[Libguestfs] [nbdkit PATCH 2/4] nbd: Normalize return values of can_*

[Eric Blake]

On 3/17/20 3:12 AM, Richard W.M. Jones wrote:
> On Mon, Mar 16, 2020 at 10:36:15PM -0500, Eric Blake wrote:
>> Although nbdkit documents that any positive value should be treated as
>> success to the .can_* callbacks, we had a window of releases where
>> anything other than 1 could trigger an assertion failure, fixed in the
>> previous patch.  Update what we return to avoid tripping the bug in
>> broken nbdkit.
>>
>> Our return value has been latent since 0e8e8eb11d (v1.1.17), but the
>> assertion failures did not occur until c306fa93ab and neighboring
>> commits (v1.15.1).  As v1.13.6 was the point where we started
>> preferring builds against libnbd, where we always returned 1 on
>> success, the problem was not detected until now; but it is still in
>> the wild for any user mixing old plugins with new libnbd.
>>
>> Signed-off-by: Eric Blake <eblake at redhat.com>
>> ---
>>   plugins/nbd/nbd-standalone.c | 12 ++++++------
>>   1 file changed, 6 insertions(+), 6 deletions(-)

> While I don't like the idea of modifying plugins to fix broken nbdkit
> behaviour, you make a good case for this being safer, so:
> 
> ACK

Think of it as working around broken nbdkit behavior, rather than fixing 
it (as the fix is in patch 1/4).  How many stable branches should we 
backport this to?  Going all the way back to stable-1.2 seems like it 
might be overkill; maybe back to stable-1.14 (where nbd-standalone was 
first split off) or stable-1.12 (the last release that did not use 
libnbd) is sufficient?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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