qcow2: Handle EAGAIN returned from update_refcount

Fixes a crash during image compression

Signed-off-by: Jindřich Makovička <makovick@gmail.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qcow2-refcount.c

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 0632fc3..b0ee42d 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -940,19 +940,21 @@
     }
 
     free_in_cluster = s->cluster_size - offset_into_cluster(s, offset);
-    if (!offset || free_in_cluster < size) {
-        int64_t new_cluster = alloc_clusters_noref(bs, s->cluster_size);
-        if (new_cluster < 0) {
-            return new_cluster;
+    do {
+        if (!offset || free_in_cluster < size) {
+            int64_t new_cluster = alloc_clusters_noref(bs, s->cluster_size);
+            if (new_cluster < 0) {
+                return new_cluster;
+            }
+
+            if (!offset || ROUND_UP(offset, s->cluster_size) != new_cluster) {
+                offset = new_cluster;
+            }
         }
 
-        if (!offset || ROUND_UP(offset, s->cluster_size) != new_cluster) {
-            offset = new_cluster;
-        }
-    }
-
-    assert(offset);
-    ret = update_refcount(bs, offset, size, 1, false, QCOW2_DISCARD_NEVER);
+        assert(offset);
+        ret = update_refcount(bs, offset, size, 1, false, QCOW2_DISCARD_NEVER);
+    } while (ret == -EAGAIN);
     if (ret < 0) {
         return ret;
     }