xcb: Steal from the pending list for GetImage

Before using some piece of SHM again, we must be sure that the X11 server is no
longer working with it. For this, we send a GetInputFocus when we are done with
the SHM locally and will only use the memory again when the reply comes in.

However, if we are allocating the memory for SHM GetImage, then we can re-use
memory earlier, because the server processes requests in order. So it will only
start writing to the memory after it is done with earlier requests for this
memory. So instead of using GetInputFocus for synchronisation, the SHM GetImage
request will automatically do this for us.

Thanks to Chris Wilson for this idea.

Signed-off-by: Uli Schlachter <psychon@znc.in>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/cairo-xcb-shm.c b/src/cairo-xcb-shm.c
index ffcaec8f1..6972720b0 100644
--- a/src/cairo-xcb-shm.c
+++ b/src/cairo-xcb-shm.c
@@ -492,6 +492,7 @@ _cairo_xcb_shm_process_pending (cairo_xcb_connection_t *connection, shm_wait_typ
 cairo_int_status_t
 _cairo_xcb_connection_allocate_shm_info (cairo_xcb_connection_t *connection,
 					 size_t size,
+					 cairo_bool_t might_reuse,
 					 cairo_xcb_shm_info_t **shm_info_out)
 {
     cairo_xcb_shm_info_t *shm_info;
@@ -506,6 +507,22 @@ _cairo_xcb_connection_allocate_shm_info (cairo_xcb_connection_t *connection,
     CAIRO_MUTEX_LOCK (connection->shm_mutex);
     _cairo_xcb_shm_process_pending (connection, PENDING_POLL);
 
+    if (might_reuse) {
+	cairo_list_foreach_entry (shm_info, cairo_xcb_shm_info_t,
+		&connection->shm_pending, pending) {
+	    if (shm_info->size >= size) {
+		cairo_list_del (&shm_info->pending);
+		CAIRO_MUTEX_UNLOCK (connection->shm_mutex);
+
+		xcb_discard_reply (connection->xcb_connection, shm_info->sync.sequence);
+		shm_info->sync.sequence = XCB_NONE;
+
+		*shm_info_out = shm_info;
+		return CAIRO_STATUS_SUCCESS;
+	    }
+	}
+    }
+
     cairo_list_foreach_entry_safe (pool, next, cairo_xcb_shm_mem_pool_t,
 				   &connection->shm_pools, link)
     {
@@ -599,6 +616,7 @@ _cairo_xcb_connection_allocate_shm_info (cairo_xcb_connection_t *connection,
     shm_info->connection = connection;
     shm_info->pool = pool;
     shm_info->shm = pool->shmseg;
+    shm_info->size = size;
     shm_info->offset = (char *) mem - (char *) pool->base;
     shm_info->mem = mem;
     shm_info->sync.sequence = XCB_NONE;