os: add support for systemd notification

It can take some time for Xorg to start. If Xorg runs as a systemd
service and other services are based on it, they have no way to
determine when Xorg is really ready to accept requests. Let's use
sd_notify() provided by libsystemd to signal systemd for readiness.
If Xorg has not been started as a systemd service, this won't do
anything.

Signed-off-by: Topi Miettinen <toiwoton@gmail.com>

man/Xserver.man

@@ -54,7 +54,8 @@ The X server may also be started directly by the user, though this
 method is usually reserved for testing and is not recommended for
 normal operation.  On some platforms, the user must have special
 permission to start the X server, often because access to certain
-devices (e.g. \fI/dev/mouse\fP) is restricted.
+devices (e.g. \fI/dev/mouse\fP) is restricted.  Where applicable, the
+X server notifies systemd when it is ready to process requests.
 .PP
 When the X server starts up, it typically takes over the display.  If
 you are running on a workstation whose console is the display, you may

os/connection.c

@@ -116,6 +116,10 @@ SOFTWARE.
 #define zoneid_t int
 #endif
 
+#ifdef HAVE_SYSTEMD_DAEMON
+#include <systemd/sd-daemon.h>
+#endif
+
 #include "probes.h"
 
 struct ospoll   *server_poll;
@@ -207,6 +211,11 @@ NotifyParentProcess(void)
     }
     if (RunFromSigStopParent)
         raise(SIGSTOP);
+#ifdef HAVE_SYSTEMD_DAEMON
+    /* If we have been started as a systemd service, tell systemd that
+       we are ready. Otherwise sd_notify() won't do anything. */
+    sd_notify(0, "READY=1");
+#endif
 #endif
 }