fetch: use batched reference updates

The reference updates performed as a part of 'git-fetch(1)', take place
one at a time. For each reference update, a new transaction is created
and committed. This is necessary to ensure we can allow individual
updates to fail without failing the entire command. The command also
supports an '--atomic' mode, which uses a single transaction to update
all of the references. But this mode has an all-or-nothing approach,
where if a single update fails, all updates would fail.

In 23fc8e4f61 (refs: implement batch reference update support,
2025-04-08), we introduced a new mechanism to batch reference updates.
Under the hood, this uses a single transaction to perform a batch of
reference updates, while allowing only individual updates to fail.
Utilize this newly introduced batch update mechanism in 'git-fetch(1)'.
This provides a significant bump in performance, especially when dealing
with repositories with large number of references.

Adding support for batched updates is simply modifying the flow to also
create a batch update transaction in the non-atomic flow.

With the reftable backend there is a 22x performance improvement, when
performing 'git-fetch(1)' with 10000 refs:

  Benchmark 1: fetch: many refs (refformat = reftable, refcount = 10000, revision = master)
    Time (mean ± σ):      3.403 s ±  0.775 s    [User: 1.875 s, System: 1.417 s]
    Range (min … max):    2.454 s …  4.529 s    10 runs

  Benchmark 2: fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD)
    Time (mean ± σ):     154.3 ms ±  17.6 ms    [User: 102.5 ms, System: 56.1 ms]
    Range (min … max):   145.2 ms … 220.5 ms    18 runs

  Summary
    fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD) ran
     22.06 ± 5.62 times faster than fetch: many refs (refformat = reftable, refcount = 10000, revision = master)

In similar conditions, the files backend sees a 1.25x performance
improvement:

  Benchmark 1: fetch: many refs (refformat = files, refcount = 10000, revision = master)
    Time (mean ± σ):     605.5 ms ±   9.4 ms    [User: 117.8 ms, System: 483.3 ms]
    Range (min … max):   595.6 ms … 621.5 ms    10 runs

  Benchmark 2: fetch: many refs (refformat = files, refcount = 10000, revision = HEAD)
    Time (mean ± σ):     485.8 ms ±   4.3 ms    [User: 91.1 ms, System: 396.7 ms]
    Range (min … max):   477.6 ms … 494.3 ms    10 runs

  Summary
    fetch: many refs (refformat = files, refcount = 10000, revision = HEAD) ran
      1.25 ± 0.02 times faster than fetch: many refs (refformat = files, refcount = 10000, revision = master)

With this we'll either be using a regular transaction or a batch update
transaction. This helps cleanup some code which is no longer needed as
we'll now always have some type of 'ref_transaction' object being
propagated.

One big change is that earlier, each individual update would propagate a
failure. Whereas now, the `ref_transaction_for_each_rejected_update`
function is called at the end of the flow to capture the exit status for
'git-fetch(1)' and also to print F/D conflict errors. This does change
the order of the errors being printed, but the behavior stays the same.

Since transaction errors are now explicitly defined as part of
76e760b999 (refs: introduce enum-based transaction error types,
2025-04-08), utilize them and get rid of custom errors defined within
'builtin/fetch.c'.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -640,9 +640,6 @@ static struct ref *get_ref_map(struct remote *remote,
 	return ref_map;
 }
 
-#define STORE_REF_ERROR_OTHER 1
-#define STORE_REF_ERROR_DF_CONFLICT 2
-
 static int s_update_ref(const char *action,
 			struct ref *ref,
 			struct ref_transaction *transaction,
@@ -650,7 +647,6 @@ static int s_update_ref(const char *action,
 {
 	char *msg;
 	char *rla = getenv("GIT_REFLOG_ACTION");
-	struct ref_transaction *our_transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
 	int ret;
 
@@ -660,43 +656,10 @@ static int s_update_ref(const char *action,
 		rla = default_rla.buf;
 	msg = xstrfmt("%s: %s", rla, action);
 
-	/*
-	 * If no transaction was passed to us, we manage the transaction
-	 * ourselves. Otherwise, we trust the caller to handle the transaction
-	 * lifecycle.
-	 */
-	if (!transaction) {
-		transaction = our_transaction = ref_store_transaction_begin(get_main_ref_store(the_repository),
-									    0, &err);
-		if (!transaction) {
-			ret = STORE_REF_ERROR_OTHER;
-			goto out;
-		}
-	}
-
 	ret = ref_transaction_update(transaction, ref->name, &ref->new_oid,
 				     check_old ? &ref->old_oid : NULL,
 				     NULL, NULL, 0, msg, &err);
-	if (ret) {
-		ret = STORE_REF_ERROR_OTHER;
-		goto out;
-	}
 
