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Re: [PATCH v11 10/13] stream: skip filters when writing backing file nam

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v11 10/13] stream: skip filters when writing backing file name to QCOW2 header
Date: Wed, 14 Oct 2020 18:40:03 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.2

14.10.2020 18:02, Max Reitz wrote:
On 12.10.20 19:43, Andrey Shinkevich wrote:
Avoid writing a filter JSON-name to QCOW2 image when the backing file
is changed after the block stream job.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
  block/stream.c | 9 +++++----
  1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index e0540ee..51462bd 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -65,6 +65,7 @@ static int stream_prepare(Job *job)
      BlockDriverState *bs = blk_bs(bjob->blk);
      BlockDriverState *unfiltered_bs = bdrv_skip_filters(bs);
      BlockDriverState *base = bdrv_filter_or_cow_bs(s->above_base);
+    BlockDriverState *base_unfiltered = bdrv_skip_filters(base);
      Error *local_err = NULL;
      int ret = 0;
@@ -73,10 +74,10 @@ static int stream_prepare(Job *job) if (bdrv_cow_child(unfiltered_bs)) {
          const char *base_id = NULL, *base_fmt = NULL;
-        if (base) {
-            base_id = s->backing_file_str;
-            if (base->drv) {
-                base_fmt = base->drv->format_name;
+        if (base_unfiltered) {
+            base_id = base_unfiltered->filename;

I think you have to leave this querying s->backing_file_str, and instead
change how qmp_block_stream() gets @base_name.  block-stream has a
backing-file parameter that can override the string that should be used

(Or perhaps you can let qmp_block_stream() just set it to NULL if no
backing-file parameter is passed and then fall back to
base_unfiltered->filename only here.  Probably better, actually, in case
base_unfiltered is changed during the job run.)

Agree with the way in brackets.

If user set backing-file parameter we should handle it here.

If backing-file is not set, we should use dynamically calculated unfiltered 
base in stream_prepare().

Best regards,

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