Re: [PATCH v2 2/4] commit: write commit-graph with Bloom filters

[Taylor Blau <me@ttaylorr.com>]

On Tue, Apr 14, 2020 at 01:40:53PM -0400, Derrick Stolee wrote:
> On 4/14/2020 1:26 PM, Junio C Hamano wrote:
> > Derrick Stolee <stolee@gmail.com> writes:
> >
> >> On 4/13/2020 6:21 PM, Junio C Hamano wrote:
> >>> Taylor Blau <me@ttaylorr.com> writes:
> >>>
> >>>> Hmm. I'm not crazy about a library function looking at 'GIT_TEST_*'
> >>>> environment variables and potentially ignoring its arguments, but given
> >>>> the discussion we had in v1, I don't feel strongly enough to recommend
> >>>> that you change this.
> >>>>
> >>>> For what it's worth, I think that 'write_commit_graph' could behave more
> >>>> purely if callers checked 'GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS' and set
> >>>> 'flags' appropriately from the outside,...
> >>>
> >>> Yeah, I agree that it would be a lot cleaner if that is easy to
> >>> arrange.  I have a suspicion that Derrick already tried and the
> >>> resulting code looked messier and was discarded?
> >>
> >> Perhaps I could fix both concerns by
> >>
> >> 1. Use a macro instead of a library call.
> >>
> >> 2. Check the _CHANGED_PATHS variable in the macro.
> >
> > I am not sure how use of a macro "fixes" purity, though.  And what
> > is the other concern?
>
> The concern was (1) checking the environment and (2) die()ing in the
> library.

I think that we might as well use a plain old function here instead of a
macro, especially since the macro is just expanding out to what that
function would be if we had written it that way.

For what it's worth, my concern was more that I don't mind dying, but
I'd rather do so in a calling function, and that by the time we're
calling 'write_commit_graph', that we should be using return values
instead of 'die()' or 'error()'.

> > How widely would this "if we are testing, write out the graph file"
> > call be sprinkled over the codebase?  I am hoping that it won't be
> > "everywhere", but only at strategic places (like "just once before
> > we leave a subcommand that creates one or bunch of commits").  And
> > how often would they be called?  I am also hoping that it won't be
> > "inside a tight loop".  In short, I am wondering if we can promise
> > our codebase that
> >
> >  - git_test_write_commit_graph_or_die() calls won't be an eyesore
> >    (and/or distraction) for developers too much.
> >
> >  - git_env_bool() call won't have performance impact.
>
> I could add a comment to the header file to say "this is only for
> improving coverage of optional features in the test suite. Do not
> call this method unless you know what you are doing."

Works for me.

> My intention right now is to only include this in 'git commit'
> and 'git merge'. Earlier discussion included thoughts about
> 'git rebase' and similar, but those are more rarely used when
> constructing an "example repo" in the test scripts.

Ditto. I think that there's benefit in having it in some places but not
all (as Stolee pointed out earlier in the thread), and that trying to
have it everywhere is a fool's errand.

> -Stolee

Thanks,
Taylor