qcow2: Flush pending discards before allocating cluster

Before a freed cluster can be reused, pending discards for this cluster
must be processed.

The original assumption was that this was not a problem because discards
are only cached during discard/write zeroes operations, which are
synchronous so that no concurrent write requests can cause cluster
allocations.

However, the discard/write zeroes operation itself can allocate a new L2
table (and it has to in order to put zero flags there), so make sure we
can cope with the situation.

This fixes https://bugs.launchpad.net/bugs/1349972.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit ecbda7a225)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block/qcow2-refcount.c

@@ -833,6 +833,11 @@ static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size)
     uint64_t i, nb_clusters, refcount;
     int ret;
 
+    /* We can't allocate clusters if they may still be queued for discard. */
+    if (s->cache_discards) {
+        qcow2_process_discards(bs, 0);
+    }
+
     nb_clusters = size_to_clusters(s, size);
 retry:
     for(i = 0; i < nb_clusters; i++) {