From ed80f025f6f87f144ee7b63931890efb4421ee78 Mon Sep 17 00:00:00 2001
From: "jan.nijtmans" <nijtmans@users.sourceforge.net>
Date: Tue, 22 Nov 2016 10:06:18 +0000
Subject: This is patch.002 from ticket
 [0b9d3ba2ba3e1e3fc33c97d5a9fa7ef85d11a696|0b9d3ba2ba], as first start of
 tip-456 implementation

---
 generic/tclIOCmd.c | 15 ++++++++++++---
 unix/tclUnixSock.c | 20 ++++++++++++++++++++
 win/tclWinSock.c   |  4 ++--
 3 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/generic/tclIOCmd.c b/generic/tclIOCmd.c
index de65da5..9dc8f07 100644
--- a/generic/tclIOCmd.c
+++ b/generic/tclIOCmd.c
@@ -1485,12 +1485,12 @@ Tcl_SocketObjCmd(
     Tcl_Obj *const objv[])	/* Argument objects. */
 {
     static const char *const socketOptions[] = {
-	"-async", "-myaddr", "-myport", "-server", NULL
+	"-async", "-myaddr", "-myport", "-server", "-reuseport", NULL
     };
     enum socketOptions {
-	SKT_ASYNC, SKT_MYADDR, SKT_MYPORT, SKT_SERVER
+	SKT_ASYNC, SKT_MYADDR, SKT_MYPORT, SKT_SERVER, SKT_REUSEPORT
     };
-    int optionIndex, a, server = 0, port, myport = 0, async = 0;
+    int optionIndex, a, server = 0, port, myport = 0, async = 0, reuseport = 0;
     const char *host, *myaddr = NULL;
     Tcl_Obj *script = NULL;
     Tcl_Channel chan;
@@ -1557,6 +1557,9 @@ Tcl_SocketObjCmd(
 	    }
 	    script = objv[a];
 	    break;
+    case SKT_REUSEPORT:
+        reuseport = 1;
+        break;
 	default:
 	    Tcl_Panic("Tcl_SocketObjCmd: bad option index to SocketOptions");
 	}
@@ -1600,6 +1603,12 @@ Tcl_SocketObjCmd(
 	Tcl_IncrRefCount(script);
 	acceptCallbackPtr->script = script;
 	acceptCallbackPtr->interp = interp;
+
+    /* Hint for Tcl_OpenTcpServer to set socket option REUSEPORT */
+    if(reuseport) {
+        port |= (1 << 16);
+    }
+
 	chan = Tcl_OpenTcpServer(interp, port, host, AcceptCallbackProc,
 		acceptCallbackPtr);
 	if (chan == NULL) {
diff --git a/unix/tclUnixSock.c b/unix/tclUnixSock.c
index 170aea9..d8a33f9 100644
--- a/unix/tclUnixSock.c
+++ b/unix/tclUnixSock.c
@@ -113,6 +113,11 @@ struct TcpState {
 
 #define SOCKET_BUFSIZE	4096
 
+#ifdef SO_REUSEPORT
+/* Bitmask to check if the setting of SO_REUSEPORT was requested by the caller. */
+#define USE_SOCK_REUSEPORT (1 << 16)
+#endif
+
 /*
  * Static routines for this file:
  */
@@ -1435,6 +1440,10 @@ Tcl_OpenTcpServer(
     char channelName[SOCK_CHAN_LENGTH];
     const char *errorMsg = NULL;
     TcpFdList *fds = NULL, *newfds;
+#ifdef SO_REUSEPORT
+    int reuseport = port & USE_SOCK_REUSEPORT;
+    CLEAR_BITS(port, USE_SOCK_REUSEPORT);
+#endif
 
     /*
      * Try to record and return the most meaningful error message, i.e. the
@@ -1512,6 +1521,17 @@ Tcl_OpenTcpServer(
 	(void) setsockopt(sock, SOL_SOCKET, SO_REUSEADDR,
 		(char *) &reuseaddr, sizeof(reuseaddr));
 
+#ifdef SO_REUSEPORT
+    /*
+     * Set up to allows multiple sockets on the same host to bind to the same port.
+     * The flag can be switched on by setting the lowest bit above the valid maximum port (0xffff).
+     */
+    if(reuseport) {
+        (void) setsockopt(sock, SOL_SOCKET, SO_REUSEPORT,
+            (char *) &reuseport, sizeof(reuseport));
+    }
+#endif
+
         /*
          * Make sure we use the same port number when opening two server
          * sockets for IPv4 and IPv6 on a random port.
diff --git a/win/tclWinSock.c b/win/tclWinSock.c
index ec881d2..af8dda1 100644
--- a/win/tclWinSock.c
+++ b/win/tclWinSock.c
@@ -2111,8 +2111,8 @@ Tcl_OpenTcpServer(
 
 	/*
 	 * Bind to the specified port. Note that we must not call
-	 * setsockopt with SO_REUSEADDR because Microsoft allows addresses
-	 * to be reused even if they are still in use.
+	 * setsockopt with SO_REUSEADDR or SO_REUSEPORT because Microsoft
+	 * allows addresses and ports to be reused even if they are still in use.
 	 *
 	 * Bind should not be affected by the socket having already been
 	 * set into nonblocking mode. If there is trouble, this is one
-- 
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