builtin/for-each-ref: factor out core logic into a helper

The implementation of `git for-each-ref` is monolithic within
`cmd_for_each_ref()`, making it impossible to share its logic with other
commands. To enable code reuse for the upcoming `git refs list`
subcommand, refactor the core logic into a shared helper function.

Introduce a new `for-each-ref.h` header to define the public interface
for this shared logic. It contains the declaration for a new helper
function, `for_each_ref_core()`, and a macro for the common usage
options.

Move the option parsing, filtering, and formatting logic from
`cmd_for_each_ref()` into a new helper function named
`for_each_ref_core()`. This helper is made generic by accepting the
command's usage string as a parameter.

The original `cmd_for_each_ref()` is simplified to a thin wrapper that
is only responsible for defining its specific usage array and calling
the shared helper.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/for-each-ref.c

@@ -2,6 +2,7 @@
 #include "commit.h"
 #include "config.h"
 #include "environment.h"
+#include "for-each-ref.h"
 #include "gettext.h"
 #include "object.h"
 #include "parse-options.h"
@@ -9,24 +10,7 @@
 #include "strbuf.h"
 #include "strvec.h"
 
-#define COMMON_USAGE_FOR_EACH_REF \
+int for_each_ref_core(int argc, const char **argv, const char *prefix, struct repository *repo, const char *const *usage)
-	"[--count=<count>] [--shell|--perl|--python|--tcl]\n" \
-	"                         [(--sort=<key>)...] [--format=<format>]\n" \
-	"                         [--include-root-refs] [--points-at=<object>]\n" \
-	"                         [--merged[=<object>]] [--no-merged[=<object>]]\n" \
-	"                         [--contains[=<object>]] [--no-contains[=<object>]]\n" \
-	"                         [(--exclude=<pattern>)...] [--start-after=<marker>]\n" \
-	"                         [ --stdin | <pattern>... ]"
-
-static char const * const for_each_ref_usage[] = {
-	"git for-each-ref " COMMON_USAGE_FOR_EACH_REF,
-	NULL
-};
-
-int cmd_for_each_ref(int argc,
-		     const char **argv,
-		     const char *prefix,
-		     struct repository *repo)
 {
 	struct ref_sorting *sorting;
 	struct string_list sorting_options = STRING_LIST_INIT_DUP;
@@ -75,17 +59,17 @@ int cmd_for_each_ref(int argc,
 	/* Set default (refname) sorting */
 	string_list_append(&sorting_options, "refname");
 
-	parse_options(argc, argv, prefix, opts, for_each_ref_usage, 0);
+	parse_options(argc, argv, prefix, opts, usage, 0);
 	if (format.array_opts.max_count < 0) {
 		error("invalid --count argument: `%d'", format.array_opts.max_count);
-		usage_with_options(for_each_ref_usage, opts);
+		usage_with_options(usage, opts);
 	}
 	if (HAS_MULTI_BITS(format.quote_style)) {
 		error("more than one quoting style?");
-		usage_with_options(for_each_ref_usage, opts);
+		usage_with_options(usage, opts);
 	}
 	if (verify_ref_format(&format))
-		usage_with_options(for_each_ref_usage, opts);
+		usage_with_options(usage, opts);
 
 	if (filter.start_after && sorting_options.nr > 1)
 		die(_("cannot use --start-after with custom sort options"));
@@ -125,3 +109,16 @@ int cmd_for_each_ref(int argc,
 	strvec_clear(&vec);
 	return 0;
 }
+
+int cmd_for_each_ref(int argc,
+		     const char **argv,
+		     const char *prefix,
+		     struct repository *repo)
+{
+	static char const * const for_each_ref_usage[] = {
+		N_("git for-each-ref " COMMON_USAGE_FOR_EACH_REF),
+		NULL
+	};
+
+	return for_each_ref_core(argc, argv, prefix, repo, for_each_ref_usage);
+}

for-each-ref.h

@@ -0,0 +1,26 @@
+#ifndef FOR_EACH_REF_H
+#define FOR_EACH_REF_H
+
+struct repository;
+
+/*
+ * Shared usage string for options common to git-for-each-ref(1)
+ * and git-refs-list(1). The command-specific part (e.g., "git refs list ")
+ * must be prepended by the caller.
+ */
+#define COMMON_USAGE_FOR_EACH_REF \
+	"[--count=<count>] [--shell|--perl|--python|--tcl]\n" \
+	"                         [(--sort=<key>)...] [--format=<format>]\n" \
+	"                         [--include-root-refs] [--points-at=<object>]\n" \
+	"                         [--merged[=<object>]] [--no-merged[=<object>]]\n" \
+	"                         [--contains[=<object>]] [--no-contains[=<object>]]\n" \
+	"                         [(--exclude=<pattern>)...] [--start-after=<marker>]\n" \
+	"                         [ --stdin | <pattern>... ]"
+
+/*
+ * The core logic for for-each-ref and its clones.
+ */
+int for_each_ref_core(int argc, const char **argv, const char *prefix,
+		      struct repository *repo, const char *const *usage);
+
+#endif /* FOR_EACH_REF_H */