Subject: Re: [PATCH v4 3/4] ident: rename commit_rewrite_person() to apply_mailmap_to_header()

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 13 Jul 2022 03:25:40 +0200

On Tue, Jul 12 2022, Siddharth Asthana wrote:

> commit_rewrite_person() takes a commit buffer and replaces the idents
> in the header with their canonical versions using the mailmap mechanism.
> The name "commit_rewrite_person()" is misleading as it doesn't convey
> what kind of rewrite are we going to do to the buffer. It also doesn't
> clearly mention that the function will limit itself to the header part
> of the buffer. The new name, "apply_mailmap_to_header()", expresses the
> functionality of the function pretty clearly.
>
> We intend to use apply_mailmap_to_header() in git-cat-file to replace
> idents in the headers of commit and tag object buffers. So, we will be
> extending this function to take tag objects buffer as well and replace
> idents on the tagger header using the mailmap mechanism.
>
> Mentored-by: Christian Couder <[email-masked]>
> Mentored-by: John Cai <[email-masked]>
> Signed-off-by: Siddharth Asthana <[email-masked]>
> ---
>  cache.h    | 6 +++---
>  ident.c    | 2 +-
>  revision.c | 2 +-
>  3 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/cache.h b/cache.h
> index c9dbe1c29a..9edb7fefd3 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1689,10 +1689,10 @@ struct ident_split {
>  int split_ident_line(struct ident_split *, const char *, int);
>  
>  /*
> - * Given a commit object buffer and the commit headers, replaces the idents
> - * in the headers with their canonical versions using the mailmap mechanism.
> + * Given a commit or tag object buffer and the commit or tag headers, replaces
> + * the idents in the headers with their canonical versions using the mailmap mechanism.
>   */
> -void commit_rewrite_person(struct strbuf *buf, const char **commit_headers, struct string_list *mailmap);
> +void apply_mailmap_to_header(struct strbuf *buf, const char **headers, struct string_list *mailmap);
>  
>  /*
>   * Compare split idents for equality or strict ordering. Note that we
> diff --git a/ident.c b/ident.c
> index 9f4f6e9071..5f17bd607d 100644
> --- a/ident.c
> +++ b/ident.c
> @@ -393,7 +393,7 @@ static ssize_t rewrite_ident_line(const char* person, struct strbuf *buf, struct
>  	return 0;
>  }
>  
> -void commit_rewrite_person(struct strbuf *buf, const char **headers, struct string_list *mailmap)
> +void apply_mailmap_to_header(struct strbuf *buf, const char **headers, struct string_list *mailmap)
>  {
>  	size_t buf_offset = 0;
>  
> diff --git a/revision.c b/revision.c
> index 14dca903b6..6ad3665204 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -3792,7 +3792,7 @@ static int commit_match(struct commit *commit, struct rev_info *opt)
>  		if (!buf.len)
>  			strbuf_addstr(&buf, message);
>  
> -		commit_rewrite_person(&buf, commit_headers, opt->mailmap);
> +		apply_mailmap_to_header(&buf, commit_headers, opt->mailmap);
>  	}
>  
>  	/* Append "fake" message parts as needed */

I can live with this so far, but I really think this is cementing the
wrong approach into place here.

We only use commit_match() to feed a commit to grep.c, which if you look
at the "header_field" struct there we take this pre-formatted output and
parse this out *again*, i.e. find "author", "reflog", "committer" etc.,
and eventually point the regex engine at that buffer.

So we really don't need to get a strbuf here, and munge the whole thing
in place to feed it to grep.c, instead we can:

 1. Not munge it at all, pass it as-is
 2. Pass the mailmap along to grep.c itself
 3. It's already parsing out the headers, so at some point it will have
    "author foo <bar>\n"
 4. In that code, we can just consult the mailmap, and then map the "foo
   <bar>" bart to "Baz <bar>" or whatever
 5. Thean search that string.

So no need for any in-place rewriting, or no?

Even with this approach this seems a bit odd, e.g. isn't your
commit_rewrite_person() largely a re-invention of find_commit_header()
in commit.c, can't we use that function there?

The replace_idents_using_mailmap() in 4/4 seems like it could be
improved in a similar way.

I.e. can't we just loop over the the object, then as we find "author"
consult the mailmap, and potentially emit a replacement, otherwise the
existing content as-is up until the next \n etc.

We should be able to "stream" all of this, instead of in-place modifying
a potentially large commit buffer, which involves memmove() etc.