF5 file still open at cleanup");
+        if (H5FDclose(session->file) < 0) {
+            mirror_log(session->loginfo, V_ERR, "H5FDclose() during cleanup!");
+            ret_value--;
+        }
+    }
+
+    /* Socket will be closed by parent side of server fork after exit */
+
+    /* Close custom logging stream */
+    if (mirror_log_term(session->loginfo) < 0) {
+        mirror_log(NULL, V_ERR, "Problem closing logging stream");
+        ret_value--;
+    }
+    session->loginfo = NULL;
+
+    /* Invalidate and release structure */
+    session->magic++;
+    HDfree(session);
+
+    return ret_value;
+} /* end session_stop() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_start
+ *
+ * Purpose:     Initiate session, open files.
+ *
+ * Return:      Success: A valid mirror_session pointer which must later be
+ *                       cleaned up with session_stop().
+ *              Failure: NULL, after cleaning up after itself.
+ * ---------------------------------------------------------------------------
+ */
+static struct mirror_session *
+session_start(int socketfd, const H5FD_mirror_xmit_open_t *xmit_open)
+{
+    struct mirror_session *session = NULL;
+    struct mirror_writer_opts opts;
+#if 0 /* TODO: behaviro option */
+    char logpath[H5FD_MIRROR_XMIT_FILEPATH_MAX] = "";
+#endif
+
+    mirror_log(NULL, V_INFO, "session_start()");
+
+    if (FALSE == H5FD_mirror_xmit_is_open(xmit_open)) {
+        mirror_log(NULL, V_ERR, "invalid OPEN xmit");
+        return NULL;
+    }
+
+    opts.magic = MW_OPTS_MAGIC;
+#if 0 /* TODO: behavior option */
+    HDsnprintf(logpath, H5FD_MIRROR_XMIT_FILEPATH_MAX, "%s.log",
+            xmit_open->filename);
+    opts.logpath = logpath;
+#else
+    opts.logpath = NULL;
+#endif
+
+    session = session_init(&opts);
+    if (NULL == session) {
+        mirror_log(NULL, V_ERR, "can't instantiate session");
+        goto error;
+    }
+
+    session->sockfd = socketfd;
+
+    if (do_open(session, xmit_open) < 0) {
+        mirror_log(NULL, V_ERR, "unable to open file");
+        goto error;
+    }
+
+    return session;
+
+error:
+    if (session != NULL) {
+        if (session_stop(session) < 0) {
+            mirror_log(NULL, V_WARN, "Can't abort session init");
+        }
+        session = NULL;
+    }
+    return NULL;
+}
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    _xmit_reply
+ *
+ * Purpose:     Common operations to send a reply xmit through the session.
+ *
+ * Return:      0 on success, -1 if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+_xmit_reply(struct mirror_session *session)
+{
+    unsigned char             xmit_buf[H5FD_MIRROR_XMIT_REPLY_SIZE];
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "_xmit_reply()");
+
+    reply->pub.xmit_count = session->xmit_count++;
+    if (H5FD_mirror_xmit_encode_reply(xmit_buf,
+            (const H5FD_mirror_xmit_reply_t *)reply)
+        != H5FD_MIRROR_XMIT_REPLY_SIZE)
+    {
+        mirror_log(session->loginfo, V_ERR, "can't encode reply");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_ALL, "reply xmit data\n```");
+    mirror_log_bytes(session->loginfo, V_ALL, H5FD_MIRROR_XMIT_REPLY_SIZE,
+            (const unsigned char *)xmit_buf);
+    mirror_log(session->loginfo, V_ALL, "```");
+
+    if (HDwrite(session->sockfd, xmit_buf, H5FD_MIRROR_XMIT_REPLY_SIZE) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't write reply to Driver");
+        return -1;
+    }
+
+    return 0;
+} /* end _xmit_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    reply_ok
+ *
+ * Purpose:     Send an OK reply through the session.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+reply_ok(struct mirror_session *session)
+{
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "reply_ok()");
+
+    reply->status = H5FD_MIRROR_STATUS_OK;
+    HDbzero(reply->message, H5FD_MIRROR_STATUS_MESSAGE_MAX);
+    return _xmit_reply(session);
+} /* end reply_ok() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    reply_error
+ *
+ * Purpose:     Send an ERROR reply with message through the session.
