Add ExternalData module

Take files we previously distributed in ITK 4.3.1:

  CMake/ExternalData.cmake
  CMake/ExternalData_config.cmake.in

and add them for distribution in upstream CMake.  Update the copyright
notice block format to follow CMake conventions.

1 files changed, 710 insertions, 0 deletions

diff --git a/Modules/ExternalData.cmake b/Modules/ExternalData.cmake
new file mode 100644
index 0000000..84020f9
--- /dev/null
+++ b/Modules/ExternalData.cmake
@@ -0,0 +1,710 @@
+# - Manage data files stored outside source tree
+# Use this module to unambiguously reference data files stored outside the
+# source tree and fetch them at build time from arbitrary local and remote
+# content-addressed locations.  Functions provided by this module recognize
+# arguments with the syntax "DATA{<name>}" as references to external data,
+# replace them with full paths to local copies of those data, and create build
+# rules to fetch and update the local copies.
+#
+# The DATA{} syntax is literal and the <name> is a full or relative path
+# within the source tree.  The source tree must contain either a real data
+# file at <name> or a "content link" at <name><ext> containing a hash of the
+# real file using a hash algorithm corresponding to <ext>.  For example, the
+# argument "DATA{img.png}" may be satisfied by either a real "img.png" file in
+# the current source directory or a "img.png.md5" file containing its MD5 sum.
+#
+# The 'ExternalData_Expand_Arguments' function evaluates DATA{} references
+# in its arguments and constructs a new list of arguments:
+#  ExternalData_Expand_Arguments(
+#    <target>   # Name of data management target
+#    <outVar>   # Output variable
+#    [args...]  # Input arguments, DATA{} allowed
+#    )
+# It replaces each DATA{} reference in an argument with the full path of a
+# real data file on disk that will exist after the <target> builds.
+#
+# The 'ExternalData_Add_Test' function wraps around the CMake add_test()
+# command but supports DATA{} references in its arguments:
+#  ExternalData_Add_Test(
+#    <target>   # Name of data management target
+#    ...        # Arguments of add_test(), DATA{} allowed
+#    )
+# It passes its arguments through ExternalData_Expand_Arguments and then
+# invokes add_test() using the results.
+#
+# The 'ExternalData_Add_Target' function creates a custom target to manage
+# local instances of data files stored externally:
+#  ExternalData_Add_Target(
+#    <target>   # Name of data management target
+#    )
+# It creates custom commands in the target as necessary to make data files
+# available for each DATA{} reference previously evaluated by other functions
+# provided by this module.  A list of URL templates must be provided in the
+# variable ExternalData_URL_TEMPLATES using the placeholders "%(algo)" and
+# "%(hash)" in each template.  Data fetch rules try each URL template in order
+# by substituting the hash algorithm name for "%(algo)" and the hash value for
+# "%(hash)".
+#
+# The following hash algorithms are supported:
+#    %(algo)     <ext>     Description
+#    -------     -----     -----------
+#    MD5         .md5      Message-Digest Algorithm 5, RFC 1321
+# Note that the hashes are used only for unique data identification and
+# download verification.  This is not security software.
+#
+# Example usage:
+#   include(ExternalData)
+#   set(ExternalData_URL_TEMPLATES "file:///local/%(algo)/%(hash)"
+#                                  "http://data.org/%(algo)/%(hash)")
+#   ExternalData_Add_Test(MyData
+#     NAME MyTest
+#     COMMAND MyExe DATA{MyInput.png}
+#     )
+#   ExternalData_Add_Target(MyData)
+# When test "MyTest" runs the "DATA{MyInput.png}" argument will be replaced by
+# the full path to a real instance of the data file "MyInput.png" on disk.  If
+# the source tree contains a content link such as "MyInput.png.md5" then the
+# "MyData" target creates a real "MyInput.png" in the build tree.
