Subject: Re: [PATCH v2 1/1] cat-file: add mailmap subcommand to --batch-command

From: Siddharth Asthana <[email-masked]>
Date: Tue, 31 Mar 2026 07:10:35 +0530

On 30/03/26 07:42, Junio C Hamano wrote:
> Siddharth Asthana <[email-masked]> writes:
> 
>> git-cat-file(1)'s --batch-command works with the --use-mailmap option,
>> but this option needs to be set when the process is created. This means
>> we cannot change this option mid-operation.
>>
>> At GitLab, Gitaly caches git-cat-file processes and it would be useful
> 
> Would "keeps interacting with a single 'cat-file' process" be more
> accurate than "caches"?  The latter gives, at least to me,

Thanks Junio

Make sense, I will update that wording in v3!

> connotations that may not be necessarily true, like (1) there is a
> pool of cat-file processes waiting for Gitaly to connect and serve,
> (2) a running Gitaly may decide to disconnect from cat-file from
> time to time, and then reconnect to one of them when it becomes
> necessary again, etc.
> 
>> if --batch-command supported toggling mailmap dynamically with existing
>> processes.
>>
>> Add a `mailmap` subcommand to --batch-command that takes a single
>> argument: `yes` to enable mailmap and `no` to disable it. When enabled,
>> mailmap data is loaded from disk on first use and kept in memory so that
>> toggling back on does not require reloading.
>>
>> Suggested-by: Junio C Hamano <[email-masked]>
> 
> This is over-crediting me.  The idea to unify the two commands into
> one may have come from me, but that is at most helped-by but it is
> perfectly fine without any credit.

Got it, thanks for pointing that out.

> 
>> Signed-off-by: Siddharth Asthana <[email-masked]>
>> ---
> 
>> diff --git a/builtin/cat-file.c b/builtin/cat-file.c
>> index b6f12f41d6..a53926d2bb 100644
>> --- a/builtin/cat-file.c
>> +++ b/builtin/cat-file.c
>> @@ -54,6 +54,7 @@ static const char *force_path;
>>   
>>   static struct string_list mailmap = STRING_LIST_INIT_NODUP;
>>   static int use_mailmap;
>> +static int mailmap_loaded;
> 
> Not part of this topic, but in case less experienced developers who
> are watching from the sidelines wonder if we want to add this
> file-scope global variable, this is perfectly fine.  Anything under
> builtin/foo.c are meant to serve a single command "git foo" and does
> not benefit from "let's sift globals into classes that belong to
> different concepts in the system; most of which will be per
> repository, so make them some part of the repository object"
> movement as much as more library-ish parts of the system.
> 
> Until a specific command starts working on multiple repositories at
> one time, that is.
> 
> 
>> @@ -692,6 +693,24 @@ static void parse_cmd_info(struct batch_options *opt,
>>   	batch_one_object(line, output, opt, data);
>>   }
>>   
>> +static void parse_cmd_mailmap(struct batch_options *opt UNUSED,
>> +			      const char *line,
>> +			      struct strbuf *output UNUSED,
>> +			      struct expand_data *data UNUSED)
>> +{
>> +	if (!strcmp(line, "yes")) {
>> +		if (!mailmap_loaded) {
>> +			read_mailmap(the_repository, &mailmap);
>> +			mailmap_loaded = 1;
>> +		}
>> +		use_mailmap = 1;
>> +	} else if (!strcmp(line, "no")) {
>> +		use_mailmap = 0;
>> +	} else {
>> +		die(_("mailmap: unknown argument '%s', expected 'yes' or 'no'"), line);
>> +	}
>> +}
> 
> OK.
> 
>> @@ -725,9 +744,10 @@ static const struct parse_cmd {
>>   	parse_cmd_fn_t fn;
>>   	unsigned takes_args;
>>   } commands[] = {
>> -	{ "contents", parse_cmd_contents, 1},
>> -	{ "info", parse_cmd_info, 1},
>> -	{ "flush", NULL, 0},
>> +	{ "contents", parse_cmd_contents, 1 },
>> +	{ "info", parse_cmd_info, 1 },
>> +	{ "flush", NULL, 0 },
>> +	{ "mailmap", parse_cmd_mailmap, 1 },
>>   };
> 
> Mixing style fixes to existing entries in the same patch that adds a
> new feature by adding a new entry to the table is annoying than
> having a preliminary clean-up patch that only fixes style and then
> the main patch that adds the feature.

Agreed. I will avoid mixing style-only cleanup with the feature change 
in v3.

> 
>>   static void batch_objects_command(struct batch_options *opt,
>> @@ -1127,8 +1147,10 @@ int cmd_cat_file(int argc,
>>   	opt_cw = (opt == 'c' || opt == 'w');
>>   	opt_epts = (opt == 'e' || opt == 'p' || opt == 't' || opt == 's');
>>   
>> -	if (use_mailmap)
>> +	if (use_mailmap) {
>>   		read_mailmap(the_repository, &mailmap);
>> +		mailmap_loaded = 1;
>> +	}
> 
> Hmph, interesting.  Two points.
> 
>   * It would make it easier to follow if these two lines are made
>     into a small helper function to be called from here and from the
>     "parse_cmd_mailmap()"?
> 
>   * Can we somehow make mailmap object itself slightly smarter so
>     that it knows if it has already been asked to read the data from
>     its sources?  It is a pretty dumb string_list, but from a cursory
>     read of the code flow, it seems that mailmap.strdup_strings is
>     initialized to be false in builtin/cat-file.c and then one of the
>     first things done in mailmap.c::read_mailmap() is to flip that
>     bit on, so the "yes" part of the parse_cmd_mailmap() might become
> 
> 	if (yes)
> 		load_mailmap();
> 
>     with the helper load_mailmap() that may look like:
> 
> 	static void load_mailmap(void)
> 	{
> 		if (mailmap.strdup_strings)
> 			return; /* we know read_mailmap() flips it on */
> 		read_mailmap(the_repository, &mailmap);
> 	}
> 
>     The first bullet point to introduce a small common helper will
>     help hiding such an ugly implementation detail there.

Thanks, this is helpful.

I will introduce a small helper and call it from both places so the flow 
is easier to follow. I will also avoid publicating the load check in 
each callsite