[PATCH libX11 1/2] xcb_io: Fix Xlib 32-bit request number wrapping

[Jonas Petersen]

By design the Xlib 32-bit internal request sequence numbers may wrap. There
is two locations within xcb_io.c that are not wrap-safe. The value of
last_flushed relies on the request to be sequential all the time. This is
not given when the sequence has just wrapped. Applications may then crash
with a "Fatal IO error 11 (Resource temporarily unavailable)".

This patch fixes this by unwrapping the sequence number when needed to retain
the sequence relative to last_flushed.

Signed-off-by: Jonas Petersen <jnsptrsn1 at gmail.com>
---
 src/xcb_io.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/xcb_io.c b/src/xcb_io.c
index 727c6c7..f2978d0 100644
--- a/src/xcb_io.c
+++ b/src/xcb_io.c
@@ -455,7 +455,7 @@ void _XSend(Display *dpy, const char *data, long size)
 	static const xReq dummy_request;
 	static char const pad[3];
 	struct iovec vec[3];
-	uint64_t requests;
+	uint64_t requests, unwrapped_request;
 	_XExtension *ext;
 	xcb_connection_t *c = dpy->xcb->connection;
 	if(dpy->flags & XlibDisplayIOError)
@@ -464,6 +464,10 @@ void _XSend(Display *dpy, const char *data, long size)
 	if(dpy->bufptr == dpy->buffer && !size)
 		return;
 
+	/* Set bit 8 of 'request' when a 32-bit wrap has just happened
+	 * so the sequence stays correct relatively to 'last_flushed'. */
+	unwrapped_request = ((uint64_t)(dpy->request < dpy->xcb->last_flushed) << 32) + dpy->request;
+
 	/* iff we asked XCB to set aside errors, we must pick those up
 	 * eventually. iff there are async handlers, we may have just
 	 * issued requests that will generate replies. in either case,
@@ -471,10 +475,10 @@ void _XSend(Display *dpy, const char *data, long size)
 	if(dpy->xcb->event_owner != XlibOwnsEventQueue || dpy->async_handlers)
 	{
 		uint64_t sequence;
-		for(sequence = dpy->xcb->last_flushed + 1; sequence <= dpy->request; ++sequence)
+		for(sequence = dpy->xcb->last_flushed + 1; sequence <= unwrapped_request; ++sequence)
 			append_pending_request(dpy, sequence);
 	}
-	requests = dpy->request - dpy->xcb->last_flushed;
+	requests = unwrapped_request - dpy->xcb->last_flushed;
 	dpy->xcb->last_flushed = dpy->request;
 
 	vec[0].iov_base = dpy->buffer;
-- 
1.7.10.4