+#
+# The DATA{} syntax can automatically recognize and fetch a file series.  If
+# the source tree contains a group of files or content links named like a
+# series then a DATA{} reference to one member adds rules to fetch all of
+# them.  Although all members of a series are fetched, only the file
+# originally named by the DATA{} argument is substituted for it.  Two
+# variables configure recognition of a series from DATA{<name>}.  First,
+# ExternalData_SERIES_PARSE is a regex of the form "^(...)(...)(...)$" to
+# parse <prefix>, <number>, and <suffix> parts from <name>.  Second,
+# ExternalData_SERIES_MATCH is a regex matching the <number> part of series
+# members named <prefix><number><suffix>.  Note that the <suffix> of a series
+# does not include a hash-algorithm extension.  Both series configuration
+# variables have default values that work well for common cases.
+#
+# The DATA{} syntax can alternatively match files associated with the named
+# file and contained in the same directory.  Associated files may be specified
+# by options using the syntax DATA{<name>,<opt1>,<opt2>,...}.  Each option may
+# specify one file by name or specify a regular expression to match file names
+# using the syntax REGEX:<regex>.  For example, the arguments
+#   DATA{MyData/MyInput.mhd,MyInput.img}                   # File pair
+#   DATA{MyData/MyFrames00.png,REGEX:MyFrames[0-9]+\\.png} # Series
+# will pass MyInput.mha and MyFrames00.png on the command line but ensure
+# that the associated files are present next to them.
+#
+# The variable ExternalData_LINK_CONTENT may be set to the name of a supported
+# hash algorithm to enable automatic conversion of real data files referenced
+# by the DATA{} syntax into content links.  For each such <file> a content
+# link named "<file><ext>" is created.  The original file is renamed to the
+# form ".ExternalData_<algo>_<hash>" to stage it for future transmission to
+# one of the locations in the list of URL templates (by means outside the
+# scope of this module).  The data fetch rule created for the content link
+# will use the staged object if it cannot be found using any URL template.
+#
+# The variable ExternalData_OBJECT_STORES may be set to a list of local
+# directories that store objects using the layout <dir>/%(algo)/%(hash).
+# These directories will be searched first for a needed object.  If the object
+# is not available in any store then it will be fetched remotely using the URL
+# templates and added to the first local store listed.  If no stores are
+# specified the default is a location inside the build tree.
+#
+# The variable ExternalData_SOURCE_ROOT may be set to the highest source
+# directory containing any path named by a DATA{} reference.  The default is
+# CMAKE_SOURCE_DIR.  ExternalData_SOURCE_ROOT and CMAKE_SOURCE_DIR must refer
+# to directories within a single source distribution (e.g. they come together
+# in one tarball).
+#
+# The variable ExternalData_BINARY_ROOT may be set to the directory to hold
+# the real data files named by expanded DATA{} references.  The default is
+# CMAKE_BINARY_DIR.  The directory layout will mirror that of content links
+# under ExternalData_SOURCE_ROOT.
+#
+# Variables ExternalData_TIMEOUT_INACTIVITY and ExternalData_TIMEOUT_ABSOLUTE
+# set the download inactivity and absolute timeouts, in seconds.  The defaults
+# are 60 seconds and 300 seconds, respectively.  Set either timeout to 0
+# seconds to disable enforcement.  The inactivity timeout is enforced only
+# with CMake >= 2.8.5.
+
+#=============================================================================
+# Copyright 2010-2013 Kitware, Inc.
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+# (To distribute this file outside of CMake, substitute the full
+#  License text for the above reference.)
+
+function(ExternalData_add_test target)
+  ExternalData_expand_arguments("${target}" testArgs ${ARGN})
+  add_test(${testArgs})
+endfunction()
+
+function(ExternalData_add_target target)
+  if(NOT ExternalData_URL_TEMPLATES)
+    message(FATAL_ERROR "ExternalData_URL_TEMPLATES is not set!")
+  endif()
+  if(NOT ExternalData_OBJECT_STORES)
+    set(ExternalData_OBJECT_STORES ${CMAKE_BINARY_DIR}/ExternalData/Objects)
+  endif()
+  set(config ${CMAKE_CURRENT_BINARY_DIR}/${target}_config.cmake)
+  configure_file(${_ExternalData_SELF_DIR}/ExternalData_config.cmake.in ${config} @ONLY)
+
+  set(files "")
+
+  # Set "_ExternalData_FILE_${file}" for each output file to avoid duplicate
+  # rules.  Use local data first to prefer real files over content links.