+ *              Message may be cut short if it would overflow the available
+ *              buffer in the xmit.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+reply_error(struct mirror_session  *session,
+            const char             *msg)
+{
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "reply_error(%s)", msg);
+
+    reply->status = H5FD_MIRROR_STATUS_ERROR;
+    HDsnprintf(reply->message, H5FD_MIRROR_STATUS_MESSAGE_MAX-1, "%s", msg);
+    return _xmit_reply(session);
+} /* end reply_error() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_close
+ *
+ * Purpose:     Handle an CLOSE operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_close(struct mirror_session *session)
+{
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_close()");
+
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "no file to close!");
+        reply_error(session, "no file to close");
+        return -1;
+    }
+
+    if (H5FDclose(session->file) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDclose()");
+        reply_error(session, "H5FDclose()");
+        return -1;
+    }
+    session->file = NULL;
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_close() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_lock
+ *
+ * Purpose:     Handle a LOCK operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_lock(struct mirror_session  *session,
+        const unsigned char    *xmit_buf)
+{
+    size_t                  decode_ret = 0;
+    H5FD_mirror_xmit_lock_t xmit_lock;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_lock()");
+
+    decode_ret = H5FD_mirror_xmit_decode_lock(&xmit_lock, xmit_buf);
+    if (H5FD_MIRROR_XMIT_LOCK_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_lock(&xmit_lock)) {
+        mirror_log(session->loginfo, V_ERR, "not a set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decode failure");
+        return -1;
+    }
+    mirror_log(session->loginfo, V_INFO, "lock rw: (%d)", xmit_lock.rw);
+
+    if (H5FDlock(session->file, (hbool_t)xmit_lock.rw) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDlock()");
+        reply_error(session, "remote H5FDlock() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_open
+ *
+ * Purpose:     Handle an OPEN operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_open(struct mirror_session  *session,
+        const H5FD_mirror_xmit_open_t *xmit_open)
+{
+    hid_t fapl_id = H5I_INVALID_HID;
+    unsigned _flags = 0;
+    haddr_t _maxaddr = HADDR_UNDEF;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_open &&
+             TRUE == H5FD_mirror_xmit_is_open(xmit_open));
+
+    mirror_log(session->loginfo, V_INFO, "do_open()");
+
+    if (0 != xmit_open->pub.xmit_count) {
+        mirror_log(session->loginfo, V_ERR, "open with xmit count not zero!");
+        reply_error(session, "initial transmission count not zero");
+        goto error;
+    }
+    if (0 != session->token) {
+        mirror_log(session->loginfo, V_ERR, "open with token already set!");
+        reply_error(session, "initial session token not zero");
+        goto error;
+    }
+
+    session->xmit_count = 1;
+    session->token = xmit_open->pub.session_token;
+    session->reply.pub.session_token = session->token;
+
+    _flags = (unsigned)xmit_open->flags;
+    _maxaddr = (haddr_t)xmit_open->maxaddr;
+
+    /* Check whether the native size_t on the remote machine (Driver) is larger
+     * than that on the local machine; if so, issue a warning.
+     * The blob is always an 8-byte bitfield -- check its contents.
+     */
+    if (xmit_open->size_t_blob > (uint64_t)((size_t)(-1))) {
+        mirror_log(session->loginfo, V_WARN,
+            "Driver size_t is larger than our own");
+    }
+
+    mirror_log(session->loginfo, V_INFO,
+            "to open file %s (flags %d) (maxaddr %d)",
+            xmit_open->filename, _flags, _maxaddr);
+
+    /* Explicitly use Sec2 as the underlying driver for now.
