receive-pack.c: add execute_commands_atomic function

This introduces the new function execute_commands_atomic which will use
one atomic transaction for all updates. The default behavior is still
the old non atomic way, one ref at a time. This is to cause as little
disruption as possible to existing clients. It is unknown if there are
client scripts that depend on the old non-atomic behavior so we make it
opt-in for now.

A later patch will add the possibility to actually use the functionality
added by this patch. For now use_atomic is always 0.

Inspired-by: Ronnie Sahlberg <sahlberg@google.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -40,6 +40,7 @@ static int transfer_unpack_limit = -1;
 static int unpack_limit = 100;
 static int report_status;
 static int use_sideband;
+static int use_atomic;
 static int quiet;
 static int prefer_ofs_delta = 1;
 static int auto_update_server_info;
@@ -1095,7 +1096,48 @@ static void execute_commands_non_atomic(struct command *commands,
 		}
 		ref_transaction_free(transaction);
 	}
+	strbuf_release(&err);
+}
+
+static void execute_commands_atomic(struct command *commands,
+					struct shallow_info *si)
+{
+	struct command *cmd;
+	struct strbuf err = STRBUF_INIT;
+	const char *reported_error = "atomic push failure";
+
+	transaction = ref_transaction_begin(&err);
+	if (!transaction) {
+		rp_error("%s", err.buf);
+		strbuf_reset(&err);
+		reported_error = "transaction failed to start";
+		goto failure;
+	}
+
+	for (cmd = commands; cmd; cmd = cmd->next) {
+		if (!should_process_cmd(cmd))
+			continue;
+
+		cmd->error_string = update(cmd, si);
+
+		if (cmd->error_string)
+			goto failure;
+	}
 
+	if (ref_transaction_commit(transaction, &err)) {
+		rp_error("%s", err.buf);
+		reported_error = "atomic transaction failed";
+		goto failure;
+	}
+	goto cleanup;
+
+failure:
+	for (cmd = commands; cmd; cmd = cmd->next)
+		if (!cmd->error_string)
+			cmd->error_string = reported_error;
+
+cleanup:
+	ref_transaction_free(transaction);
 	strbuf_release(&err);
 }
 
@@ -1133,7 +1175,10 @@ static void execute_commands(struct command *commands,
 	free(head_name_to_free);
 	head_name = head_name_to_free = resolve_refdup("HEAD", 0, sha1, NULL);
 
-	execute_commands_non_atomic(commands, si);
+	if (use_atomic)
+		execute_commands_atomic(commands, si);
+	else
+		execute_commands_non_atomic(commands, si);
 
 	if (shallow_update)
 		warn_if_skipped_connectivity_check(commands, si);