[Libguestfs] [libnbd PATCH] tests: Enhance errors test

[Eric Blake]

On 7/2/19 9:01 AM, Eric Blake wrote:

>> This test fails when run under valgrind.  An abbreviated log shows
>> what's happening:
>>

>> I wonder if we could remove the race using a custom nbdkit-sh-plugin
>> which would block on writes until (eg) a local trigger file was
>> touched?  Even that seems as if it would depend on the amount of data
>> that the kernel is able to buffer.
> 
> I don't know how to make an nbdkit plugin stop the code in nbdkit/server
> from read()ing from the client (the plugin code doesn't get to run until
> the core has learned that the client wants a command serviced).  But it
> may be possible to tweak things to send back-to-back write requests,
> where even if the first write request gets sent completely, the plugin
> can delay responding to that first write and use --filter=noparallel to
> prevent the second command from reaching nbdkit.  I'll play with that,
> to see if I can reproduce the valgrind race, as well as work around it
> with back-to-back write commands to increase the likelihood of actually
> preventing nbdkit from consuming the second command.

Should be fixed now.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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