Re: [PATCH v5 14/15] convert: add filter.<driver>.process option

From: Stefan Beller <sbeller@google.com>
Date: Aug 12, 2016, 16:48 UTC
Message-ID: <CAGZ79kaRKROUganF838w29rCkJ592sZvu+q9fo+h4a4sPeDXMQ@mail.gmail.com>
In-Reply-To: <20160812163809.3wdkuqegxfjam2yn@sigill.intra.peff.net>

On Fri, Aug 12, 2016 at 9:38 AM, Jeff King <peff@peff.net> wrote:

Show 22 quoted lines

> On Fri, Aug 12, 2016 at 09:33:18AM -0700, Stefan Beller wrote:
>
>> > If the result content is empty then the filter is expected to respond
>> > with a success status and an empty list.
>> > ------------------------
>> > packet:          git< status=success\n
>> > packet:          git< 0000
>> > packet:          git< 0000  # empty content!
>> > packet:          git< 0000  # empty list!
>> > ------------------------
>>
>> Why do we need the last flush packet? We'd expect as many successes
>> as we send out contents? Do we plan on interleaving operation, i.e.
>> Git sends out 10 files but the filter process is not as fast as Git sending
>> out and the answers trickle in slowly?
>
> There was prior discussion in the thread, but basically, it is there to
> be able to signal an error that is encountered midway through sending
> the file (i.e., to say "status=error"). If you do not have a final
> flush, then you would send nothing, and the receiver would be left
> wondering if you were successful, or if it simply did not get your error
> report yet.

    I did not follow the prior discussion, so I approached this review with
    no prior knowledge from prior reviews, but instead read through and
    was asking a lot of questions that came up immediately. In case my
    questions are too dumb just omit them, but I thought they were good
    material to answer in a commit message ("Why did we do it this way
    and not differently").

Thanks for the explanation. So this is similar as the situation below that we wait for the flush and then an error/success report?

Show 17 quoted lines

>
>> > If the filter experiences an error during processing, then it can
>> > send the status "error". Depending on the `filter.<driver>.required`
>> > flag Git will interpret that as error but it will not stop or restart
>> > the filter process.
>> > ------------------------
>> > packet:          git< status=success\n
>>
>> So the first success is meaningless essentially?
>> Would it make sense to move the sucess behind the content sending
>> in all cases?
>
> No, the first success says "good so far, here's the file content". The
> second says "I succeeded in sending you the file content".
>
> You _can_ drop the first one, but it may be more convenient for the
> receiver to know up-front that there was a failure.

If there was a failure upfront, it would become

packet: git< 0000 # no content is encapsulated here packet: git< 0000 packet: git< status=error\n packet: git< 0000

so from a protocol side I'd claim it doesn't look bad. I assume with convenient you mean the implementation side of things?

If we do the success first and then error out halfway, we still have to clean up, so I do not see how this impacts implementation?

Thanks, Stefan