Subject: Re: [PATCH v2 1/8] for-each-ref: add --stdin option

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 15 Mar 2023 14:37:39 +0100

On Fri, Mar 10 2023, Derrick Stolee via GitGitGadget wrote:

> From: Derrick Stolee <[email-masked]>
>
> When a user wishes to input a large list of patterns to 'git
> for-each-ref' (likely a long list of exact refs) there are frequently
> system limits on the number of command-line arguments.

Okey, and the current API assumes you just assign "argv" to this, but...

> When reading from stdin, we populate the filter.name_patterns array
> dynamically as opposed to pointing to the 'argv' array directly. This
> requires a careful cast while freeing the individual strings,
> conditioned on the --stdin option.

..sounds potentially nasty...

> @@ -75,7 +77,27 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
>  	ref_sorting_set_sort_flags_all(sorting, REF_SORTING_ICASE, icase);
>  	filter.ignore_case = icase;
>  
> -	filter.name_patterns = argv;
> +	if (from_stdin) {
> +		struct strbuf line = STRBUF_INIT;
> +		size_t nr = 0, alloc = 16;
> +
> +		if (argv[0])
> +			die(_("unknown arguments supplied with --stdin"));
> +
> +		CALLOC_ARRAY(filter.name_patterns, alloc);
> +
> +		while (strbuf_getline(&line, stdin) != EOF) {
> +			ALLOC_GROW(filter.name_patterns, nr + 1, alloc);
> +			filter.name_patterns[nr++] = strbuf_detach(&line, NULL);
> +		}
> +
> +		/* Add a terminating NULL string. */
> +		ALLOC_GROW(filter.name_patterns, nr + 1, alloc);
> +		filter.name_patterns[nr + 1] = NULL;
> +	} else {
> +		filter.name_patterns = argv;
> +	}
> +
>  	filter.match_as_path = 1;
>  	filter_refs(&array, &filter, FILTER_REFS_ALL);
>  	ref_array_sort(sorting, &array);
> @@ -97,5 +119,10 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
>  	free_commit_list(filter.with_commit);
>  	free_commit_list(filter.no_commit);
>  	ref_sorting_release(sorting);
> +	if (from_stdin) {
> +		for (size_t i = 0; filter.name_patterns[i]; i++)
> +			free((char *)filter.name_patterns[i]);
> +		free(filter.name_patterns);
> +	}
>  	return 0;
>  }

Why do we need to seemingly re-invent a "struct strvec" here? I tried to
simplify this on top of this (well, "seen"), and we can get rid of all
of this manual memory management & trailing NULL juggling as a result:
	
	diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
	index cf5ba6ffc12..13b75eff28c 100644
	--- a/builtin/for-each-ref.c
	+++ b/builtin/for-each-ref.c
	@@ -7,6 +7,7 @@
	 #include "parse-options.h"
	 #include "ref-filter.h"
	 #include "commit-reach.h"
	+#include "strvec.h"
	 
	 static char const * const for_each_ref_usage[] = {
	 	N_("git for-each-ref [<options>] [<pattern>]"),
	@@ -27,6 +28,7 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
	 	struct ref_format format = REF_FORMAT_INIT;
	 	struct strbuf output = STRBUF_INIT;
	 	struct strbuf err = STRBUF_INIT;
	+	struct strvec stdin_pat = STRVEC_INIT;
	 	int from_stdin = 0;
	 
	 	struct option opts[] = {
	@@ -81,21 +83,13 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
	 
	 	if (from_stdin) {
	 		struct strbuf line = STRBUF_INIT;
	-		size_t nr = 0, alloc = 16;
	 
	 		if (argv[0])
	 			die(_("unknown arguments supplied with --stdin"));
	 
	-		CALLOC_ARRAY(filter.name_patterns, alloc);
	-
	-		while (strbuf_getline(&line, stdin) != EOF) {
	-			ALLOC_GROW(filter.name_patterns, nr + 1, alloc);
	-			filter.name_patterns[nr++] = strbuf_detach(&line, NULL);
	-		}
	-
	-		/* Add a terminating NULL string. */
	-		ALLOC_GROW(filter.name_patterns, nr + 1, alloc);
	-		filter.name_patterns[nr + 1] = NULL;
	+		while (strbuf_getline(&line, stdin) != EOF)
	+			strvec_push(&stdin_pat, line.buf);
	+		filter.name_patterns = stdin_pat.v;
	 	} else {
	 		filter.name_patterns = argv;
	 	}
	@@ -123,10 +117,6 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
	 	free_commit_list(filter.with_commit);
	 	free_commit_list(filter.no_commit);
	 	ref_sorting_release(sorting);
	-	if (from_stdin) {
	-		for (size_t i = 0; filter.name_patterns[i]; i++)
	-			free(filter.name_patterns[i]);
	-		free(filter.name_patterns);
	-	}
	+	strvec_clear(&stdin_pat);
	 	return 0;
	 }

It *is* an extra copy though, as your implementation re-uses the strbuf
we already allocated.

But presumably that's trivial in this case, and if we care I think we
should resurrect something like [1] instead, i.e. we could just teach
the strvec API to have a strvec_push_nodup(). But I doubt that in this
case it'll matter.

In any case, if you don't want to take this as-is, please fix this so
that we're not reaching into the "filter.name_patterns" and casting its
"const char" to "char".

If we're going to add a hack here that API should instead know how to
free its own resources (so we could clean up the free_commit_list() here
seen in the context), and we could carry some "my argv needs free-ing".

But none of that seems needed in this case, this is just another case
where we can pretend that we have a "normal" argv, and then clean up our
own strvec, no?

1. https://lore.kernel.org/git/65a6[email-masked]/