update-ref: implement interactive transaction handling

The git-update-ref(1) command can only handle queueing transactions
right now via its "--stdin" parameter, but there is no way for users to
handle the transaction itself in a more explicit way. E.g. in a
replicated scenario, one may imagine a coordinator that spawns
git-update-ref(1) for multiple repositories and only if all agree that
an update is possible will the coordinator send a commit. Such a
transactional session could look like

    > start
    < start: ok
    > update refs/heads/master $OLD $NEW
    > prepare
    < prepare: ok
    # All nodes have returned "ok"
    > commit
    < commit: ok

or

    > start
    < start: ok
    > create refs/heads/master $OLD $NEW
    > prepare
    < fatal: cannot lock ref 'refs/heads/master': reference already exists
    # On all other nodes:
    > abort
    < abort: ok

In order to allow for such transactional sessions, this commit
introduces four new commands for git-update-ref(1), which matches those
we have internally already with the exception of "start":

    - start: start a new transaction

    - prepare: prepare the transaction, that is try to lock all
               references and verify their current value matches the
               expected one

    - commit: explicitly commit a session, that is update references to
              match their new expected state

    - abort: abort a session and roll back all changes

By design, git-update-ref(1) will commit as soon as standard input is
being closed. While fine in a non-transactional world, it is definitely
unexpected in a transactional world. Because of this, as soon as any of
the new transactional commands is used, the default will change to
aborting without an explicit "commit". To avoid a race between queueing
updates and the first "prepare" that starts a transaction, the "start"
command has been added to start an explicit transaction.

Add some tests to exercise this new functionality.

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

3 files changed, 255 insertions, 8 deletions

diff --git a/Documentation/git-update-ref.txt b/Documentation/git-update-ref.txt
index 9bd039ce08..3e737c2360 100644
--- a/Documentation/git-update-ref.txt
+++ b/Documentation/git-update-ref.txt
@@ -66,6 +66,10 @@ performs all modifications together.  Specify commands of the form:
 	delete SP <ref> [SP <oldvalue>] LF
 	verify SP <ref> [SP <oldvalue>] LF
 	option SP <opt> LF
+	start LF
+	prepare LF
+	commit LF
+	abort LF
 
 With `--create-reflog`, update-ref will create a reflog for each ref
 even if one would not ordinarily be created.
@@ -83,6 +87,10 @@ quoting:
 	delete SP <ref> NUL [<oldvalue>] NUL
 	verify SP <ref> NUL [<oldvalue>] NUL
 	option SP <opt> NUL
+	start NUL
+	prepare NUL
+	commit NUL
+	abort NUL
 
 In this format, use 40 "0" to specify a zero value, and use the empty
 string to specify a missing value.
@@ -114,6 +122,24 @@ option::
 	The only valid option is `no-deref` to avoid dereferencing
 	a symbolic ref.
 
+start::
+	Start a transaction. In contrast to a non-transactional session, a
+	transaction will automatically abort if the session ends without an
+	explicit commit.
+
+prepare::
+	Prepare to commit the transaction. This will create lock files for all
+	queued reference updates. If one reference could not be locked, the
+	transaction will be aborted.
+
+commit::
+	Commit all reference updates queued for the transaction, ending the
+	transaction.
+
+abort::
+	Abort the transaction, releasing all locks if the transaction is in
+	prepared state.
+
 If all <ref>s can be locked with matching <oldvalue>s
 simultaneously, all modifications are performed.  Otherwise, no
 modifications are performed.  Note that while each individual
diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 348407b896..b74dd9a69d 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -312,21 +312,80 @@ static void parse_cmd_option(struct ref_transaction *transaction,
 		die("option unknown: %s", next);
 }
 
