[PATCH v2 4/4] block: Use blk_make_empty() after commits

[Max Reitz]

bdrv_commit() already has a BlockBackend pointing to the BDS that we
want to empty, it just has the wrong permissions.

qemu-img commit has no BlockBackend pointing to the old backing file
yet, but introducing one is simple.

After this commit, bdrv_make_empty() is the only remaining caller of
BlockDriver.bdrv_make_empty().

Signed-off-by: Max Reitz <address@hidden>
---
 block/commit.c | 16 +++++++++-------
 qemu-img.c     | 19 ++++++++++++++-----
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/block/commit.c b/block/commit.c
index 87f6096d90..ba60fb7955 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -414,7 +414,9 @@ int bdrv_commit(BlockDriverState *bs)
     }
 
     ctx = bdrv_get_aio_context(bs);
-    src = blk_new(ctx, BLK_PERM_CONSISTENT_READ, BLK_PERM_ALL);
+    /* WRITE_UNCHANGED is required for bdrv_make_empty() */
+    src = blk_new(ctx, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED,
+                  BLK_PERM_ALL);
     backing = blk_new(ctx, BLK_PERM_WRITE | BLK_PERM_RESIZE, BLK_PERM_ALL);
 
     ret = blk_insert_bs(src, bs, &local_err);
@@ -492,14 +494,14 @@ int bdrv_commit(BlockDriverState *bs)
         }
     }
 
-    if (drv->bdrv_make_empty) {
-        ret = drv->bdrv_make_empty(bs);
-        if (ret < 0) {
-            goto ro_cleanup;
-        }
-        blk_flush(src);
+    ret = blk_make_empty(src, NULL);
+    /* Ignore -ENOTSUP */
+    if (ret < 0 && ret != -ENOTSUP) {
+        goto ro_cleanup;
     }
 
+    blk_flush(src);
+
     /*
      * Make sure all data we wrote to the backing device is actually
      * stable on disk.
diff --git a/qemu-img.c b/qemu-img.c
index 7f52742ef2..77f3575538 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1065,11 +1065,20 @@ static int img_commit(int argc, char **argv)
         goto unref_backing;
     }
 
-    if (!drop && bs->drv->bdrv_make_empty) {
-        ret = bs->drv->bdrv_make_empty(bs);
-        if (ret) {
-            error_setg_errno(&local_err, -ret, "Could not empty %s",
-                             filename);
+    if (!drop) {
+        BlockBackend *old_backing_blk;
+
+        old_backing_blk = blk_new_with_bs(bs, BLK_PERM_WRITE, BLK_PERM_ALL,
+                                          &local_err);
+        if (!old_backing_blk) {
+            goto unref_backing;
+        }
+        ret = blk_make_empty(old_backing_blk, &local_err);
+        blk_unref(old_backing_blk);
+        if (ret == -ENOTSUP) {
+            error_free(local_err);
+            local_err = NULL;
+        } else if (ret < 0) {
             goto unref_backing;
         }
     }
-- 
2.25.4