[PATCH 1/2] qcow2: do not clear the dirty bit when reopening a read-only node

qcow2_reopen_prepare() clears the dirty bit whenever the node is
reopened read-only, with an unguarded header write. A read-only node
can still be dirty, inherited from an earlier writable session, and it
holds no BLK_PERM_WRITE to resolve that. A read-only to read-only
reopen of a dirty image therefore fails outright:

  $ qemu-io -r -f qcow2 dirty.qcow2 <<< $'reopen -r\nquit'
  qemu-io: failed while preparing to reopen image 'dirty.qcow2'

Clear it only for a node that is writable now, the predicate
qcow2_do_open() already uses for the repair. bdrv_is_writable() also
excludes an inactive node, whose header must not be touched either.

Signed-off-by: Denis V. Lunev <[email protected]>
CC: Kevin Wolf <[email protected]>
CC: Hanna Reitz <[email protected]>
---
 block/qcow2.c              |  8 +++++---
 tests/qemu-iotests/039     | 11 +++++++++++
 tests/qemu-iotests/039.out |  5 +++++
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 7292dd036c..1543255eba 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2102,9 +2102,11 @@ qcow2_reopen_prepare(BDRVReopenState *state,BlockReopenQueue *queue,
             goto fail;
         }
 
-        ret = qcow2_mark_clean(state->bs);
-        if (ret < 0) {
-            goto fail;
+        if (bdrv_is_writable(state->bs)) {
+            ret = qcow2_mark_clean(state->bs);
+            if (ret < 0) {
+                goto fail;
+            }
         }
     }
 
diff --git a/tests/qemu-iotests/039 b/tests/qemu-iotests/039
index 94a8bfe754..a5be81bc4a 100755
--- a/tests/qemu-iotests/039
+++ b/tests/qemu-iotests/039
@@ -95,6 +95,17 @@ $QEMU_IMG info --image-opts \
 # The dirty bit must still be set: this open never wrote any guest data
 _qcow2_dump_header | grep incompatible_features
 
+echo
+echo "== Read-only reopen must not clear the dirty bit =="
+
+# Reopening a read-only node must not try to write the QCOW2 header either,
+# and must leave the dirty bit for whoever ends up repairing the image.
+$QEMU_IO -r -c "reopen -r" -c "read -P 0x5a 0 512" "$TEST_IMG" \
+    | _filter_qemu_io
+
+# The dirty bit must still be set
+_qcow2_dump_header | grep incompatible_features
+
 echo
 echo "== Repairing the image file must succeed =="
 
diff --git a/tests/qemu-iotests/039.out b/tests/qemu-iotests/039.out
index c66361128f..3c71e5a3dd 100644
--- a/tests/qemu-iotests/039.out
+++ b/tests/qemu-iotests/039.out
@@ -27,6 +27,11 @@ incompatible_features     [0]
 == Read-only open must not crash on close ==
 incompatible_features     [0]
 
+== Read-only reopen must not clear the dirty bit ==
+read 512/512 bytes at offset 0
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+incompatible_features     [0]
+
 == Repairing the image file must succeed ==
 ERROR cluster 5 refcount=0 reference=1
 Rebuilding refcount structure
-- 
2.53.0