[PATCH for-4.2?] block/qcow2-bitmap: fix crash bug in qcow2_co_remove_persistent_dirty_bitmap

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Here is double bug:

First, return error but not set errp. This may lead to:
qmp block-dirty-bitmap-remove may report success when actually failed

block-dirty-bitmap-remove used in a transaction will crash, as
qmp_transaction will think that it returned success and will cal
block_dirty_bitmap_remove_commit which will crash, as state->bitmap is
NULL

Second (like in anecdote), this case is not an error at all. As it is
documented in the comment above bdrv_co_remove_persistent_dirty_bitmap
definition, absence of bitmap is not an error, and similar case handled
at start of qcow2_co_remove_persistent_dirty_bitmap, it returns 0 when
there is no bitmaps at all..

But when there are some bitmaps, but not the requested one, it return
error with errp unset.

Fix that.

Fixes: b56a1e31759b750
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---

Hi all!

Ohm, suddenly we faced this bug. It's a regression in 4.2. I'm very
sorry for introducing it, and it sad that it's found so late..

Personally, I think that this one worth rc5, as it makes new bitmap
interfaces unusable. But the decision is yours.

Last minute edit: hmm, actually, transaction action introduced in
4.2, so crash is not a regression, only broken block-dirty-bitmap-remove
command is a regression... Maybe it's OK for stable.

 block/qcow2-bitmap.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
index 8abaf632fc..c6c8ebbe89 100644
--- a/block/qcow2-bitmap.c
+++ b/block/qcow2-bitmap.c
@@ -1469,8 +1469,10 @@ int coroutine_fn qcow2_co_remove_persistent_dirty_bitmap(BlockDriverState *bs,
     Qcow2BitmapList *bm_list;
 
     if (s->nb_bitmaps == 0) {
-        /* Absence of the bitmap is not an error: see explanation above
-         * bdrv_remove_persistent_dirty_bitmap() definition. */
+        /*
+         * Absence of the bitmap is not an error: see explanation above
+         * bdrv_co_remove_persistent_dirty_bitmap() definition.
+         */
         return 0;
     }
 
@@ -1485,7 +1487,8 @@ int coroutine_fn qcow2_co_remove_persistent_dirty_bitmap(BlockDriverState *bs,
 
     bm = find_bitmap_by_name(bm_list, name);
     if (bm == NULL) {
-        ret = -EINVAL;
+        /* Absence of the bitmap is not an error, see above. */
+        ret = 0;
         goto out;
     }
 
-- 
2.21.0