Re: [Qemu-devel] [PATCH v5 10/10] test-qga: Actually test 0xff sync bytes

[Markus Armbruster]

Michael Roth <address@hidden> writes:

> Quoting Markus Armbruster (2017-05-03 03:57:41)
>> Michael Roth <address@hidden> writes:
>> 
>> > Quoting Michael Roth (2017-05-02 11:46:36)
>> >> Quoting Eric Blake (2017-04-27 16:58:21)
>> >> > Commit 62c39b3 introduced test-qga, and at face value, appears
>> >> > to be testing the 'guest-sync' behavior that is recommended for
>> >> > guests in sending 0xff to QGA to force the parser to reset.  But
>> >> > this aspect of the test has never actually done anything: the
>> >> > qmp_fd() call chain converts its string argument into QObject,
>> >> > then converts that QObject back to the actual string that is
>> >> > sent over the wire - and the conversion process silently drops
>> >> > the 0xff byte from the string sent to QGA, thus never resetting
>> >> > the QGA parser.
>> >> > 
>> >> > An upcoming patch will get rid of the wasteful round trip
>> >> > through QObject, at which point the string in test-qga will be
>> >> > directly sent over the wire.
>> >> > 
>> >> > But fixing qmp_fd() to actually send 0xff over the wire is not
>> >> > all we have to do - the actual QMP parser loudly complains that
>> >> > 0xff is not valid JSON, and sends an error message _prior_ to
>> >> > actually parsing the 'guest-sync' or 'guest-sync-delimited'
>> >> > command.  With 'guest-sync', we cannot easily tell if this error
>> >> > message is a result of our command - which is WHY we invented
>> >> > the 'guest-sync-delimited' command.  So for the testsuite, fix
>> >> > things to only check 0xff behavior on 'guest-sync-delimited',
>> >> > and to loop until we've consumed all garbage prior to the
>> >> > requested delimiter, which matches the documented actions that
>> >> > a real QGA client is supposed to do.
>> >> 
>> >> The full re-sync sequence is actually to perform that process,
>> >> check if the response matches the client-provided sync token,
>> >> and then repeat the procedure if it doesn't (in the odd case
>> >> that a previous client initiated a guest-sync-delimited
>> >> sequence and never consumed the response). The current
>> >> implementation only performs one iteration so it's not quite
>> >> a full implementation of the documentation procedure.
>> >
>> > Well, to be more accurate it's a full implementation of the
>> > documented procedure, it's just that the procedure isn't
>> > fully documented properly. I'll send a patch to address that.
>> 
>> Good.
>> 
>> >> For the immediate purpose of improving the handling to deal
>> >> with the 0xFF-generated error the patch seems sound though,
>> >> maybe just something worth noting in the commit msg or
>> >> comments so that we might eventually test the full procedure.
>> 
>> Feel free to suggest something for me to add to the commit message.
>
> Maybe change:
>
>   "which matches the documented actions that a real QGA client
>    is supposed to do."
>
> to
>
>   "which is compatible with the documented actions that a real
>    QGA client is supposed to do."
>
> and add the following comment to test_qga_sync_delimited
>
>   /* 
>    * Note that the full reset sequence would involve checking the
>    * response of guest-sync-delimited and repeating the loop if
>    * 'id' field of the response does not match the 'id' field of 
>    * the request. Testing this fully would require inserting
>    * garbage in the response stream and is left as a future test
>    * to implement.
>    */

Eric, want me to squash that in?