Re: [PATCH v7 29/47] blockdev: Use CAF in external_snapshot_prepare()

[Max Reitz]

On 20.07.20 18:08, Andrey Shinkevich wrote:
> On 25.06.2020 18:21, Max Reitz wrote:
>> This allows us to differentiate between filters and nodes with COW
>> backing files: Filters cannot be used as overlays at all (for this
>> function).
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   blockdev.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/blockdev.c b/blockdev.c
>> index 1eb0fcdea2..aabe51036d 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -1549,7 +1549,12 @@ static void
>> external_snapshot_prepare(BlkActionState *common,
>>           goto out;
>>       }
>>   -    if (state->new_bs->backing != NULL) {
>> +    if (state->new_bs->drv->is_filter) {
> 
> 
> Is there a chance to get a filter here? If so, is that when a user
> specifies the file name of such a kind “filter[filter-name]:foo.qcow2”
> or somehow else?

It would be with blockdev-snapshot and by specifying a filter for
@overlay.  Technically that’s already caught by the check whether the
overlay supports backing images (whether drv->supports_backing is true),
but we might as well give a nicer error message.

Example:

{"execute":"qmp_capabilities"}

{"execute":"blockdev-add","arguments":
 {"node-name":"overlay","driver":"copy-on-read",
  "file":{"driver":"null-co"}}}

{"execute":"blockdev-add","arguments":
 {"node-name":"base","driver":"null-co"}}

{"execute":"blockdev-snapshot","arguments":
 {"node":"base","overlay":"overlay"}}

Max

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