+
+  # Custom commands to copy or link local data.
+  get_property(data_local GLOBAL PROPERTY _ExternalData_${target}_LOCAL)
+  foreach(entry IN LISTS data_local)
+    string(REPLACE "|" ";" tuple "${entry}")
+    list(GET tuple 0 file)
+    list(GET tuple 1 name)
+    if(NOT DEFINED "_ExternalData_FILE_${file}")
+      set("_ExternalData_FILE_${file}" 1)
+      add_custom_command(
+        COMMENT "Generating ${file}"
+        OUTPUT "${file}"
+        COMMAND ${CMAKE_COMMAND} -Drelative_top=${CMAKE_BINARY_DIR}
+                                 -Dfile=${file} -Dname=${name}
+                                 -DExternalData_ACTION=local
+                                 -DExternalData_CONFIG=${config}
+                                 -P ${_ExternalData_SELF}
+        DEPENDS "${name}"
+        )
+      list(APPEND files "${file}")
+    endif()
+  endforeach()
+
+  # Custom commands to fetch remote data.
+  get_property(data_fetch GLOBAL PROPERTY _ExternalData_${target}_FETCH)
+  foreach(entry IN LISTS data_fetch)
+    string(REPLACE "|" ";" tuple "${entry}")
+    list(GET tuple 0 file)
+    list(GET tuple 1 name)
+    list(GET tuple 2 ext)
+    set(stamp "${ext}-stamp")
+    if(NOT DEFINED "_ExternalData_FILE_${file}")
+      set("_ExternalData_FILE_${file}" 1)
+      add_custom_command(
+        # Users care about the data file, so hide the hash/timestamp file.
+        COMMENT "Generating ${file}"
+        # The hash/timestamp file is the output from the build perspective.
+        # List the real file as a second output in case it is a broken link.
+        # The files must be listed in this order so CMake can hide from the
+        # make tool that a symlink target may not be newer than the input.
+        OUTPUT "${file}${stamp}" "${file}"
+        # Run the data fetch/update script.
+        COMMAND ${CMAKE_COMMAND} -Drelative_top=${CMAKE_BINARY_DIR}
+                                 -Dfile=${file} -Dname=${name} -Dext=${ext}
+                                 -DExternalData_ACTION=fetch
+                                 -DExternalData_CONFIG=${config}
+                                 -P ${_ExternalData_SELF}
+        # Update whenever the object hash changes.
+        DEPENDS "${name}${ext}"
+        )
+      list(APPEND files "${file}${stamp}")
+    endif()
+  endforeach()
+
+  # Custom target to drive all update commands.
+  add_custom_target(${target} ALL DEPENDS ${files})
+endfunction()
+
+function(ExternalData_expand_arguments target outArgsVar)
+  # Replace DATA{} references with real arguments.
+  set(data_regex "DATA{([^{}\r\n]*)}")
+  set(other_regex "([^D]|D[^A]|DA[^T]|DAT[^A]|DATA[^{])+|.")
+  set(outArgs "")
+  foreach(arg IN LISTS ARGN)
+    if("x${arg}" MATCHES "${data_regex}")
+      # Split argument into DATA{}-pieces and other pieces.
+      string(REGEX MATCHALL "${data_regex}|${other_regex}" pieces "${arg}")
+      # Compose output argument with DATA{}-pieces replaced.
+      set(outArg "")
+      foreach(piece IN LISTS pieces)
+        if("x${piece}" MATCHES "^x${data_regex}$")
+          # Replace this DATA{}-piece with a file path.
+          string(REGEX REPLACE "${data_regex}" "\\1" data "${piece}")
+          _ExternalData_arg("${target}" "${piece}" "${data}" file)
+          set(outArg "${outArg}${file}")
+        else()
+          # No replacement needed for this piece.
+          set(outArg "${outArg}${piece}")
+        endif()
+      endforeach()
+      list(APPEND outArgs "${outArg}")
+    else()
+      # No replacements needed in this argument.
