builtin/remote: only iterate through refs that are to be renamed

When renaming a remote we also need to rename all references
accordingly. But while we only need to rename references that are
contained in the "refs/remotes/$OLDNAME/" namespace, we end up using
`refs_for_each_rawref()` that iterates through _all_ references. We know
to exit early in the callback in case we see an irrelevant reference,
but ultimately this is still a waste of compute as we knowingly iterate
through references that we won't ever care about.

Improve this by using `refs_for_each_rawref_in()`, which knows to only
iterate through (potentially broken) references in a given prefix.

The following benchmark renames a remote with a single reference in a
repository that has 100k unrelated references. This shows a sizeable
improvement with the "files" backend:

    Benchmark 1: rename remote (refformat = files, revision = HEAD~)
      Time (mean ± σ):      42.6 ms ±   0.9 ms    [User: 29.1 ms, System: 8.4 ms]
      Range (min … max):    40.1 ms …  43.3 ms    10 runs

    Benchmark 2: rename remote (refformat = files, revision = HEAD)
      Time (mean ± σ):      31.7 ms ±   4.0 ms    [User: 19.6 ms, System: 6.9 ms]
      Range (min … max):    27.1 ms …  36.0 ms    10 runs

    Summary
      rename remote (refformat = files, revision = HEAD) ran
        1.35 ± 0.17 times faster than rename remote (refformat = files, revision = HEAD~)

The "reftable" backend shows roughly the same absolute improvement, but
given that it's already significantly faster than the "files" backend
this translates to a much larger relative improvement:

    Benchmark 1: rename remote (refformat = reftable, revision = HEAD~)
      Time (mean ± σ):      18.2 ms ±   0.5 ms    [User: 12.7 ms, System: 3.0 ms]
      Range (min … max):    17.3 ms …  21.4 ms    110 runs

    Benchmark 2: rename remote (refformat = reftable, revision = HEAD)
      Time (mean ± σ):       8.8 ms ±   0.5 ms    [User: 3.8 ms, System: 2.9 ms]
      Range (min … max):     7.5 ms …   9.9 ms    167 runs

    Summary
      rename remote (refformat = reftable, revision = HEAD) ran
        2.07 ± 0.12 times faster than rename remote (refformat = reftable, revision = HEAD~)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/remote.c

@@ -720,16 +720,8 @@ static int rename_one_ref(const char *old_refname, const char *referent,
 	struct strbuf new_referent = STRBUF_INIT;
 	struct strbuf new_refname = STRBUF_INIT;
 	struct rename_info *rename = cb_data;
-	const char *ptr = old_refname;
 	int error;
 
-	if (!skip_prefix(ptr, "refs/remotes/", &ptr) ||
-	    !skip_prefix(ptr, rename->old_name, &ptr) ||
-	    !skip_prefix(ptr, "/", &ptr)) {
-		error = 0;
-		goto out;
-	}
-
 	compute_renamed_ref(rename, old_refname, &new_refname);
 
 	if (flags & REF_ISSYMREF) {
@@ -932,7 +924,10 @@ static int mv(int argc, const char **argv, const char *prefix,
 			rename.progress = start_delayed_progress(the_repository,
 								 _("Renaming remote references"), 0);
 
-		result = refs_for_each_rawref(get_main_ref_store(the_repository),
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "refs/remotes/%s/", rename.old_name);
+
+		result = refs_for_each_rawref_in(get_main_ref_store(the_repository), buf.buf,
 				rename_one_ref, &rename);
 		if (result < 0)
 			die(_("queueing remote ref renames failed: %s"), rename.err->buf);

refs.c

@@ -1839,7 +1839,13 @@ int refs_for_each_namespaced_ref(struct ref_store *refs,
 
 int refs_for_each_rawref(struct ref_store *refs, each_ref_fn fn, void *cb_data)
 {
-	return do_for_each_ref(refs, "", NULL, fn, 0,
+	return refs_for_each_rawref_in(refs, "", fn, cb_data);
+}
+
+int refs_for_each_rawref_in(struct ref_store *refs, const char *prefix,
+			    each_ref_fn fn, void *cb_data)
+{
+	return do_for_each_ref(refs, prefix, NULL, fn, 0,
 			       DO_FOR_EACH_INCLUDE_BROKEN, cb_data);
 }
 

refs.h

@@ -428,6 +428,8 @@ int refs_for_each_namespaced_ref(struct ref_store *refs,
 
 /* can be used to learn about broken ref and symref */
 int refs_for_each_rawref(struct ref_store *refs, each_ref_fn fn, void *cb_data);
+int refs_for_each_rawref_in(struct ref_store *refs, const char *prefix,
+			    each_ref_fn fn, void *cb_data);
 
 /*
  * Iterates over all refs including root refs, i.e. pseudorefs and HEAD.