qcow2: Don't write with BDRV_O_INCOMING

qcow2_open() causes writes when repairing an image with the dirty flag
set and when clearing autoclear flags. It shouldn't do this when another
qemu instance is still actively working on this image file.

One effect of the bug is that images may have a cleared dirty flag while
the migration source host still has it in use with lazy refcounts
enabled, so refcounts are not accurate and the dirty flag must remain
set.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2.c

@@ -644,7 +644,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     /* Clear unknown autoclear feature bits */
-    if (!bs->read_only && s->autoclear_features != 0) {
+    if (!bs->read_only && !(flags & BDRV_O_INCOMING) && s->autoclear_features) {
         s->autoclear_features = 0;
         ret = qcow2_update_header(bs);
         if (ret < 0) {
@@ -657,7 +657,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     qemu_co_mutex_init(&s->lock);
 
     /* Repair image if dirty */
-    if (!(flags & BDRV_O_CHECK) && !bs->read_only &&
+    if (!(flags & (BDRV_O_CHECK | BDRV_O_INCOMING)) && !bs->read_only &&
         (s->incompatible_features & QCOW2_INCOMPAT_DIRTY)) {
         BdrvCheckResult result = {0};
 
@@ -1137,10 +1137,12 @@ static void qcow2_close(BlockDriverState *bs)
     /* else pre-write overlap checks in cache_destroy may crash */
     s->l1_table = NULL;
 
-    qcow2_cache_flush(bs, s->l2_table_cache);
-    qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (!(bs->open_flags & BDRV_O_INCOMING)) {
+        qcow2_cache_flush(bs, s->l2_table_cache);
+        qcow2_cache_flush(bs, s->refcount_block_cache);
 
-    qcow2_mark_clean(bs);
+        qcow2_mark_clean(bs);
+    }
 
     qcow2_cache_destroy(bs, s->l2_table_cache);
     qcow2_cache_destroy(bs, s->refcount_block_cache);