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Re: [PATCH v12 13/14] stream: skip filters when writing backing file nam

From: Andrey Shinkevich
Subject: Re: [PATCH v12 13/14] stream: skip filters when writing backing file name to QCOW2 header
Date: Tue, 27 Oct 2020 19:01:01 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 27.10.2020 18:09, Vladimir Sementsov-Ogievskiy wrote:
22.10.2020 21:13, Andrey Shinkevich wrote:
Avoid writing a filter JSON file name and a filter format name to QCOW2
image when the backing file is changed after the block stream job.
A user is still able to assign the 'backing-file' parameter for a
block-stream job keeping in mind the possible issue mentioned above.
If the user does not specify the 'backing-file' parameter, QEMU will
assign it automatically.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
  block/stream.c | 15 +++++++++++++--
  blockdev.c     |  9 ++-------
  2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index e0540ee..1ba74ab 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 = NULL;
      Error *local_err = NULL;
      int ret = 0;
@@ -75,8 +76,18 @@ static int stream_prepare(Job *job)
          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_id) {
+                if (base->drv) {
+                    base_fmt = base->drv->format_name;

hmm. this doesn't make real sense: so, we assume that user specified backing_file_str, which may not relate to base, but we use base->drv->format_name? But it may be name of the filter driver, which would be wrong..

Any ideas?

1. we can use base_fmt=NULL, to provoke probing on next open of the qcow2 file..

I would choose this item #1 but have to check the probing code logic... Particularly, I do not remember now if the probing is able to recognize a protocol. The logic for the format_name in the QEMU existent code (I has kept it here in the patch) is a slippery way for an imprudent user. That's why I staked on the backing_file_str deprication in the previous version.

2. we can do probing now
3. we can at least check, if backing_file_str ==

Not bad for the sanity check but we will search a node by the file name again - not good ((

base_unfiltered->filename, in this case we can use base_unfiltered->drv->format_name

+                }
+            } else {
+                base_unfiltered = bdrv_skip_filters(base);
+                if (base_unfiltered) {
+                    base_id = base_unfiltered->filename;
+                    if (base_unfiltered->drv) {
+                        base_fmt = base_unfiltered->drv->format_name;
+                    }
+                }
          bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
diff --git a/blockdev.c b/blockdev.c
index c917625..0e9c783 100644
--- a/blockdev.c
+++ b/blockdev.c


-    stream_start(has_job_id ? job_id : NULL, bs, base_bs, base_name,
+    stream_start(has_job_id ? job_id : NULL, bs, base_bs,
+                 has_backing_file ? backing_file : NULL,

backing_file should be NULL if has_backing_file is false, so you can use just backing_file instead of ternary operator.

Yes, if reliable. I has kept the conformation with the ternary operator at the first parameter above.


                   job_flags, has_speed ? speed : 0, on_error,
                   filter_node_name, &local_err);
      if (local_err) {

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