+     */
+    fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (fapl_id < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't create FAPL");
+        reply_error(session, "H5Pcreate() failure");
+        goto error;
+    }
+    if (H5Pset_fapl_sec2(fapl_id) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't set FAPL as Sec2");
+        reply_error(session, "H5Pset_fapl_sec2() failure");
+        goto error;
+    }
+
+    session->file = H5FDopen(xmit_open->filename, _flags, fapl_id, _maxaddr);
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "H5FDopen()");
+        reply_error(session, "remote H5FDopen() failure");
+        goto error;
+    }
+
+    /* FAPL is set and in use; clean up */
+    if (H5Pclose(fapl_id) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't set FAPL as Sec2");
+        reply_error(session, "H5Pset_fapl_sec2() failure");
+        goto error;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+
+error:
+    if (fapl_id > 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Pclose(fapl_id);
+        } H5E_END_TRY;
+    }
+    return -1;
+} /* end do_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_set_eoa
+ *
+ * Purpose:     Handle a SET_EOA operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_set_eoa(struct mirror_session  *session,
+           const unsigned char    *xmit_buf)
+{
+    size_t                 decode_ret = 0;
+    H5FD_mirror_xmit_eoa_t xmit_seoa;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_set_eoa()");
+
+    decode_ret = H5FD_mirror_xmit_decode_set_eoa(&xmit_seoa, xmit_buf);
+    if (H5FD_MIRROR_XMIT_EOA_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_set_eoa(&xmit_seoa)) {
+        mirror_log(session->loginfo, V_ERR, "not a set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decode failure");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "set EOA addr %d",
+            xmit_seoa.eoa_addr);
+
+    if (H5FDset_eoa(session->file, (H5FD_mem_t)xmit_seoa.type,
+            (haddr_t)xmit_seoa.eoa_addr)
+        < 0)
+    {
+        mirror_log(session->loginfo, V_ERR, "H5FDset_eoa()");
+        reply_error(session, "remote H5FDset_eoa() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_truncate
+ *
+ * Purpose:     Handle a TRUNCATE operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_truncate(struct mirror_session  *session)
+{
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_truncate()");
+
+    /* default DXPL ID (0), 0 for "FALSE" closing -- both probably unused */
+    if (H5FDtruncate(session->file, 0, 0) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDtruncate()");
+        reply_error(session, "remote H5FDtruncate() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_truncate() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_unlock
+ *
+ * Purpose:     Handle an UNLOCK operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_unlock(struct mirror_session  *session)
+{
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_unlock()");
+
+    if (H5FDunlock(session->file) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDunlock()");
+        reply_error(session, "remote H5FDunlock() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_unlock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_write
+ *
+ * Purpose:     Handle a WRITE operation.
+ *              Receives command, replies; receives & writes data, replies.
+ *
+ *              It is known that this results in suboptimal performance,
+ *              but handling both small and very, very large write buffers
+ *              with a single "over the wire" exchange
+ *              poses design challenges not worth tackling as of March 2020.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_write(struct mirror_session  *session,
+         const unsigned char    *xmit_buf)
+{
+    size_t      decode_ret        = 0;
+    haddr_t     addr              = 0;
+    haddr_t     sum_bytes_written = 0;
+    H5FD_mem_t  type              = 0;
+    char       *buf               = NULL;
+    ssize_t     nbytes_in_packet  = 0;
+    H5FD_mirror_xmit_write_t xmit_write;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_write()");
+
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "no open file!");
+        reply_error(session, "no file open on remote");
+        return -1;
+    }
+
+    decode_ret = H5FD_mirror_xmit_decode_write(&xmit_write, xmit_buf);
+    if (H5FD_MIRROR_XMIT_WRITE_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode write xmit");
+        reply_error(session, "remote xmit_write_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_write(&xmit_write)) {
+        mirror_log(session->loginfo, V_ERR, "not a write xmit");
+        reply_error(session, "remote xmit_write_t decode failure");
+        return -1;
+    }
+
+    addr = (haddr_t)xmit_write.offset;
+    type = (H5FD_mem_t)xmit_write.type;
+
+    /* Allocate the buffer once -- re-use between loops.
+     */
+    buf = (char *)HDmalloc(sizeof(char) * H5FD_MIRROR_DATA_BUFFER_MAX);
+    if (NULL == buf) {
+        mirror_log(session->loginfo, V_ERR, "can't allocate databuffer");
+        reply_error(session, "can't allocate buffer for receiving data");
+        return -1;
+    }
+
+    /* got write signal; ready for data */
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "to write %zu bytes at %zu",
+            xmit_write.size,
+            addr);
+
+    /* The given write may be:
+     * 1. larger than the allowed single buffer size
+     * 2. larger than the native size_t of this system
+     *
+     * Handle all cases by looping, ingesting as much of the stream as possible
+     * and writing that part to the file.