+      list(APPEND outArgs "${arg}")
+    endif()
+  endforeach()
+  set("${outArgsVar}" "${outArgs}" PARENT_SCOPE)
+endfunction()
+
+#-----------------------------------------------------------------------------
+# Private helper interface
+
+set(_ExternalData_SELF "${CMAKE_CURRENT_LIST_FILE}")
+get_filename_component(_ExternalData_SELF_DIR "${_ExternalData_SELF}" PATH)
+
+function(_ExternalData_compute_hash var_hash algo file)
+  if("${algo}" STREQUAL "MD5")
+    # TODO: Errors
+    execute_process(COMMAND "${CMAKE_COMMAND}" -E md5sum "${file}"
+      OUTPUT_VARIABLE output)
+    string(SUBSTRING "${output}" 0 32 hash)
+    set("${var_hash}" "${hash}" PARENT_SCOPE)
+  else()
+    # TODO: Other hashes.
+    message(FATAL_ERROR "Hash algorithm ${algo} unimplemented.")
+  endif()
+endfunction()
+
+function(_ExternalData_random var)
+  if(NOT ${CMAKE_VERSION} VERSION_LESS 2.8.5)
+    string(RANDOM LENGTH 6 random)
+  elseif(EXISTS /dev/urandom)
+    file(READ /dev/urandom random LIMIT 4 HEX)
+  else()
+    message(FATAL_ERROR "CMake >= 2.8.5 required in this environment")
+  endif()
+  set("${var}" "${random}" PARENT_SCOPE)
+endfunction()
+
+function(_ExternalData_exact_regex regex_var string)
+  string(REGEX REPLACE "([][+.*()^])" "\\\\\\1" regex "${string}")
+  set("${regex_var}" "${regex}" PARENT_SCOPE)
+endfunction()
+
+function(_ExternalData_atomic_write file content)
+  _ExternalData_random(random)
+  set(tmp "${file}.tmp${random}")
+  file(WRITE "${tmp}" "${content}")
+  file(RENAME "${tmp}" "${file}")
+endfunction()
+
+function(_ExternalData_link_content name var_ext)
+  if("${ExternalData_LINK_CONTENT}" MATCHES "^(MD5)$")
+    set(algo "${ExternalData_LINK_CONTENT}")
+  else()
+    message(FATAL_ERROR
+      "Unknown hash algorithm specified by ExternalData_LINK_CONTENT:\n"
+      "  ${ExternalData_LINK_CONTENT}")
+  endif()
+  _ExternalData_compute_hash(hash "${algo}" "${name}")
+  get_filename_component(dir "${name}" PATH)
+  set(staged "${dir}/.ExternalData_${algo}_${hash}")
+  set(ext ".md5")
+  _ExternalData_atomic_write("${name}${ext}" "${hash}\n")
+  file(RENAME "${name}" "${staged}")
+  set("${var_ext}" "${ext}" PARENT_SCOPE)
+
+  file(RELATIVE_PATH relname "${ExternalData_SOURCE_ROOT}" "${name}${ext}")
+  message(STATUS "Linked ${relname} to ExternalData ${algo}/${hash}")
+endfunction()
+
+function(_ExternalData_arg target arg options var_file)
+  # Separate data path from the options.
+  string(REPLACE "," ";" options "${options}")
+  list(GET options 0 data)
+  list(REMOVE_AT options 0)
+
+  # Convert to full path.
+  if(IS_ABSOLUTE "${data}")
+    set(absdata "${data}")
+  else()
+    # TODO: If ${data} does not start in "./" or "../" then use search path?
+    get_filename_component(absdata "${CMAKE_CURRENT_SOURCE_DIR}/${data}" ABSOLUTE)
+  endif()
+
+  # Convert to relative path under the source tree.
