Subject: Re: [PATCH 0/5] run-command API: get rid of "argv"

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 22 Nov 2021 19:26:26 +0100

On Mon, Nov 22 2021, Jeff King wrote:

> On Mon, Nov 22, 2021 at 05:04:02PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
>> This series is an alternate but more thorough way to solve the pager
>> segfault reported by Enzo Matsumiya[1], and more generally avoids
>> similar issues in the future.
>> 
>> That the run-command API exposed two subtly different ways of doing
>> the same thing wouldn't only lead to the sort of bug reported in [1],
>> but also made memory management around it rather painful. As noted by
>> Jeff King in[2]:
>> 
>>     I'd like to eventually get rid of the argv interface entirely
>>     because it has memory-ownership semantics that are easy to get
>>     wrong.
>
> Yeah, unsurprisingly I'm in favor of this direction (and in fact started
> looking at myself before seeing your responses). It's big and complex
> enough that I do worry about prepending it in front of the segfault bug
> fix being discussed.
>
>> As noted in 5/5 we've still got a similar issue with "env" and
>> "env_array". I've got a follow-up series that similarly removes "env"
>> which we can do at some point (it's much smaller than this one), but
>> for now let's focus on "argv".
>
> I think we should probably do both, though I am OK with doing it
> separately. There are fewer callers for "env", but I found more
> ancillary cleanup necessary (e.g., "const char **" versus "const char
> *const *" headaches).
>
>> Ævar Arnfjörð Bjarmason (5):
>>   archive-tar: use our own cmd.buf in error message
>>   upload-archive: use regular "struct child_process" pattern
>>   run-command API users: use strvec_pushv(), not argv assignment
>>   run-command API users: use strvec_pushl(), not argv construction
>>   run-command API: remove "argv" member, always use "args"
>
> I left a few comments on individual patches. I had done a rough cut at
> this, too. One big difference is that I used the opportunity to clean up
> some ugly and error-prone uses of argv that are now unnecessary. For
> instance:
>
> diff --git a/builtin/notes.c b/builtin/notes.c
> index 2b2bac43f3..85d1abad88 100644
> --- a/builtin/notes.c
> +++ b/builtin/notes.c
> @@ -134,14 +134,13 @@ static void copy_obj_to_fd(int fd, const struct object_id *oid)
>  
>  static void write_commented_object(int fd, const struct object_id *object)
>  {
> -	const char *show_args[5] =
> -		{"show", "--stat", "--no-notes", oid_to_hex(object), NULL};
>  	struct child_process show = CHILD_PROCESS_INIT;
>  	struct strbuf buf = STRBUF_INIT;
>  	struct strbuf cbuf = STRBUF_INIT;
>  
>  	/* Invoke "git show --stat --no-notes $object" */
> -	strvec_pushv(&show.args, show_args);
> +	strvec_pushl(&show.args, "show", "--stat", "--no-notes",
> +		     oid_to_hex(object), NULL);
>  	show.no_stdin = 1;
>  	show.out = -1;
>  	show.err = 0;
>
> The show_args variable is error-prone in two ways:
>
>   - the magic number "5" must be in sync with the rest of the array. In
>     this case it's superfluous and could just be removed, but I'll give
>     a related example below.
>
>   - we have to remember to include the trailing NULL. We have to for
>     pushl(), too, but in that case the compiler will warn us when we
>     omit it.
>
> Here's another one:
>
> @@ -943,23 +941,22 @@ static int run_receive_hook(struct command *commands,
>  
>  static int run_update_hook(struct command *cmd)
>  {
> -	const char *argv[5];
> +	const char *hook_cmd;
>  	struct child_process proc = CHILD_PROCESS_INIT;
>  	int code;
>  
> -	argv[0] = find_hook("update");
> -	if (!argv[0])
> +	hook_cmd = find_hook("update");
> +	if (!hook_cmd)
>  		return 0;
>  
> -	argv[1] = cmd->ref_name;
> -	argv[2] = oid_to_hex(&cmd->old_oid);
> -	argv[3] = oid_to_hex(&cmd->new_oid);
> -	argv[4] = NULL;
> +	strvec_push(&proc.args, hook_cmd);
> +	strvec_push(&proc.args, cmd->ref_name);
> +	strvec_push(&proc.args, oid_to_hex(&cmd->old_oid));
> +	strvec_push(&proc.args, oid_to_hex(&cmd->new_oid));
>  
>  	proc.no_stdin = 1;
>  	proc.stdout_to_stderr = 1;
>  	proc.err = use_sideband ? -1 : 0;
> -	strvec_pushv(&proc.args, argv);
>  	proc.trace2_hook_name = "update";
>
> In this case the magic "5" really is important, and we get rid of it
> (and again don't need to worry about the terminating NULL).
>
> I'm on the fence on how important it is to do these cleanups. IMHO they
> are half of what really sells the change in the first place (since the
> other bug can pretty easily be fixed without it).
>
> But maybe it is piling too much onto what is already a pretty big
> change. The cleanups could be done individually later.

Yeah, those are nice. I did do most/all those initially myself, but
ended up ejecting them in anticipation of getting comments about runaway
refactoring, as they're not strictly necessary. But I can include them
again if you/Junio would like...

> diff --git a/daemon.c b/daemon.c
> index cc278077d2..4a000ee4af 100644
> --- a/daemon.c
> +++ b/daemon.c
> @@ -329,10 +329,15 @@ static int run_access_hook(struct daemon_service *service, const char *dir,
>  	char *eol;
>  	int seen_errors = 0;
>  
> +	strvec_push(&child.args, access_hook);
> +	strvec_push(&child.args, service->name);
> +	strvec_push(&child.args, path);
> +	strvec_push(&child.args, hi->hostname.buf);
> +	strvec_push(&child.args, get_canon_hostname(hi));
> +	strvec_push(&child.args, get_ip_address(hi));
> +	strvec_push(&child.args, hi->tcp_port.buf);
> +
>  	child.use_shell = 1;
> -	strvec_pushl(&child.args, access_hook, service->name, path,
> -		     hi->hostname.buf, get_canon_hostname(hi),
> -		     get_ip_address(hi), hi->tcp_port.buf, NULL);
>  	child.no_stdin = 1;
>  	child.no_stderr = 1;
>  	child.out = -1;
>
> I had other changes from yours like this. This is purely cosmetic, and I
> could see arguments either way. I find the one-per-line version a bit
> easier to read. Even though it repeats child.args over and over, it's
> easy to look past since it's all aligned.
>
> I'm OK calling that bike-shedding, but I offer it mostly in case you
> didn't try it the other way and actually like my color. ;)

I do like it better :) It's another thing I did like that initiall, but
ended up moving to strvec_pushl(). IIRC because I got the opposite
request on a recent bundle.c topic of mine (now landed). I.e. it used
multiple aligned strvec_push() initailly, and it was suggested to use
strvec_pushl() instead...