+     */
+    sum_bytes_written = 0;
+    do {
+        nbytes_in_packet = HDread(session->sockfd, buf,
+                H5FD_MIRROR_DATA_BUFFER_MAX);
+        if (-1 == nbytes_in_packet) {
+            mirror_log(session->loginfo, V_ERR, "can't read into databuffer");
+            reply_error(session, "can't read data buffer");
+            return -1;
+        }
+
+        mirror_log(session->loginfo, V_INFO, "received %zd bytes",
+            nbytes_in_packet);
+        if (HEXDUMP_WRITEDATA) {
+            mirror_log(session->loginfo, V_ALL, "DATA:\n```");
+            mirror_log_bytes(session->loginfo, V_ALL, nbytes_in_packet,
+                    (const unsigned char *)buf);
+            mirror_log(session->loginfo, V_ALL, "```");
+        }
+
+        mirror_log(session->loginfo, V_INFO, "writing %zd bytes at %zu",
+                 nbytes_in_packet,
+                 (addr + sum_bytes_written));
+
+        if (H5FDwrite(session->file, type, H5P_DEFAULT,
+                (addr + sum_bytes_written), (size_t)nbytes_in_packet, buf)
+            < 0)
+        {
+            mirror_log(session->loginfo, V_ERR, "H5FDwrite()");
+            reply_error(session, "remote H5FDwrite() failure");
+            return -1;
+        }
+
+        sum_bytes_written += (haddr_t)nbytes_in_packet;
+
+    } while (sum_bytes_written < xmit_write.size); /* end while ingesting */
+
+    HDfree(buf);
+
+    /* signal that we're done here and a-ok */
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    receive_communique
+ *
+ * Purpose:     Accept bytes from the socket, check for emergency shutdown, and
+ *              sanity-check received bytes.
+ *              The raw bytes read are stored in the sock_comm structure at
+ *              comm->raw.
+ *              The raw bytes are decoded and a xmit_t (header) struct pointer
+ *              in comm is populated at comm->xmit_recd.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+receive_communique(
+        struct mirror_session  *session,
+        struct sock_comm       *comm)
+{
+    ssize_t     read_ret = 0;
+    size_t      decode_ret;
+    H5FD_mirror_xmit_t *X = comm->xmit_recd;
+
+    HDassert((session != NULL)                    && \
+             (session->magic == MW_SESSION_MAGIC) && \
+             (comm != NULL)                       && \
+             (comm->magic == MW_SOCK_COMM_MAGIC)  && \
+             (comm->xmit_recd != NULL)            && \
+             (comm->raw != NULL)                  && \
+             (comm->raw_size >= H5FD_MIRROR_XMIT_BUFFER_MAX) );
+
+    mirror_log(session->loginfo, V_INFO, "receive_communique()");
+
+    HDbzero(comm->raw, comm->raw_size);
+    comm->recd_die = 0;
+
+    mirror_log(session->loginfo, V_INFO, "ready to receive"); /* TODO */
+
+    read_ret = HDread(session->sockfd, comm->raw, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if (-1 == read_ret) {
+        mirror_log(session->loginfo, V_ERR, "read:%zd", read_ret);
+        goto error;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "received %zd bytes", read_ret);
+    if (HEXDUMP_XMITS) {
+        mirror_log(session->loginfo, V_ALL, "```", read_ret);
+        mirror_log_bytes(session->loginfo, V_ALL, (size_t)read_ret,
+                (const unsigned char *)comm->raw);
+        mirror_log(session->loginfo, V_ALL, "```");
+    } /* end if hexdump transmissions received */
+
+    /* old-fashioned manual kill (for debugging) */
+    if (!HDstrncmp("GOODBYE", comm->raw, 7)) {
+        mirror_log(session->loginfo, V_INFO, "received GOODBYE");
+        comm->recd_die = 1;
+        goto done;
+    }
+
+    decode_ret = H5FD_mirror_xmit_decode_header(X,
+            (const unsigned char *)comm->raw);
+    if (H5FD_MIRROR_XMIT_HEADER_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR,
+                "header decode size mismatch: expected (%z), got (%z)",
+                H5FD_MIRROR_XMIT_HEADER_SIZE, decode_ret);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "xmit size mismatch");
+        goto error;
+    }
+
+    if (!H5FD_mirror_xmit_is_xmit(X)) {
