Re: [PATCH 1/2] pretty: add %(describe)

[Junio C Hamano <gitster@pobox.com>]

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> On Sun, Feb 14 2021, René Scharfe wrote:
>
>> +'%(describe)':: human-readable name, like linkgit:git-describe[1];
>> +		empty string for undescribable commits
>
> In the case of undescribable we've got the subcommand exiting non-zero
> and we ignore it. The right thing in this case given how the rest of
> format arguments work, but maybe something to explicitly test for?
>>
>> +	if (skip_prefix(placeholder, "(describe)", &arg)) {
>> +		struct child_process cmd = CHILD_PROCESS_INIT;
>> +		struct strbuf out = STRBUF_INIT;
>> +		struct strbuf err = STRBUF_INIT;
>> +
>> +		cmd.git_cmd = 1;
>> +		strvec_push(&cmd.args, "describe");
>> +		strvec_push(&cmd.args, oid_to_hex(&commit->object.oid));
>> +		pipe_command(&cmd, NULL, 0, &out, 0, &err, 0);
>> +		strbuf_rtrim(&out);
>> +		strbuf_addbuf(sb, &out);
>> +		strbuf_release(&out);
>> +		strbuf_release(&err);
>> +		return arg - placeholder;
>> +	}
>
> There's another edge case in this: if you do "%(describe)%(describe)"
> it'll be run twice for the rev, 3 times if you add another "%(describe)"
> etc. I don't know if pretty.c has an easy way to cache/avoid that.

Just like for-each-ref that has the "atom" system to lazily parse
and cache a computed result so that multiple invocations of the same
placeholder will have to prepare a string only once, the pretty side
has the "format_commit_context" structure that can be used to cache
values that are expensive to compute in case it is used more than
once, so we could use it.

I however suspect that we may already be leaking some cacheed values
in the current code.  For example, there is "signature_check"
instance that is used to handle %G and we do use it to call
check_signature(), but a call to signature_check_clear() to release
resources is nowhere to be seen as far as I can tell.