+  if(NOT ExternalData_SOURCE_ROOT)
+    set(ExternalData_SOURCE_ROOT "${CMAKE_SOURCE_DIR}")
+  endif()
+  set(top_src "${ExternalData_SOURCE_ROOT}")
+  file(RELATIVE_PATH reldata "${top_src}" "${absdata}")
+  if(IS_ABSOLUTE "${reldata}" OR "${reldata}" MATCHES "^\\.\\./")
+    message(FATAL_ERROR "Data file referenced by argument\n"
+      "  ${arg}\n"
+      "does not lie under the top-level source directory\n"
+      "  ${top_src}\n")
+  endif()
+  if(NOT ExternalData_BINARY_ROOT)
+    set(ExternalData_BINARY_ROOT "${CMAKE_BINARY_DIR}")
+  endif()
+  set(top_bin "${ExternalData_BINARY_ROOT}")
+
+  # Handle in-source builds gracefully.
+  if("${top_src}" STREQUAL "${top_bin}")
+    if(ExternalData_LINK_CONTENT)
+      message(WARNING "ExternalData_LINK_CONTENT cannot be used in-source")
+      set(ExternalData_LINK_CONTENT 0)
+    endif()
+    set(top_same 1)
+  endif()
+
+  set(external "") # Entries external to the source tree.
+  set(internal "") # Entries internal to the source tree.
+  set(have_original 0)
+
+  # Process options.
+  set(associated_files "")
+  set(associated_regex "")
+  foreach(opt ${options})
+    if("x${opt}" MATCHES "^xREGEX:[^:/]+$")
+      # Regular expression to match associated files.
+      string(REGEX REPLACE "^REGEX:" "" regex "${opt}")
+      list(APPEND associated_regex "${regex}")
+    elseif("x${opt}" MATCHES "^[^][:/*?]+$")
+      # Specific associated file.
+      list(APPEND associated_files "${opt}")
+    else()
+      message(FATAL_ERROR "Unknown option \"${opt}\" in argument\n"
+        "  ${arg}\n")
+    endif()
+  endforeach()
+
+  if(associated_files OR associated_regex)
+    # Load the named data file and listed/matching associated files.
+    _ExternalData_arg_single()
+    _ExternalData_arg_associated()
+  elseif("${reldata}" MATCHES "(^|/)[^/.]+$")
+    # Files with no extension cannot be a series.
+    _ExternalData_arg_single()
+  else()
+    # Match a whole file series by default.
+    _ExternalData_arg_series()
+  endif()
+
+  if(NOT have_original)
+    message(FATAL_ERROR "Data file referenced by argument\n"
+      "  ${arg}\n"
+      "corresponds to source tree path\n"
+      "  ${reldata}\n"
+      "that does not exist (with or without an extension)!")
+  endif()
+
+  if(external)
+    # Make the series available in the build tree.
+    set_property(GLOBAL APPEND PROPERTY
+      _ExternalData_${target}_FETCH "${external}")
+    set_property(GLOBAL APPEND PROPERTY
+      _ExternalData_${target}_LOCAL "${internal}")
+    set("${var_file}" "${top_bin}/${reldata}" PARENT_SCOPE)
+  else()
+    # The whole series is in the source tree.
+    set("${var_file}" "${top_src}/${reldata}" PARENT_SCOPE)
+  endif()
+endfunction()
+
+macro(_ExternalData_arg_associated)
+  # Associated files lie in the same directory.
+  get_filename_component(reldir "${reldata}" PATH)
+  if(reldir)
+    set(reldir "${reldir}/")
+  endif()
+  _ExternalData_exact_regex(reldir_regex "${reldir}")
+
+  # Find files named explicitly.
+  foreach(file ${associated_files})
+    _ExternalData_exact_regex(file_regex "${file}")
+    _ExternalData_arg_find_files("${reldir}${file}" "${reldir_regex}${file_regex}")
+  endforeach()
+
+  # Find files matching the given regular expressions.
+  set(all "")
+  set(sep "")
+  foreach(regex ${associated_regex})
+    set(all "${all}${sep}${reldir_regex}${regex}")
+    set(sep "|")
+  endforeach()
+  _ExternalData_arg_find_files("${reldir}" "${all}")
+endmacro()
+
+macro(_ExternalData_arg_single)
+  # Match only the named data by itself.
+  _ExternalData_exact_regex(data_regex "${reldata}")
+  _ExternalData_arg_find_files("${reldata}" "${data_regex}")
+endmacro()
+
+macro(_ExternalData_arg_series)
+  # Configure series parsing and matching.
