remote prune: optimize "dangling symref" check/warning

When 'git remote prune' was used to delete many refs in a repository
with many refs, a lot of time was spent checking for (now) dangling
symbolic refs pointing to the deleted ref, since warn_dangling_symref()
was once per deleted ref to check all other refs in the repository.

Avoid this using the new warn_dangling_symrefs() function which
makes one pass over all refs and checks for all the deleted refs in
one go, after they have all been deleted.

Signed-off-by: Jens Lindström <jl@opera.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/remote.c

@@ -1313,6 +1313,7 @@ static int prune_remote(const char *remote, int dry_run)
 {
 	int result = 0, i;
 	struct ref_states states;
+	struct string_list delete_refs_list = STRING_LIST_INIT_NODUP;
 	const char **delete_refs;
 	const char *dangling_msg = dry_run
 		? _(" %s will become dangling!")
@@ -1339,6 +1340,8 @@ static int prune_remote(const char *remote, int dry_run)
 	for (i = 0; i < states.stale.nr; i++) {
 		const char *refname = states.stale.items[i].util;
 
+		string_list_insert(&delete_refs_list, refname);
+
 		if (!dry_run)
 			result |= delete_ref(refname, NULL, 0);
 
@@ -1348,9 +1351,11 @@ static int prune_remote(const char *remote, int dry_run)
 		else
 			printf_ln(_(" * [pruned] %s"),
 			       abbrev_ref(refname, "refs/remotes/"));
-		warn_dangling_symref(stdout, dangling_msg, refname);
 	}
 
+	warn_dangling_symrefs(stdout, dangling_msg, &delete_refs_list);
+	string_list_clear(&delete_refs_list, 0);
+
 	free_remote_ref_states(&states);
 	return result;
 }

refs.c

@@ -1611,6 +1611,7 @@ int peel_ref(const char *refname, unsigned char *sha1)
 struct warn_if_dangling_data {
 	FILE *fp;
 	const char *refname;
+	const struct string_list *refnames;
 	const char *msg_fmt;
 };
 
@@ -1625,8 +1626,12 @@ static int warn_if_dangling_symref(const char *refname, const unsigned char *sha
 		return 0;
 
 	resolves_to = resolve_ref_unsafe(refname, junk, 0, NULL);
-	if (!resolves_to || strcmp(resolves_to, d->refname))
+	if (!resolves_to
+	    || (d->refname
+		? strcmp(resolves_to, d->refname)
+		: !string_list_has_string(d->refnames, resolves_to))) {
 		return 0;
+	}
 
 	fprintf(d->fp, d->msg_fmt, refname);
 	fputc('\n', d->fp);
@@ -1639,6 +1644,18 @@ void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname)
 
 	data.fp = fp;
 	data.refname = refname;
+	data.refnames = NULL;
+	data.msg_fmt = msg_fmt;
+	for_each_rawref(warn_if_dangling_symref, &data);
+}
+
+void warn_dangling_symrefs(FILE *fp, const char *msg_fmt, const struct string_list *refnames)
+{
+	struct warn_if_dangling_data data;
+
+	data.fp = fp;
+	data.refname = NULL;
+	data.refnames = refnames;
 	data.msg_fmt = msg_fmt;
 	for_each_rawref(warn_if_dangling_symref, &data);
 }

refs.h

@@ -89,6 +89,7 @@ static inline const char *has_glob_specials(const char *pattern)
 extern int for_each_rawref(each_ref_fn, void *);
 
 extern void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname);
+extern void warn_dangling_symrefs(FILE *fp, const char *msg_fmt, const struct string_list* refnames);
 
 /*
  * Lock the packed-refs file for writing.  Flags is passed to