[v2] os: Add -displayfd option

diff --git a/dix/globals.c b/dix/globals.c
index a564575..332b91f 100644
--- a/dix/globals.c
+++ b/dix/globals.c
@@ -128,6 +128,7 @@  int defaultColorVisualClass = -1;
 int monitorResolution = 0;
 
 char *display;
+int displayfd;
 char *ConnectionInfo;
 
 CARD32 TimeOutValue = DEFAULT_TIMEOUT * MILLI_PER_SECOND;
diff --git a/include/opaque.h b/include/opaque.h
index 9ca408a..b76ab6e 100644
--- a/include/opaque.h
+++ b/include/opaque.h
@@ -50,6 +50,7 @@  extern _X_EXPORT int ScreenSaverAllowExposures;
 extern _X_EXPORT int defaultScreenSaverBlanking;
 extern _X_EXPORT int defaultScreenSaverAllowExposures;
 extern _X_EXPORT char *display;
+extern _X_EXPORT int displayfd;
 
 extern _X_EXPORT int defaultBackingStore;
 extern _X_EXPORT Bool disableBackingStore;
diff --git a/man/Xserver.man b/man/Xserver.man
index 0cd9b94..8d243d6 100644
--- a/man/Xserver.man
+++ b/man/Xserver.man
@@ -127,6 +127,13 @@  Not obeyed by all servers.
 .B \-core
 causes the server to generate a core dump on fatal errors.
 .TP 8
+.B \-displayfd \fIfd\fP
+specifies a file descriptor in the launching process.  Rather than specify
+a display number, the X server will attempt to listen on successively higher
+display numbers, and upon finding a free one, will write the port number back
+on this file descriptor as a newline-terminated string.  The \-pn option is
+ignored when using \-displayfd.
+.TP 8
 .B \-deferglyphs \fIwhichfonts\fP
 specifies the types of fonts for which the server should attempt to use
 deferred glyph loading.  \fIwhichfonts\fP can be all (all fonts),
diff --git a/os/connection.c b/os/connection.c
index 1099752..028f588 100644
--- a/os/connection.c
+++ b/os/connection.c
@@ -142,6 +142,7 @@  Bool AnyClientsWriteBlocked;    /* true if some client blocked on write */
 static Bool RunFromSmartParent; /* send SIGUSR1 to parent process */
 Bool RunFromSigStopParent;      /* send SIGSTOP to our own process; Upstart (or
                                    equivalent) will send SIGCONT back. */
+static char dynamic_display[7]; /* display name */
 Bool PartialNetwork;            /* continue even if unable to bind all addrs */
 static Pid_t ParentProcess;
 
@@ -350,6 +351,10 @@  void
 NotifyParentProcess(void)
 {
 #if !defined(WIN32)
+    if (dynamic_display[0]) {
+        write(displayfd, dynamic_display, strlen(dynamic_display));
+        close(displayfd);
+    }
     if (RunFromSmartParent) {
         if (ParentProcess > 1) {
             kill(ParentProcess, SIGUSR1);
@@ -360,6 +365,18 @@  NotifyParentProcess(void)
 #endif
 }
 
+static Bool
+TryCreateSocket(int num, int *partial)
+{
+    char port[20];
+
+    sprintf(port, "%d", num);
+
+    return (_XSERVTransMakeAllCOTSServerListeners(port, partial,
+                                                  &ListenTransCount,
+                                                  &ListenTransConns) >= 0);
+}
+
 /*****************
  * CreateWellKnownSockets
  *    At initialization, create the sockets to listen on for new clients.
@@ -370,7 +387,6 @@  CreateWellKnownSockets(void)
 {
     int i;
     int partial;
-    char port[20];
 
     FD_ZERO(&AllSockets);
     FD_ZERO(&AllClients);
@@ -386,29 +402,38 @@  CreateWellKnownSockets(void)
 
     FD_ZERO(&WellKnownConnections);
 
-    snprintf(port, sizeof(port), "%d", atoi(display));
-
-    if ((_XSERVTransMakeAllCOTSServerListeners(port, &partial,
-                                               &ListenTransCount,
-                                               &ListenTransConns) >= 0) &&
-        (ListenTransCount >= 1)) {
-        if (!PartialNetwork && partial) {
-            FatalError("Failed to establish all listening sockets");
+    if (display) {
+        if (TryCreateSocket(atoi(display), &partial) &&
+            ListenTransCount >= 1)
+            if (!PartialNetwork && partial)
+                FatalError ("Failed to establish all listening sockets");
+    }
+    else { /* -displayfd */
+        Bool found = 0;
+        for (i = 0; i < 65535 - X_TCP_PORT; i++) {
+            if (TryCreateSocket(i, &partial) && !partial) {
+                found = 1;
+                break;
+            }
+            else
+                CloseWellKnownConnections();
         }
-        else {
-            ListenTransFds = malloc(ListenTransCount * sizeof(int));
+        if (!found)
+            FatalError("Failed to find a socket to listen on");
+        sprintf(dynamic_display, "%d\n", i);
+        display = dynamic_display;
+    }
 
-            for (i = 0; i < ListenTransCount; i++) {
-                int fd = _XSERVTransGetConnectionNumber(ListenTransConns[i]);
+    ListenTransFds = malloc(ListenTransCount * sizeof (int));
 
-                ListenTransFds[i] = fd;
-                FD_SET(fd, &WellKnownConnections);
+    for (i = 0; i < ListenTransCount; i++) {
+        int fd = _XSERVTransGetConnectionNumber(ListenTransConns[i]);
 
-                if (!_XSERVTransIsLocal(ListenTransConns[i])) {
-                    DefineSelf(fd);
-                }
-            }
-        }
+        ListenTransFds[i] = fd;
+        FD_SET(fd, &WellKnownConnections);
+
+        if (!_XSERVTransIsLocal(ListenTransConns[i]))
+            DefineSelf (fd);
     }
 
     if (!XFD_ANYSET(&WellKnownConnections))
diff --git a/os/utils.c b/os/utils.c
index 30592d2..3a1ef93 100644
--- a/os/utils.c
+++ b/os/utils.c
@@ -659,6 +659,15 @@  ProcessCommandLine(int argc, char *argv[])
             else
                 UseMsg();
         }
+        else if (strcmp(argv[i], "-displayfd") == 0) {
+            if (++i < argc) {
+                displayfd = atoi(argv[i]);
+                display = NULL;
+                nolock = TRUE;
+            }
+            else
+                UseMsg();
+        }
 #ifdef DPMSExtension
         else if (strcmp(argv[i], "dpms") == 0)
             /* ignored for compatibility */ ;