+  if(ExternalData_SERIES_PARSE)
+    if(NOT "${ExternalData_SERIES_PARSE}" MATCHES
+        "^\\^\\([^()]*\\)\\([^()]*\\)\\([^()]*\\)\\$$")
+      message(FATAL_ERROR
+        "ExternalData_SERIES_PARSE is set to\n"
+        "  ${ExternalData_SERIES_PARSE}\n"
+        "which is not of the form\n"
+        "  ^(...)(...)(...)$\n")
+    endif()
+    set(series_parse "${ExternalData_SERIES_PARSE}")
+  else()
+    set(series_parse "^(.*)()(\\.[^./]*)$")
+  endif()
+  if(ExternalData_SERIES_MATCH)
+    set(series_match "${ExternalData_SERIES_MATCH}")
+  else()
+    set(series_match "[_.]?[0-9]*")
+  endif()
+
+  # Parse the base, number, and extension components of the series.
+  string(REGEX REPLACE "${series_parse}" "\\1;\\2;\\3" tuple "${reldata}")
+  list(LENGTH tuple len)
+  if(NOT "${len}" EQUAL 3)
+    message(FATAL_ERROR "Data file referenced by argument\n"
+      "  ${arg}\n"
+      "corresponds to path\n"
+      "  ${reldata}\n"
+      "that does not match regular expression\n"
+      "  ${series_parse}")
+  endif()
+  list(GET tuple 0 relbase)
+  list(GET tuple 2 ext)
+
+  # Glob files that might match the series.
+  # Then match match base, number, and extension.
+  _ExternalData_exact_regex(series_base "${relbase}")
+  _ExternalData_exact_regex(series_ext "${ext}")
+  _ExternalData_arg_find_files("${relbase}*${ext}"
+    "${series_base}${series_match}${series_ext}")
+endmacro()
+
+function(_ExternalData_arg_find_files pattern regex)
+  file(GLOB globbed RELATIVE "${top_src}" "${top_src}/${pattern}*")
+  foreach(entry IN LISTS globbed)
+    string(REGEX REPLACE "^(${regex})(\\.md5|)$" "\\1;\\2" tuple "${entry}")
+    list(LENGTH tuple len)
+    if("${len}" EQUAL 2)
+      list(GET tuple 0 relname)
+      list(GET tuple 1 alg)
+      set(name "${top_src}/${relname}")
+      set(file "${top_bin}/${relname}")
+      if(alg)
+        list(APPEND external "${file}|${name}|${alg}")
+      elseif(ExternalData_LINK_CONTENT)
+        _ExternalData_link_content("${name}" alg)
+        list(APPEND external "${file}|${name}|${alg}")
+      elseif(NOT top_same)
+        list(APPEND internal "${file}|${name}")
+      endif()
+      if("${relname}" STREQUAL "${reldata}")
+        set(have_original 1)
+      endif()
+    endif()
+  endforeach()
+  set(external "${external}" PARENT_SCOPE)
+  set(internal "${internal}" PARENT_SCOPE)
+  set(have_original "${have_original}" PARENT_SCOPE)
+endfunction()
+
+#-----------------------------------------------------------------------------
+# Private script mode interface
+
+if(CMAKE_GENERATOR OR NOT ExternalData_ACTION)
+  return()
+endif()
+
+if(ExternalData_CONFIG)
+  include(${ExternalData_CONFIG})
+endif()
+if(NOT ExternalData_URL_TEMPLATES)
+  message(FATAL_ERROR "No ExternalData_URL_TEMPLATES set!")
+endif()
+
+function(_ExternalData_link_or_copy src dst)
+  # Create a temporary file first.
+  get_filename_component(dst_dir "${dst}" PATH)
+  file(MAKE_DIRECTORY "${dst_dir}")
+  _ExternalData_random(random)
+  set(tmp "${dst}.tmp${random}")
+  if(UNIX)
+    # Create a symbolic link.
+    set(tgt "${src}")
+    if(relative_top)
+      # Use relative path if files are close enough.
