block: Manage backing file references in bdrv_set_backing_hd()

This simplifies the code somewhat, especially when dropping whole
backing file subchains.

The exception is the mirroring code that does adventurous things with
bdrv_swap() and in order to keep it working, I had to duplicate most of
bdrv_set_backing_hd() locally. We'll get rid again of this ugliness
shortly.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

3 files changed, 18 insertions, 34 deletions

diff --git a/block/mirror.c b/block/mirror.c
index 5d3b8bb..6247b27 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -370,10 +370,18 @@ static void mirror_exit(BlockJob *job, void *opaque)
         bdrv_swap(s->target, to_replace);
         if (s->common.driver->job_type == BLOCK_JOB_TYPE_COMMIT) {
             /* drop the bs loop chain formed by the swap: break the loop then
-             * trigger the unref from the top one */
-            BlockDriverState *p = backing_bs(s->base);
-            bdrv_set_backing_hd(s->base, NULL);
-            bdrv_unref(p);
+             * trigger the unref */
+            /* FIXME This duplicates bdrv_set_backing_hd(), except for the
+             * actual detach/unref so that the loop can be broken. When
+             * bdrv_swap() gets replaced, this will become sane again. */
+            BlockDriverState *backing = s->base->backing->bs;
+            assert(s->base->backing_blocker);
+            bdrv_op_unblock_all(backing, s->base->backing_blocker);
+            error_free(s->base->backing_blocker);
+            s->base->backing_blocker = NULL;
+            bdrv_detach_child(s->base->backing);
+            s->base->backing = NULL;
+            bdrv_unref(backing);
         }
     }
     if (s->to_replace) {
diff --git a/block/stream.c b/block/stream.c
index ba535c5..3f64fa2 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -52,34 +52,6 @@ static int coroutine_fn stream_populate(BlockDriverState *bs,
     return bdrv_co_copy_on_readv(bs, sector_num, nb_sectors, &qiov);
 }
 
-static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
-                                const char *base_id)
-{
-    BlockDriverState *intermediate;
-    intermediate = backing_bs(top);
-
-    /* Must assign before bdrv_delete() to prevent traversing dangling pointer
-     * while we delete backing image instances.
-     */
-    bdrv_set_backing_hd(top, base);
-
-    while (intermediate) {
-        BlockDriverState *unused;
-
-        /* reached base */
-        if (intermediate == base) {
-            break;
-        }
-
-        unused = intermediate;
-        intermediate = backing_bs(intermediate);
-        bdrv_set_backing_hd(unused, NULL);
-        bdrv_unref(unused);
-    }
-
-    bdrv_refresh_limits(top, NULL);
-}
-
 typedef struct {
     int ret;
     bool reached_end;
@@ -101,7 +73,7 @@ static void stream_complete(BlockJob *job, void *opaque)
             }
         }
         data->ret = bdrv_change_backing_file(job->bs, base_id, base_fmt);
-        close_unused_images(job->bs, base, base_id);
+        bdrv_set_backing_hd(job->bs, base);
     }
 
     g_free(s->backing_file_str);
diff --git a/block/vvfat.c b/block/vvfat.c
index 7c4b0f5..b41055a 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2923,6 +2923,7 @@ static BlockDriver vvfat_write_target = {
 static int enable_write_target(BDRVVVFATState *s, Error **errp)
 {
     BlockDriver *bdrv_qcow = NULL;
+    BlockDriverState *backing;
     QemuOpts *opts = NULL;
     int ret;
     int size = sector2cluster(s, s->sector_count);
@@ -2971,7 +2972,10 @@ static int enable_write_target(BDRVVVFATState *s, Error **errp)
     unlink(s->qcow_filename);
 #endif
 
-    bdrv_set_backing_hd(s->bs, bdrv_new());
+    backing = bdrv_new();
+    bdrv_set_backing_hd(s->bs, backing);
+    bdrv_unref(backing);
+
     s->bs->backing->bs->drv = &vvfat_write_target;
     s->bs->backing->bs->opaque = g_new(void *, 1);
     *(void**)s->bs->backing->bs->opaque = s;