fetch: allow passing a transaction to s_update_ref()

The handling of ref updates is completely handled by `s_update_ref()`,
which will manage the complete lifecycle of the reference transaction.
This is fine right now given that git-fetch(1) does not support atomic
fetches, so each reference gets its own transaction. It is quite
inflexible though, as `s_update_ref()` only knows about a single
reference update at a time, so it doesn't allow us to alter the
strategy.

This commit prepares `s_update_ref()` and its only caller
`update_local_ref()` to allow passing an external transaction. If none
is given, then the existing behaviour is triggered which creates a new
transaction and directly commits it. Otherwise, if the caller provides a
transaction, then we only queue the update but don't commit it. This
optionally allows the caller to manage when a transaction will be
committed.

Given that `update_local_ref()` is always called with a `NULL`
transaction for now, no change in behaviour is expected from this
change.

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

builtin/fetch.c

@@ -583,11 +583,12 @@ static struct ref *get_ref_map(struct remote *remote,
 
 static int s_update_ref(const char *action,
 			struct ref *ref,
+			struct ref_transaction *transaction,
 			int check_old)
 {
 	char *msg;
 	char *rla = getenv("GIT_REFLOG_ACTION");
-	struct ref_transaction *transaction;
+	struct ref_transaction *our_transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int ret;
 
@@ -597,10 +598,17 @@ static int s_update_ref(const char *action,
 		rla = default_rla.buf;
 	msg = xstrfmt("%s: %s", rla, action);
 
-	transaction = ref_transaction_begin(&err);
+	/*
+	 * If no transaction was passed to us, we manage the transaction
+	 * ourselves. Otherwise, we trust the caller to handle the transaction
+	 * lifecycle.
+	 */
 	if (!transaction) {
-		ret = STORE_REF_ERROR_OTHER;
+		transaction = our_transaction = ref_transaction_begin(&err);
-		goto out;
+		if (!transaction) {
+			ret = STORE_REF_ERROR_OTHER;
+			goto out;
+		}
 	}
 
 	ret = ref_transaction_update(transaction, ref->name, &ref->new_oid,
@@ -611,19 +619,21 @@ static int s_update_ref(const char *action,
 		goto out;
 	}
 
-	switch (ref_transaction_commit(transaction, &err)) {
+	if (our_transaction) {
-	case 0:
+		switch (ref_transaction_commit(our_transaction, &err)) {
-		break;
+		case 0:
-	case TRANSACTION_NAME_CONFLICT:
+			break;
-		ret = STORE_REF_ERROR_DF_CONFLICT;
+		case TRANSACTION_NAME_CONFLICT:
-		goto out;
+			ret = STORE_REF_ERROR_DF_CONFLICT;
-	default:
+			goto out;
-		ret = STORE_REF_ERROR_OTHER;
+		default:
-		goto out;
+			ret = STORE_REF_ERROR_OTHER;
+			goto out;
+		}
 	}
 
 out:
-	ref_transaction_free(transaction);
+	ref_transaction_free(our_transaction);
 	if (ret)
 		error("%s", err.buf);
 	strbuf_release(&err);
@@ -766,6 +776,7 @@ static void format_display(struct strbuf *display, char code,
 }
 
 static int update_local_ref(struct ref *ref,
+			    struct ref_transaction *transaction,
 			    const char *remote,
 			    const struct ref *remote_ref,
 			    struct strbuf *display,
@@ -806,7 +817,7 @@ static int update_local_ref(struct ref *ref,
 	    starts_with(ref->name, "refs/tags/")) {
 		if (force || ref->force) {
 			int r;
-			r = s_update_ref("updating tag", ref, 0);
+			r = s_update_ref("updating tag", ref, transaction, 0);
 			format_display(display, r ? '!' : 't', _("[tag update]"),
 				       r ? _("unable to update local ref") : NULL,
 				       remote, pretty_ref, summary_width);
@@ -843,7 +854,7 @@ static int update_local_ref(struct ref *ref,
 			what = _("[new ref]");
 		}
 
-		r = s_update_ref(msg, ref, 0);
+		r = s_update_ref(msg, ref, transaction, 0);
 		format_display(display, r ? '!' : '*', what,
 			       r ? _("unable to update local ref") : NULL,
 			       remote, pretty_ref, summary_width);
@@ -865,7 +876,7 @@ static int update_local_ref(struct ref *ref,
 		strbuf_add_unique_abbrev(&quickref, &current->object.oid, DEFAULT_ABBREV);
 		strbuf_addstr(&quickref, "..");
 		strbuf_add_unique_abbrev(&quickref, &ref->new_oid, DEFAULT_ABBREV);
-		r = s_update_ref("fast-forward", ref, 1);
+		r = s_update_ref("fast-forward", ref, transaction, 1);
 		format_display(display, r ? '!' : ' ', quickref.buf,
 			       r ? _("unable to update local ref") : NULL,
 			       remote, pretty_ref, summary_width);
@@ -877,7 +888,7 @@ static int update_local_ref(struct ref *ref,
 		strbuf_add_unique_abbrev(&quickref, &current->object.oid, DEFAULT_ABBREV);
 		strbuf_addstr(&quickref, "...");
 		strbuf_add_unique_abbrev(&quickref, &ref->new_oid, DEFAULT_ABBREV);
-		r = s_update_ref("forced-update", ref, 1);
+		r = s_update_ref("forced-update", ref, transaction, 1);
 		format_display(display, r ? '!' : '+', quickref.buf,
 			       r ? _("unable to update local ref") : _("forced update"),
 			       remote, pretty_ref, summary_width);
@@ -1094,8 +1105,8 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 
 			strbuf_reset(&note);
 			if (ref) {
-				rc |= update_local_ref(ref, what, rm, &note,
+				rc |= update_local_ref(ref, NULL, what,
-						       summary_width);
+						       rm, &note, summary_width);
 				free(ref);
 			} else if (write_fetch_head || dry_run) {
 				/*