From: Max Reitz <mreitz@redhat.com>

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 <mreitz@redhat.com>
Message-Id: <20200429141126.85159-5-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[kwolf: Fixed up reference output for 098]
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/commit.c             | 16 +++++++++-------
 qemu-img.c                 | 19 ++++++++++++++-----
 tests/qemu-iotests/098.out |  8 ++++----
 3 files changed, 27 insertions(+), 16 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 6a4327aaba..f1795c788c 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1104,11 +1104,20 @@ 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;
         }
     }
diff --git a/tests/qemu-iotests/098.out b/tests/qemu-iotests/098.out
index 7634d0e8b0..23cf371f53 100644
--- a/tests/qemu-iotests/098.out
+++ b/tests/qemu-iotests/098.out
@@ -6,7 +6,7 @@ Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
+qemu-img: Failed to empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
 No errors were found on the image.
 
 === empty_image_prepare ===
@@ -15,7 +15,7 @@ Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
+qemu-img: Failed to empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
 Leaked cluster 4 refcount=1 reference=0
 Leaked cluster 5 refcount=1 reference=0
 Repairing cluster 4 refcount=1 reference=0
@@ -28,7 +28,7 @@ Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
+qemu-img: Failed to empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
 ERROR cluster 0 refcount=0 reference=1
 ERROR cluster 1 refcount=0 reference=1
 ERROR cluster 3 refcount=0 reference=1
@@ -42,7 +42,7 @@ Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
+qemu-img: Failed to empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
 ERROR cluster 0 refcount=0 reference=1
 ERROR cluster 1 refcount=0 reference=1
 ERROR cluster 3 refcount=0 reference=1
-- 
2.25.4