xlib/shm: Wrap the detection of shm with locking

As we access a global error variable, we need to hold a mutex against
simultaneous checking of multiple Displays. This should already be true
as we hold our display mutex to serialize initialisation, so just add an
assertion. As the client may mix use of cairo in one thread with X from
another, we need to hold the Display lock and serialise whilst
manipulating the low-level state of the Display.

Suggested-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/cairo-xlib-surface-shm.c b/src/cairo-xlib-surface-shm.c
index ad36ce1d0..2d201272a 100644
--- a/src/cairo-xlib-surface-shm.c
+++ b/src/cairo-xlib-surface-shm.c
@@ -231,6 +231,7 @@ _pqueue_pop (struct pqueue *pq)
 }
 
 static cairo_bool_t _x_error_occurred;
+
 static int
 _check_error_handler (Display     *display,
 		     XErrorEvent *event)
@@ -263,7 +264,11 @@ can_use_shm (Display *dpy, int *has_pixmap)
 	return FALSE;
     }
 
+    assert (CAIRO_MUTEX_IS_LOCKED (_cairo_xlib_display_mutex));
     _x_error_occurred = FALSE;
+
+    XLockDisplay (dpy);
+    XSync (dpy, False);
     old_handler = XSetErrorHandler (_check_error_handler);
 
     success = XShmAttach (dpy, &shm);
@@ -272,6 +277,7 @@ can_use_shm (Display *dpy, int *has_pixmap)
 
     XSync (dpy, False);
     XSetErrorHandler (old_handler);
+    XUnlockDisplay (dpy);
 
     shmctl (shm.shmid, IPC_RMID, NULL);
     shmdt (shm.shmaddr);