Subject: Re: [PATCH] builtin/commit.c: memoize git-path for COMMIT_EDITMSG

From: Junio C Hamano <[email-masked]>
Date: Mon, 23 May 2016 12:16:43 -0700

Pranit Bauva <[email-masked]> writes:

> This is a follow up commit for f932729c (memoize common git-path
> "constant" files, 10-Aug-2015).
>
> It serves two purposes:
>   1. It reduces the number of calls to git_path() .
>
>   2. It serves the benefits of using GIT_PATH_FUNC as mentioned in the
>      commit message of f932729c.

All of that is a good idea, but I have huge doubts about its use.

> diff --git a/builtin/commit.c b/builtin/commit.c
> index 391126e..ffa242c 100644
> --- a/builtin/commit.c
> +++ b/builtin/commit.c
> @@ -92,8 +92,10 @@ N_("If you wish to skip this commit, use:\n"
>  "Then \"git cherry-pick --continue\" will resume cherry-picking\n"
>  "the remaining commits.\n");
>  
> +static GIT_PATH_FUNC(git_path_commit_editmsg, "COMMIT_EDITMSG")
> +
>  static const char *use_message_buffer;
> -static const char commit_editmsg[] = "COMMIT_EDITMSG";
> +static const char commit_editmsg_path[] = git_path_commit_editmsg();

The function defined with the macro looks like

	const char *git_path_commit_editmsg(void)
        {
		static char *ret;
                if (!ret)
                	ret = git_pathdup("COMMIT_EDITMSG");
		return ret;
	}

so receiving its result to "const char v[]" looks somewhat
suspicious.

More importantly, when is this function evaluated and returned value
used to fill commit_editmsg_path[]?  In order for git_pathdup() to
produce a meaningful result, it needs to know where .git/ directory
is, which (roughly) means setup_git_dir() must have been called from
a callchain from main() somewhere already.

But I do not think the linker knows that fact.

> @@ -771,9 +773,9 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
>  		hook_arg2 = "";
>  	}
>  

Instead, what you could do is to call git_path_commit_editmsg() when
you refer to that global variable whose initialization is suspect.

> -	s->fp = fopen_for_writing(git_path(commit_editmsg));
> +	s->fp = fopen_for_writing(commit_editmsg_path);

i.e.

	s->fp = fopen_for_writing(git_path_commit_editmsg());

As you can see in its definition, when the original code used to
call git_path(), it is safe to call git_path_commit_editmsg(),
because for the original git_path() to be correct, the code should
already have established where $GIT_DIR is, so it is safe to call
git_pathdup(), too.  Also, as you can see in its definition, calling
the function many times would not cause git_path() called many
times.  The first invocation will keep its value that is constant
within the program that works with a constant $GIT_DIR.

And you do not free its return value.