+        mirror_log(session->loginfo, V_ERR, "bad magic: 0x%X", X->magic);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "bad magic");
+        goto error;
+    }
+
+    if (session->xmit_count != X->xmit_count) {
+        mirror_log(session->loginfo, V_ERR,
+                "xmit_count mismatch exp:%d recd:%d",
+                session->xmit_count, X->xmit_count);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "xmit_count mismatch");
+        goto error;
+    }
+
+    if ( (session->token > 0) && (session->token != X->session_token) ) {
+        mirror_log(session->loginfo, V_ERR, "wrong session");
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "wrong session");
+        goto error;
+    }
+
+    session->xmit_count++;
+
+done:
+    return 0;
+
+error:
+    return -1;
+} /* end receive_communique() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    process_instructions
+ *
+ * Purpose:     Receive and handle all instructions from Driver.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+process_instructions(struct mirror_session *session)
+{
+    struct sock_comm    comm;
+    char                xmit_buf[H5FD_MIRROR_XMIT_BUFFER_MAX]; /* raw bytes */
+    H5FD_mirror_xmit_t  xmit_recd; /* for decoded xmit header */
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "process_instructions()");
+
+    comm.magic = MW_SOCK_COMM_MAGIC;
+    comm.recd_die = 0; /* Flag for program to terminate */
+    comm.xmit_recd = &xmit_recd;
+    comm.raw = xmit_buf;
+    comm.raw_size = sizeof(xmit_buf);
+
+    while (1) { /* sill-listening infinite loop */
+
+        /* Use convenience structure for raw/decoded info in/out */
+        if (receive_communique(session, &comm) < 0) {
+            mirror_log(session->loginfo, V_ERR, "problem reading socket");
+            return -1;
+        }
+
+        if (comm.recd_die) {
+            goto done;
+        }
+
+        switch(xmit_recd.op) {
+        case H5FD_MIRROR_OP_CLOSE:
+            if (do_close(session) < 0) {
+                return -1;
+            }
+            goto done;
+        case H5FD_MIRROR_OP_LOCK:
+            if (do_lock(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_OPEN:
+            mirror_log(session->loginfo, V_ERR, "OPEN xmit during session");
+            reply_error(session, "illegal OPEN xmit during session");
+            return -1;
+        case H5FD_MIRROR_OP_SET_EOA:
+            if (do_set_eoa(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_TRUNCATE:
+            if (do_truncate(session) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_UNLOCK:
+            if (do_unlock(session) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_WRITE:
+            if (do_write(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        default:
+            mirror_log(session->loginfo, V_ERR, "unrecognized transmission");
+            reply_error(session, "unrecognized transmission");
+            return -1;
+        } /* end switch (xmit_recd.op) */
+
+    } /* end while still listening */
+
+done:
+    comm.magic = 0; /* invalidate structure, on principle */
+    xmit_recd.magic = 0; /* invalidate structure, on principle */
+    return 0;
+} /* end process_instructions() */
+
+
+/* ------------------------------------------------------------------------- */
+herr_t
+run_writer(int socketfd, H5FD_mirror_xmit_open_t *xmit_open)
+{
+    struct mirror_session *session = NULL;
+    int ret_value = SUCCEED;
+
+    session = session_start(socketfd, xmit_open);
+    if (NULL == session) {
+        mirror_log(NULL, V_ERR, "Can't start session -- aborting");
+        ret_value = FAIL;
+    }
+    else {
+        if (process_instructions(session) < 0) {
+            mirror_log(session->loginfo, V_ERR,
+                    "problem processing instructions");
+            ret_value = FAIL;
+        }
+        if (session_stop(session) < 0) {
+            mirror_log(NULL, V_ERR, "Can't stop session -- going down hard");
+            ret_value = FAIL;
+        }
+    }
+
+    return ret_value;
+} /* end run_writer */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+