[PATCH xserver] xwayland: Make it possible to force CLOCK_MONOTONIC

[Jonas Ådahl]

By default the X server will try CLOCK_MONOTONIC_COARSE before
CLOCK_MONOTONIC. This causes various issues for Wayland display
servers who may get their internal timestamps from the CLOCK_MONOTONIC,
since it may very well happen that a timestamp from CLOCK_MONOTONIC
retrieved before a sending an X request will still be "later" than the
timestamp the X server than gets after receiving the request, due to
the fact that CLOCK_MONOTONIC_COARSE has a lower resolution.

To make it possible for Wayland display servers to have more control
over the clock the X server uses, add a '-forceMonotonic' command line
argument to Xwayland, that forces the X server to always use the
CLOCK_MONOTONIC clock as its source for timestamps.

This commit also makes it a build requirement to have clock_gettime()
and CLOCK_MONOTONIC support, and a runtime requirement to have
CLOCK_MONOTONIC support if the clock was forced to be monotonic.

Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
---

Hi,

This patch fixes various issues[0] related to the race condition described in
the commit message. An alternative solution discussed has been to allow a
"grace period" as long as the clock resolution difference for future
timestamps, or rounding down timestamps from CLOCK_MONOTONIC in the display
server. I think both these solutions have their own problems, while just
changing the clock here only has a (most likely negliable) performance loss.

FWIW, I would be just as fine with always forcing CLOCK_MONOTONIC for Xwayland
if the command line argument seems unnecessary.


Jonas

[0] https://bugzilla.gnome.org/show_bug.cgi?id=756272


 configure.ac           |  4 ++++
 hw/xwayland/xwayland.c |  6 ++++++
 include/os.h           |  7 +++++++
 os/utils.c             | 26 ++++++++++++++++++++++++--
 4 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index f4de90f..8cc508c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2471,6 +2471,10 @@ if test "x$XWAYLAND" = xyes; then
 	AC_SUBST([XWAYLAND_LIBS])
 	AC_SUBST([XWAYLAND_SYS_LIBS])
 
+	if test "x$MONOTONIC_CLOCK" != xyes; then
+		AC_MSG_ERROR([Xwayland requires CLOCK_MONOTONIC support.])
+	fi
+
 	WAYLAND_PREFIX=`$PKG_CONFIG --variable=prefix wayland-client`
 	AC_PATH_PROG([WAYLAND_SCANNER], [wayland-scanner],,
 		     [${WAYLAND_PREFIX}/bin$PATH_SEPARATOR$PATH])
diff --git a/hw/xwayland/xwayland.c b/hw/xwayland/xwayland.c
index 3d36205..fa0143e 100644
--- a/hw/xwayland/xwayland.c
+++ b/hw/xwayland/xwayland.c
@@ -32,6 +32,7 @@
 #include <misyncshm.h>
 #include <compositeext.h>
 #include <glx_extinit.h>
+#include <os.h>
 
 void
 ddxGiveUp(enum ExitCode error)
@@ -68,6 +69,7 @@ ddxUseMsg(void)
     ErrorF("-rootless              run rootless, requires wm support\n");
     ErrorF("-wm fd                 create X client for wm on given fd\n");
     ErrorF("-listen fd             add give fd as a listen socket\n");
+    ErrorF("-forceMonotonic        force usage of CLOCK_MONOTONIC\n");
 }
 
 int
@@ -86,6 +88,10 @@ ddxProcessArgument(int argc, char *argv[], int i)
     else if (strcmp(argv[i], "-shm") == 0) {
         return 1;
     }
+    else if (strcmp(argv[i], "-forceMonotonic") == 0) {
+        ForceClockId(CLOCK_MONOTONIC);
+        return 1;
+    }
 
     return 0;
 }
diff --git a/include/os.h b/include/os.h
index 461d5d6..f37ad5d 100644
--- a/include/os.h
+++ b/include/os.h
@@ -51,6 +51,9 @@ SOFTWARE.
 #include <stdarg.h>
 #include <stdint.h>
 #include <string.h>
+#ifdef MONOTONIC_CLOCK
+#include <time.h>
+#endif
 
 #define SCREEN_SAVER_ON   0
 #define SCREEN_SAVER_OFF  1
@@ -180,6 +183,10 @@ extern _X_EXPORT void ListenOnOpenFD(int /* fd */ , int /* noxauth */ );
 
 extern _X_EXPORT Bool AddClientOnOpenFD(int /* fd */ );
 
+#ifdef MONOTONIC_CLOCK
+extern _X_EXPORT void ForceClockId(clockid_t /* forced_clockid */);
+#endif
+
 extern _X_EXPORT CARD32 GetTimeInMillis(void);
 extern _X_EXPORT CARD64 GetTimeInMicros(void);
 
diff --git a/os/utils.c b/os/utils.c
index e48d9f8..85e7b07 100644
--- a/os/utils.c
+++ b/os/utils.c
@@ -210,6 +210,10 @@ sig_atomic_t inSignalContext = FALSE;
 #define HAS_SAVED_IDS_AND_SETEUID
 #endif
 
+#ifdef MONOTONIC_CLOCK
+static clockid_t clockid;
+#endif
+
 OsSigHandlerPtr
 OsSignal(int sig, OsSigHandlerPtr handler)
 {
@@ -427,6 +431,26 @@ GiveUp(int sig)
     errno = olderrno;
 }
 
+#ifdef MONOTONIC_CLOCK
+void
+ForceClockId(clockid_t forced_clockid)
+{
+    struct timespec tp;
+
+    if (clockid) {
+        ErrorF("Warning: clock id was forced after it was initialized.\n");
+        return;
+    }
+    clockid = forced_clockid;
+
+    if (clock_gettime(clockid, &tp) != 0) {
+        FatalError("Forced clock id failed to retrieve current time: %s\n",
+                   strerror(errno));
+        return;
+    }
+}
+#endif
+
 #if (defined WIN32 && defined __MINGW32__) || defined(__CYGWIN__)
 CARD32
 GetTimeInMillis(void)
@@ -446,7 +470,6 @@ GetTimeInMillis(void)
 
 #ifdef MONOTONIC_CLOCK
     struct timespec tp;
-    static clockid_t clockid;
 
     if (!clockid) {
 #ifdef CLOCK_MONOTONIC_COARSE
@@ -475,7 +498,6 @@ GetTimeInMicros(void)
     struct timeval tv;
 #ifdef MONOTONIC_CLOCK
     struct timespec tp;
-    static clockid_t clockid;
 
     if (!clockid) {
         if (clock_gettime(CLOCK_MONOTONIC, &tp) == 0)
-- 
2.4.3