Subject: Re: [PATCH 2/3] transport.c: introduce core.alternateRefsCommand

From: Taylor Blau <[email-masked]>
Date: Thu, 20 Sep 2018 16:00:34 -0400

On Thu, Sep 20, 2018 at 03:37:51PM -0400, Jeff King wrote:
> On Thu, Sep 20, 2018 at 02:04:11PM -0400, Taylor Blau wrote:
>
> > diff --git a/Documentation/config.txt b/Documentation/config.txt
> > index 112041f407..b908bc5825 100644
> > --- a/Documentation/config.txt
> > +++ b/Documentation/config.txt
> > @@ -616,6 +616,12 @@ core.preferSymlinkRefs::
> >  	This is sometimes needed to work with old scripts that
> >  	expect HEAD to be a symbolic link.
> >
> > +core.alternateRefsCommand::
> > +	When listing references from an alternate (e.g., in the case of ".have"), use
> > +	the shell to execute the specified command instead of
> > +	linkgit:git-for-each-ref[1]. The first argument is the path of the alternate.
> > +	Output must be of the form: `%(objectname) SPC %(refname)`.
>
> We discussed off-list the notion that this could just be the objectname,
> since the ".have" mechanism doesn't care about the actual refnames.
>
> There's a little prior discussion from the list:
>
>   https://public-inbox.org/git/[email-masked]/
>
> My "rev-list --alternate-refs" patches _do_ use the refnames, since you
> could do something like "--source" that cares about them. But there's
> some awkwardness there, because the names are in a different namespace
> than the rest of the refs. If we were to just say "nope, you do not get
> to see the names of the alternates" then that awkwardness goes away. But
> it also loses some information that could _possibly_ be of use to a
> caller.
>
> Back in that earlier discussion I did not have a strong opinion, but
> here we are cementing that decision into a user-visible interface. So it
> probably makes sense to revisit and decide once and for all.

Interesting, and thanks for the link to the prior discussion. I think
that I agree mostly with your rationale in [1], which boils down (for
me) to:

  - Other callers (like 'rev-list --alternate-refs') might care about
    them. Even if we don't have those patches in Git today, it's worth
    keeping their use case(s) in mind.

  - I didn't measure either, but I can't imagine that we're paying a
    huge price for this. So, it might be easy enough to keep saying,
    "please write output as '%(objectname) SP %(refname)'", even if we
    end up throwing out the refname, anyway.

> > +test_description='git receive-pack test'
> > +
> > +. ./test-lib.sh
> > +
> > +test_expect_success 'setup' '
> > +	test_commit one &&
> > +	git update-ref refs/heads/a HEAD &&
> > +	test_commit two &&
> > +	git update-ref refs/heads/b HEAD &&
> > +	test_commit three &&
> > +	git update-ref refs/heads/c HEAD &&
> > +	git clone --bare . fork &&
> > +	git clone fork pusher &&
> > +	(
> > +		cd fork &&
> > +		git config receive.advertisealternates true &&
> > +		git update-ref -d refs/heads/a &&
> > +		git update-ref -d refs/heads/b &&
> > +		git update-ref -d refs/heads/c &&
> > +		git update-ref -d refs/heads/master &&
> > +		git update-ref -d refs/tags/one &&
> > +		git update-ref -d refs/tags/two &&
> > +		git update-ref -d refs/tags/three &&
>
> Probably not worth nit-picking process count, but this could done with a
> single "update-ref --stdin".

Sure, I don't think that 7 `update-ref`'s vs 2 (`cat` + `git update-ref
--stdin`) will make or break the series, but I can happily shorten it as
you suggest ;-).

> > +		printf "../../.git/objects" >objects/info/alternates
>
> Also a nitpick, but I think "echo" would be more usual here (we handle
> the lack of a trailing newline just fine, but any use of printf makes me
> wonder if something tricky is going on with line endings).

'echo' indeed seems to be the way to go. This 'printf' preference is a
Git LFS-ism ;-).

> > +test_expect_success 'with core.alternateRefsCommand' '
> > +	test_config -C fork core.alternateRefsCommand \
> > +		"git --git-dir=\"\$1\" for-each-ref \
> > +		--format=\"%(objectname) %(refname)\" \
> > +		refs/heads/a refs/heads/c;:" &&
>
> This is cute and all, but might it be more readable to use
> write_script() to stick it into its own script?

Good idea, I'll do that.

> > +	cat >expect <<-EOF &&
> > +	$(git rev-parse a) .have
> > +	$(git rev-parse c) .have
> > +	EOF
> > +	printf "0000" | git receive-pack fork | extract_haves >actual &&
>
> There's been a push lately to avoid having git on the left-hand side of
> a fork, since we might otherwise miss its exit code (including things
> like asan/valgrind errors). So maybe:
>
>    ... receive-pack fork >actual &&
>    extract_haves <actual >actual.haves &&
>    test_cmp expect actual.haves
>
> or similar?

Sure, I agree that it's a good idea to not miss the exit code (since we
don't have pipefail on), etc. I adopted your suggestion into my local
copy.

> > diff --git a/transport.c b/transport.c
> > index 24ae3f375d..e7d2cdf00b 100644
> > --- a/transport.c
> > +++ b/transport.c
> > @@ -1328,10 +1328,21 @@ char *transport_anonymize_url(const char *url)
> >  static void fill_alternate_refs_command(struct child_process *cmd,
> >  					const char *repo_path)
> >  {
> > -	cmd->git_cmd = 1;
> > -	argv_array_pushf(&cmd->args, "--git-dir=%s", repo_path);
> > -	argv_array_push(&cmd->args, "for-each-ref");
> > -	argv_array_push(&cmd->args, "--format=%(objectname) %(refname)");
> > +	const char *value;
> > +
> > +	if (!git_config_get_value("core.alternateRefsCommand", &value)) {
> > +		cmd->use_shell = 1;
> > +
> > +		argv_array_push(&cmd->args, value);
> > +		argv_array_push(&cmd->args, repo_path);
>
> Setting use_shell allows the shell trickery in your test, and matches
> the modern way we run config-based commands. Good.
>
> > +	} else {
> > +		cmd->git_cmd = 1;
> > +
> > +		argv_array_pushf(&cmd->args, "--git-dir=%s", repo_path);
> > +		argv_array_push(&cmd->args, "for-each-ref");
> > +		argv_array_push(&cmd->args, "--format=%(objectname) %(refname)");
> > +	}
> > +
> >  	cmd->env = local_repo_env;
> >  	cmd->out = -1;
>
> And we still clear local_repo_env for the custom command, which is good
> to avoid confusion like $GIT_DIR being set when the custom command does
> "cd $1 && git ...". Good.

Thanks,
Taylor

[1]: https://public-inbox.org/git/[email-masked]/