[PATCH] qemu-img: Support bitmap --merge into backing image

[Eric Blake <eblake@redhat.com>]

If you have the chain 'base.qcow2 <- top.qcow2' and want to merge a
bitmap from top into base, qemu-img was failing with:

qemu-img: Could not open 'top.qcow2': Could not open backing file: Failed to get shared "write" lock
Is another process using the image [base.qcow2]?

The easiest fix is to not open the entire backing chain of the source
image, so that we aren't worrying about competing BDS visiting the
backing image as both a read-only backing of the source and the
writeable destination.

Fixes: http://bugzilla.redhat.com/1877209
Reported-by: Eyal Shenitzky <eshenitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 qemu-img.c                 |  3 +-
 tests/qemu-iotests/291     | 12 ++++++++
 tests/qemu-iotests/291.out | 56 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index eb2fc1f86243..b15098a2f9b3 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -4755,7 +4755,8 @@ static int img_bitmap(int argc, char **argv)
     }
     bs = blk_bs(blk);
     if (src_filename) {
-        src = img_open(false, src_filename, src_fmt, 0, false, false, false);
+        src = img_open(false, src_filename, src_fmt, BDRV_O_NO_BACKING,
+                       false, false, false);
         if (!src) {
             goto out;
         }
diff --git a/tests/qemu-iotests/291 b/tests/qemu-iotests/291
index 1e0bb76959bb..4f837b205655 100755
--- a/tests/qemu-iotests/291
+++ b/tests/qemu-iotests/291
@@ -91,6 +91,15 @@ $QEMU_IMG bitmap --remove --image-opts \
     driver=$IMGFMT,file.driver=file,file.filename="$TEST_IMG" tmp
 _img_info --format-specific

+echo
+echo "=== Merge from top layer into backing image ==="
+echo
+
+$QEMU_IMG rebase -u -F qcow2 -b "$TEST_IMG.base" "$TEST_IMG"
+$QEMU_IMG bitmap --add --merge b2 -b "$TEST_IMG" -F $IMGFMT \
+     -f $IMGFMT "$TEST_IMG.base" b3
+_img_info --format-specific --backing-chain
+
 echo
 echo "=== Check bitmap contents ==="
 echo
@@ -107,6 +116,9 @@ $QEMU_IMG map --output=json --image-opts \
 nbd_server_start_unix_socket -r -f qcow2 -B b2 "$TEST_IMG"
 $QEMU_IMG map --output=json --image-opts \
     "$IMG,x-dirty-bitmap=qemu:dirty-bitmap:b2" | _filter_qemu_img_map
+nbd_server_start_unix_socket -r -f qcow2 -B b3 "$TEST_IMG"
+$QEMU_IMG map --output=json --image-opts \
+    "$IMG,x-dirty-bitmap=qemu:dirty-bitmap:b3" | _filter_qemu_img_map

 # success, all done
 echo '*** done'
diff --git a/tests/qemu-iotests/291.out b/tests/qemu-iotests/291.out
index ee89a728855f..3990f7aacc7b 100644
--- a/tests/qemu-iotests/291.out
+++ b/tests/qemu-iotests/291.out
@@ -68,6 +68,59 @@ Format specific information:
     corrupt: false
     extended l2: false

+=== Merge from top layer into backing image ===
+
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 10 MiB (10485760 bytes)
+cluster_size: 65536
+backing file: TEST_DIR/t.IMGFMT.base
+backing file format: IMGFMT
+Format specific information:
+    compat: 1.1
+    compression type: zlib
+    lazy refcounts: false
+    bitmaps:
+        [0]:
+            flags:
+            name: b1
+            granularity: 524288
+        [1]:
+            flags:
+                [0]: auto
+            name: b2
+            granularity: 65536
+        [2]:
+            flags:
+            name: b0
+            granularity: 65536
+    refcount bits: 16
+    corrupt: false
+    extended l2: false
+
+image: TEST_DIR/t.IMGFMT.base
+file format: IMGFMT
+virtual size: 10 MiB (10485760 bytes)
+cluster_size: 65536
+Format specific information:
+    compat: 1.1
+    compression type: zlib
+    lazy refcounts: false
+    bitmaps:
+        [0]:
+            flags:
+                [0]: auto
+            name: b0
+            granularity: 65536
+        [1]:
+            flags:
+                [0]: auto
+            name: b3
+            granularity: 65536
+    refcount bits: 16
+    corrupt: false
+    extended l2: false
+
 === Check bitmap contents ===

 [{ "start": 0, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
@@ -79,4 +132,7 @@ Format specific information:
 [{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
 { "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+[{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
+{ "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done
-- 
2.28.0