git: remote, Do not iterate all references on update.

The current code iterates all the references in the remote to check if
they match the refspec. This is OK when the refspec is a wildcard but
is a waste of time when they are not.

A hash with references is generated for fast access before starting the
update and used only when the refspec is not a wildcard.

In a repository with 7800 references this meant 7800 * 7800 checks. With
the current code it took 8m30s to update the references. With the new
code it takes less than 0.5s.

References are already extensively tested in remote_test.go.

Signed-off-by: Javi Fontan <jfontan@gmail.com>

Author: jfontan

remote.go

@@ -371,14 +371,22 @@ func (r *Remote) addReferencesToUpdate(
 	refspecs []config.RefSpec,
 	localRefs []*plumbing.Reference,
 	remoteRefs storer.ReferenceStorer,
-	req *packp.ReferenceUpdateRequest) error {
+	req *packp.ReferenceUpdateRequest,
+) error {
+	// This references dictionary will be used to search references by name.
+	refsDict := make(map[string]*plumbing.Reference)
+	for _, ref := range localRefs {
+		refsDict[ref.Name().String()] = ref
+	}
+
 	for _, rs := range refspecs {
 		if rs.IsDelete() {
 			if err := r.deleteReferences(rs, remoteRefs, req); err != nil {
 				return err
 			}
 		} else {
-			if err := r.addOrUpdateReferences(rs, localRefs, remoteRefs, req); err != nil {
+			err := r.addOrUpdateReferences(rs, localRefs, refsDict, remoteRefs, req)
+			if err != nil {
 				return err
 			}
 		}
@@ -390,9 +398,21 @@ func (r *Remote) addReferencesToUpdate(
 func (r *Remote) addOrUpdateReferences(
 	rs config.RefSpec,
 	localRefs []*plumbing.Reference,
+	refsDict map[string]*plumbing.Reference,
 	remoteRefs storer.ReferenceStorer,
 	req *packp.ReferenceUpdateRequest,
 ) error {
+	// If it is not a wilcard refspec we can directly search for the reference
+	// in the references dictionary.
+	if !rs.IsWildcard() {
+		ref, ok := refsDict[rs.Src()]
+		if !ok {
+			return nil
+		}
+
+		return r.addReferenceIfRefSpecMatches(rs, remoteRefs, ref, req)
+	}
+
 	for _, ref := range localRefs {
 		err := r.addReferenceIfRefSpecMatches(rs, remoteRefs, ref, req)
 		if err != nil {