Subject: Re: [PATCH v2 0/5] Inspect reflog data programmatically in more tests

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 30 Nov 2021 00:28:39 +0100

On Mon, Nov 29 2021, Han-Wen Nienhuys wrote:

> On Mon, Nov 29, 2021 at 11:14 AM Ævar Arnfjörð Bjarmason
> <[email-masked]> wrote:
>> > This helps for reftable support, and will help if we want to reconsider
>> > under which conditions reflogs get created/updated.
>>
>> Having looked at this in a bit more detail than last time
>> (https://lore.kernel.org/git/[email-masked]/)
>> I applaud the goals, but to be blunt the specific approach just seems a
>> bit backwards to me.
>>
>> As noted in that message I have patches to tweak the "verbose" mode to
>> be backend-independent, which as we see from your series is one thing in
>> the files backend that consumes the "message" and assumes things about
>> newlines.
>
> In v2, I went with Jun's suggestion, and left the newlines alone, just
> trimming them in refs/debug.c .  I think that makes most of your mail
> irrelevant?

To whatever immediate problem you're trying to solve? Probably. I'm
mainly pointing out that we can make some of these APIs nicer & a bit
more abstract from the details of the file backend.

Or, as is the case with "ident" v.s. "committer_name/committer_email" in
these reflog callbacks no existing caller actually cares about that
format, so if/when we get to that reftable integration (which seems to
have the two as seperate fields) splitting those up in refs.[ch]
probably makes sense.

>> Perhaps reftable is capable of just handing the underlying code pointers
>> into the mmap()'d file, so we could even skip all (re)allocations? Or if
>> not, that certainly seems like a sensible thing to anticipate in a
>> backend-independent interface. We could do that in the file backend if
>> we were a bit smarter and used mmap() instead of the
>> fopen()/read-in-a-loop pattern.
>
> It sounds like premature optimization. Reading reflogs is not usually
> a performance sensitive operation. Also, if you hand out mmap'd
> pointers, how would the reftable storage code know when it is safe to
> close and munmap the file?

The same way we do the fopen/fclose lifetime now, i.e. you could rely on
them for the iteration, which seems to be a common pattern in code that
needs this.

I was aiming more for the lack of premature optimization there,
i.e. instead of byte-twiddling things by injecting \0s we could just
have char */size_t offsets (as noted elsewhere), which would also nicely
allow binary storage formats, if those formats happen to have an
embedded string. Is that true of reftable?