+      file(RELATIVE_PATH relsrc "${relative_top}" "${src}")
+      file(RELATIVE_PATH relfile "${relative_top}" "${dst}")
+      if(NOT IS_ABSOLUTE "${relsrc}" AND NOT "${relsrc}" MATCHES "^\\.\\./" AND
+          NOT IS_ABSOLUTE "${reldst}" AND NOT "${reldst}" MATCHES "^\\.\\./")
+        file(RELATIVE_PATH tgt "${dst_dir}" "${src}")
+      endif()
+    endif()
+    execute_process(COMMAND "${CMAKE_COMMAND}" -E create_symlink "${tgt}" "${tmp}" RESULT_VARIABLE result)
+  else()
+    # Create a copy.
+    execute_process(COMMAND "${CMAKE_COMMAND}" -E copy "${src}" "${tmp}" RESULT_VARIABLE result)
+  endif()
+  if(result)
+    file(REMOVE "${tmp}")
+    message(FATAL_ERROR "Failed to create\n  ${tmp}\nfrom\n  ${obj}")
+  endif()
+
+  # Atomically create/replace the real destination.
+  file(RENAME "${tmp}" "${dst}")
+endfunction()
+
+function(_ExternalData_download_file url file err_var msg_var)
+  set(retry 3)
+  while(retry)
+    math(EXPR retry "${retry} - 1")
+    if("${CMAKE_VERSION}" VERSION_GREATER 2.8.4.20110602)
+      if(ExternalData_TIMEOUT_INACTIVITY)
+        set(inactivity_timeout INACTIVITY_TIMEOUT ${ExternalData_TIMEOUT_INACTIVITY})
+      elseif(NOT "${ExternalData_TIMEOUT_INACTIVITY}" EQUAL 0)
+        set(inactivity_timeout INACTIVITY_TIMEOUT 60)
+      else()
+        set(inactivity_timeout "")
+      endif()
+    else()
+      set(inactivity_timeout "")
+    endif()
+    if(ExternalData_TIMEOUT_ABSOLUTE)
+      set(absolute_timeout TIMEOUT ${ExternalData_TIMEOUT_ABSOLUTE})
+    elseif(NOT "${ExternalData_TIMEOUT_ABSOLUTE}" EQUAL 0)
+      set(absolute_timeout TIMEOUT 300)
+    else()
+      set(absolute_timeout "")
+    endif()
+    file(DOWNLOAD "${url}" "${file}" STATUS status LOG log ${inactivity_timeout} ${absolute_timeout} SHOW_PROGRESS)
+    list(GET status 0 err)
+    list(GET status 1 msg)
+    if(err)
+      if("${msg}" MATCHES "HTTP response code said error" AND
+          "${log}" MATCHES "error: 503")
+        set(msg "temporarily unavailable")
+      endif()
+    elseif("${log}" MATCHES "\nHTTP[^\n]* 503")
+      set(err TRUE)
+      set(msg "temporarily unavailable")
+    endif()
+    if(NOT err OR NOT "${msg}" MATCHES "partial|timeout|temporarily")
+      break()
+    elseif(retry)
+      message(STATUS "[download terminated: ${msg}, retries left: ${retry}]")
+    endif()
+  endwhile()
+  set("${err_var}" "${err}" PARENT_SCOPE)
+  set("${msg_var}" "${msg}" PARENT_SCOPE)
+endfunction()
+
+function(_ExternalData_download_object name hash algo var_obj)
+  # Search all object stores for an existing object.
+  foreach(dir ${ExternalData_OBJECT_STORES})
+    set(obj "${dir}/${algo}/${hash}")
+    if(EXISTS "${obj}")
+      message(STATUS "Found object: \"${obj}\"")
+      set("${var_obj}" "${obj}" PARENT_SCOPE)
+      return()
+    endif()
+  endforeach()
+
+  # Download object to the first store.
