submodule: rewrite module_list shell function in C

Most of the submodule operations work on a set of submodules.
Calculating and using this set is usually done via:

       module_list "$@" | {
           while read mode sha1 stage sm_path
           do
                # the actual operation
           done
       }

Currently the function `module_list` is implemented in the
git-submodule.sh as a shell script wrapping a perl script.
The rewrite is in C, such that it is faster and can later be
easily adapted when other functions are rewritten in C.

git-submodule.sh, similar to the builtin commands, will navigate
to the top-most directory of the repository and keep the
subdirectory as a variable. As the helper is called from
within the git-submodule.sh script, we are already navigated
to the root level, but the path arguments are still relative
to the subdirectory we were in when calling git-submodule.sh.
That's why there is a `--prefix` option pointing to an alternative
path which to anchor relative path arguments.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -0,0 +1,128 @@
+#include "builtin.h"
+#include "cache.h"
+#include "parse-options.h"
+#include "quote.h"
+#include "pathspec.h"
+#include "dir.h"
+#include "utf8.h"
+
+struct module_list {
+	const struct cache_entry **entries;
+	int alloc, nr;
+};
+#define MODULE_LIST_INIT { NULL, 0, 0 }
+
+static int module_list_compute(int argc, const char **argv,
+			       const char *prefix,
+			       struct pathspec *pathspec,
+			       struct module_list *list)
+{
+	int i, result = 0;
+	char *max_prefix, *ps_matched = NULL;
+	int max_prefix_len;
+	parse_pathspec(pathspec, 0,
+		       PATHSPEC_PREFER_FULL |
+		       PATHSPEC_STRIP_SUBMODULE_SLASH_CHEAP,
+		       prefix, argv);
+
+	/* Find common prefix for all pathspec's */
+	max_prefix = common_prefix(pathspec);
+	max_prefix_len = max_prefix ? strlen(max_prefix) : 0;
+
+	if (pathspec->nr)
+		ps_matched = xcalloc(pathspec->nr, 1);
+
+	if (read_cache() < 0)
+		die(_("index file corrupt"));
+
+	for (i = 0; i < active_nr; i++) {
+		const struct cache_entry *ce = active_cache[i];
+
+		if (!S_ISGITLINK(ce->ce_mode) ||
+		    !match_pathspec(pathspec, ce->name, ce_namelen(ce),
+				    max_prefix_len, ps_matched, 1))
+			continue;
+
+		ALLOC_GROW(list->entries, list->nr + 1, list->alloc);
+		list->entries[list->nr++] = ce;
+		while (i + 1 < active_nr &&
+		       !strcmp(ce->name, active_cache[i + 1]->name))
+			/*
+			 * Skip entries with the same name in different stages
+			 * to make sure an entry is returned only once.
+			 */
+			i++;
+	}
+	free(max_prefix);
+
+	if (ps_matched && report_path_error(ps_matched, pathspec, prefix))
+		result = -1;
+
+	free(ps_matched);
+
+	return result;
+}
+
+static int module_list(int argc, const char **argv, const char *prefix)
+{
+	int i;
+	struct pathspec pathspec;
+	struct module_list list = MODULE_LIST_INIT;
+
+	struct option module_list_options[] = {
+		OPT_STRING(0, "prefix", &prefix,
+			   N_("path"),
+			   N_("alternative anchor for relative paths")),
+		OPT_END()
+	};
+
+	const char *const git_submodule_helper_usage[] = {
+		N_("git submodule--helper list [--prefix=<path>] [<path>...]"),
+		NULL
+	};
+
+	argc = parse_options(argc, argv, prefix, module_list_options,
+			     git_submodule_helper_usage, 0);
+
+	if (module_list_compute(argc, argv, prefix, &pathspec, &list) < 0) {
+		printf("#unmatched\n");
+		return 1;
+	}
+
+	for (i = 0; i < list.nr; i++) {
+		const struct cache_entry *ce = list.entries[i];
+
+		if (ce_stage(ce))
+			printf("%06o %s U\t", ce->ce_mode, sha1_to_hex(null_sha1));
+		else
+			printf("%06o %s %d\t", ce->ce_mode, sha1_to_hex(ce->sha1), ce_stage(ce));
+
+		utf8_fprintf(stdout, "%s\n", ce->name);
+	}
+	return 0;
+}
+
+
+struct cmd_struct {
+	const char *cmd;
+	int (*fn)(int, const char **, const char *);
+};
+
+static struct cmd_struct commands[] = {
+	{"list", module_list},
+};
+
+int cmd_submodule__helper(int argc, const char **argv, const char *prefix)
+{
+	int i;
+	if (argc < 2)
+		die(_("fatal: submodule--helper subcommand must be "
+		      "called with a subcommand"));
+
+	for (i = 0; i < ARRAY_SIZE(commands); i++)
+		if (!strcmp(argv[1], commands[i].cmd))
+			return commands[i].fn(argc - 1, argv + 1, prefix);
+
+	die(_("fatal: '%s' is not a valid submodule--helper "
+	      "subcommand"), argv[1]);
+}