-	if (our_transaction) {
-		switch (ref_transaction_commit(our_transaction, &err)) {
-		case 0:
-			break;
-		case REF_TRANSACTION_ERROR_NAME_CONFLICT:
-			ret = STORE_REF_ERROR_DF_CONFLICT;
-			goto out;
-		default:
-			ret = STORE_REF_ERROR_OTHER;
-			goto out;
-		}
-	}
-
-out:
-	ref_transaction_free(our_transaction);
 	if (ret)
 		error("%s", err.buf);
 	strbuf_release(&err);
@@ -1139,7 +1102,6 @@ N_("it took %.2f seconds to check forced updates; you can use\n"
    "to avoid this check\n");
 
 static int store_updated_refs(struct display_state *display_state,
-			      const char *remote_name,
 			      int connectivity_checked,
 			      struct ref_transaction *transaction, struct ref *ref_map,
 			      struct fetch_head *fetch_head,
@@ -1277,11 +1239,6 @@ static int store_updated_refs(struct display_state *display_state,
 		}
 	}
 
-	if (rc & STORE_REF_ERROR_DF_CONFLICT)
-		error(_("some local refs could not be updated; try running\n"
-		      " 'git remote prune %s' to remove any old, conflicting "
-		      "branches"), remote_name);
-
 	if (advice_enabled(ADVICE_FETCH_SHOW_FORCED_UPDATES)) {
 		if (!config->show_forced_updates) {
 			warning(_(warn_show_forced_updates));
@@ -1365,9 +1322,8 @@ static int fetch_and_consume_refs(struct display_state *display_state,
 	}
 
 	trace2_region_enter("fetch", "consume_refs", the_repository);
-	ret = store_updated_refs(display_state, transport->remote->name,
-				 connectivity_checked, transaction, ref_map,
-				 fetch_head, config);
+	ret = store_updated_refs(display_state, connectivity_checked,
+				 transaction, ref_map, fetch_head, config);
 	trace2_region_leave("fetch", "consume_refs", the_repository);
 
 out:
@@ -1687,6 +1643,36 @@ cleanup:
 	return result;
 }
 
+struct ref_rejection_data {
+	int *retcode;
+	int conflict_msg_shown;
+	const char *remote_name;
+};
+
+static void ref_transaction_rejection_handler(const char *refname,
+					      const struct object_id *old_oid UNUSED,
+					      const struct object_id *new_oid UNUSED,
+					      const char *old_target UNUSED,
+					      const char *new_target UNUSED,
+					      enum ref_transaction_error err,
+					      void *cb_data)
+{
+	struct ref_rejection_data *data = cb_data;
+
+	if (err == REF_TRANSACTION_ERROR_NAME_CONFLICT && !data->conflict_msg_shown) {
+		error(_("some local refs could not be updated; try running\n"
+			" 'git remote prune %s' to remove any old, conflicting "
+			"branches"), data->remote_name);
+		data->conflict_msg_shown = 1;
+	} else {
+		const char *reason = ref_transaction_error_msg(err);
+
+		error(_("fetching ref %s failed: %s"), refname, reason);
+	}
+
+	*data->retcode = 1;
+}
+
 static int do_fetch(struct transport *transport,
 		    struct refspec *rs,
 		    const struct fetch_config *config)
@@ -1807,6 +1793,24 @@ static int do_fetch(struct transport *transport,
 			retcode = 1;
 	}
 
+	/*
+	 * If not atomic, we can still use batched updates, which would be much
+	 * more performant. We don't initiate the transaction before pruning,
+	 * since pruning must be an independent step, to avoid F/D conflicts.
+	 *
+	 * TODO: if reference transactions gain logical conflict resolution, we
+	 * can delete and create refs (with F/D conflicts) in the same transaction
+	 * and this can be moved above the 'prune_refs()' block.
+	 */
+	if (!transaction) {
+		transaction = ref_store_transaction_begin(get_main_ref_store(the_repository),
+							  REF_TRANSACTION_ALLOW_FAILURE, &err);
+		if (!transaction) {
+			retcode = -1;
+			goto cleanup;
+		}
+	}
+
 	if (fetch_and_consume_refs(&display_state, transport, transaction, ref_map,
 				   &fetch_head, config)) {
 		retcode = 1;
@@ -1838,16 +1842,31 @@ static int do_fetch(struct transport *transport,
 		free_refs(tags_ref_map);
 	}
 
-	if (transaction) {
-		if (retcode)
-			goto cleanup;
+	if (retcode)
+		goto cleanup;
 
-		retcode = ref_transaction_commit(transaction, &err);
+	retcode = ref_transaction_commit(transaction, &err);
+	if (retcode) {
+		/*
+		 * Explicitly handle transaction cleanup to avoid
+		 * aborting an already closed transaction.
+		 */
+		ref_transaction_free(transaction);
+		transaction = NULL;
+		goto cleanup;
+	}
+
+	if (!atomic_fetch) {
+		struct ref_rejection_data data = {
+			.retcode = &retcode,
+			.conflict_msg_shown = 0,
+			.remote_name = transport->remote->name,
+		};
+
+		ref_transaction_for_each_rejected_update(transaction,
+							 ref_transaction_rejection_handler,
+							 &data);
 		if (retcode) {
-			/*
-			 * Explicitly handle transaction cleanup to avoid
-			 * aborting an already closed transaction.
-			 */
 			ref_transaction_free(transaction);
 			transaction = NULL;
 			goto cleanup;