+  list(GET ExternalData_OBJECT_STORES 0 store)
+  set(obj "${store}/${algo}/${hash}")
+
+  _ExternalData_random(random)
+  set(tmp "${obj}.tmp${random}")
+  set(found 0)
+  set(tried "")
+  foreach(url_template IN LISTS ExternalData_URL_TEMPLATES)
+    string(REPLACE "%(hash)" "${hash}" url_tmp "${url_template}")
+    string(REPLACE "%(algo)" "${algo}" url "${url_tmp}")
+    message(STATUS "Fetching \"${url}\"")
+    _ExternalData_download_file("${url}" "${tmp}" err errMsg)
+    set(tried "${tried}\n  ${url}")
+    if(err)
+      set(tried "${tried} (${errMsg})")
+    else()
+      # Verify downloaded object.
+      _ExternalData_compute_hash(dl_hash "${algo}" "${tmp}")
+      if("${dl_hash}" STREQUAL "${hash}")
+        set(found 1)
+        break()
+      else()
+        set(tried "${tried} (wrong hash ${algo}=${dl_hash})")
+        if("$ENV{ExternalData_DEBUG_DOWNLOAD}" MATCHES ".")
+          file(RENAME "${tmp}" "${store}/${algo}/${dl_hash}")
+        endif()
+      endif()
+    endif()
+    file(REMOVE "${tmp}")
+  endforeach()
+
+  get_filename_component(dir "${name}" PATH)
+  set(staged "${dir}/.ExternalData_${algo}_${hash}")
+
+  if(found)
+    file(RENAME "${tmp}" "${obj}")
+    message(STATUS "Downloaded object: \"${obj}\"")
+  elseif(EXISTS "${staged}")
+    set(obj "${staged}")
+    message(STATUS "Staged object: \"${obj}\"")
+  else()
+    message(FATAL_ERROR "Object ${algo}=${hash} not found at:${tried}")
+  endif()
+
+  set("${var_obj}" "${obj}" PARENT_SCOPE)
+endfunction()
+
+if("${ExternalData_ACTION}" STREQUAL "fetch")
+  foreach(v ExternalData_OBJECT_STORES file name ext)
+    if(NOT DEFINED "${v}")
+      message(FATAL_ERROR "No \"-D${v}=\" value provided!")
+    endif()
+  endforeach()
+
+  file(READ "${name}${ext}" hash)
+  string(STRIP "${hash}" hash)
+
+  if("${ext}" STREQUAL ".md5")
+    set(algo "MD5")
+  else()
+    message(FATAL_ERROR "Unknown hash algorithm extension \"${ext}\"")
+  endif()
+
+  _ExternalData_download_object("${name}" "${hash}" "${algo}" obj)
+
+  # Check if file already corresponds to the object.
+  set(stamp "${ext}-stamp")
+  set(file_up_to_date 0)
+  if(EXISTS "${file}" AND EXISTS "${file}${stamp}")
+    file(READ "${file}${stamp}" f_hash)
+    string(STRIP "${f_hash}" f_hash)
+    if("${f_hash}" STREQUAL "${hash}")
+      #message(STATUS "File already corresponds to object")
+      set(file_up_to_date 1)
+    endif()
+  endif()
+
+  if(file_up_to_date)
+    # Touch the file to convince the build system it is up to date.
+    execute_process(COMMAND "${CMAKE_COMMAND}" -E touch "${file}")
+  else()
+    _ExternalData_link_or_copy("${obj}" "${file}")
+  endif()
+
+  # Atomically update the hash/timestamp file to record the object referenced.
+  _ExternalData_atomic_write("${file}${stamp}" "${hash}\n")
+elseif("${ExternalData_ACTION}" STREQUAL "local")
+  foreach(v file name)
+    if(NOT DEFINED "${v}")
+      message(FATAL_ERROR "No \"-D${v}=\" value provided!")
+    endif()
+  endforeach()
+  _ExternalData_link_or_copy("${name}" "${file}")
+elseif("${ExternalData_ACTION}" STREQUAL "store")
+  foreach(v dir file)
+    if(NOT DEFINED "${v}")
+      message(FATAL_ERROR "No \"-D${v}=\" value provided!")
+    endif()
+  endforeach()
+  if(NOT DEFINED algo)
+    set(algo "MD5")
+  endif()
+  _ExternalData_compute_hash(hash "${algo}" "${file}")
+else()
+  message(FATAL_ERROR "Unknown ExternalData_ACTION=[${ExternalData_ACTION}]")
+endif()