+static void parse_cmd_start(struct ref_transaction *transaction,
+			    const char *next, const char *end)
+{
+	if (*next != line_termination)
+		die("start: extra input: %s", next);
+	puts("start: ok");
+}
+
+static void parse_cmd_prepare(struct ref_transaction *transaction,
+			      const char *next, const char *end)
+{
+	struct strbuf error = STRBUF_INIT;
+	if (*next != line_termination)
+		die("prepare: extra input: %s", next);
+	if (ref_transaction_prepare(transaction, &error))
+		die("prepare: %s", error.buf);
+	puts("prepare: ok");
+}
+
+static void parse_cmd_abort(struct ref_transaction *transaction,
+			    const char *next, const char *end)
+{
+	struct strbuf error = STRBUF_INIT;
+	if (*next != line_termination)
+		die("abort: extra input: %s", next);
+	if (ref_transaction_abort(transaction, &error))
+		die("abort: %s", error.buf);
+	puts("abort: ok");
+}
+
+static void parse_cmd_commit(struct ref_transaction *transaction,
+			     const char *next, const char *end)
+{
+	struct strbuf error = STRBUF_INIT;
+	if (*next != line_termination)
+		die("commit: extra input: %s", next);
+	if (ref_transaction_commit(transaction, &error))
+		die("commit: %s", error.buf);
+	puts("commit: ok");
+	ref_transaction_free(transaction);
+}
+
+enum update_refs_state {
+	/* Non-transactional state open for updates. */
+	UPDATE_REFS_OPEN,
+	/* A transaction has been started. */
+	UPDATE_REFS_STARTED,
+	/* References are locked and ready for commit */
+	UPDATE_REFS_PREPARED,
+	/* Transaction has been committed or closed. */
+	UPDATE_REFS_CLOSED,
+};
+
 static const struct parse_cmd {
 	const char *prefix;
 	void (*fn)(struct ref_transaction *, const char *, const char *);
 	unsigned args;
+	enum update_refs_state state;
 } command[] = {
-	{ "update", parse_cmd_update, 3 },
-	{ "create", parse_cmd_create, 2 },
-	{ "delete", parse_cmd_delete, 2 },
-	{ "verify", parse_cmd_verify, 2 },
-	{ "option", parse_cmd_option, 1 },
+	{ "update",  parse_cmd_update,  3, UPDATE_REFS_OPEN },
+	{ "create",  parse_cmd_create,  2, UPDATE_REFS_OPEN },
+	{ "delete",  parse_cmd_delete,  2, UPDATE_REFS_OPEN },
+	{ "verify",  parse_cmd_verify,  2, UPDATE_REFS_OPEN },
+	{ "option",  parse_cmd_option,  1, UPDATE_REFS_OPEN },
+	{ "start",   parse_cmd_start,   0, UPDATE_REFS_STARTED },
+	{ "prepare", parse_cmd_prepare, 0, UPDATE_REFS_PREPARED },
+	{ "abort",   parse_cmd_abort,   0, UPDATE_REFS_CLOSED },
+	{ "commit",  parse_cmd_commit,  0, UPDATE_REFS_CLOSED },
 };
 
 static void update_refs_stdin(void)
 {
 	struct strbuf input = STRBUF_INIT, err = STRBUF_INIT;
+	enum update_refs_state state = UPDATE_REFS_OPEN;
 	struct ref_transaction *transaction;
 	int i, j;
 
@@ -374,14 +433,45 @@ static void update_refs_stdin(void)
 			if (strbuf_appendwholeline(&input, stdin, line_termination))
 				break;
 
+		switch (state) {
+		case UPDATE_REFS_OPEN:
+		case UPDATE_REFS_STARTED:
+			/* Do not downgrade a transaction to a non-transaction. */
+			if (cmd->state >= state)
+				state = cmd->state;
+			break;
+		case UPDATE_REFS_PREPARED:
+			if (cmd->state != UPDATE_REFS_CLOSED)
+				die("prepared transactions can only be closed");
+			state = cmd->state;
+			break;
+		case UPDATE_REFS_CLOSED:
+			die("transaction is closed");
+			break;
+		}
+
 		cmd->fn(transaction, input.buf + strlen(cmd->prefix) + !!cmd->args,
 			input.buf + input.len);
 	}
 
-	if (ref_transaction_commit(transaction, &err))
-		die("%s", err.buf);
+	switch (state) {
+	case UPDATE_REFS_OPEN:
+		/* Commit by default if no transaction was requested. */
+		if (ref_transaction_commit(transaction, &err))
+			die("%s", err.buf);
+		ref_transaction_free(transaction);
+		break;
+	case UPDATE_REFS_STARTED:
+	case UPDATE_REFS_PREPARED:
+		/* If using a transaction, we want to abort it. */
+		if (ref_transaction_abort(transaction, &err))
+			die("%s", err.buf);
+		break;
+	case UPDATE_REFS_CLOSED:
+		/* Otherwise no need to do anything, the transaction was closed already. */
+		break;
+	}
 
-	ref_transaction_free(transaction);
 	strbuf_release(&err);
 	strbuf_release(&input);
 }
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index a6224ef65f..48d0d42afd 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -1404,4 +1404,135 @@ test_expect_success 'handle per-worktree refs in refs/bisect' '
 	! test_cmp main-head worktree-head
 '
 
+test_expect_success 'transaction handles empty commit' '
+	cat >stdin <<-EOF &&
+	start
+	prepare
+	commit
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start prepare commit >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction handles empty commit with missing prepare' '
+	cat >stdin <<-EOF &&
+	start
+	commit
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start commit >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction handles sole commit' '
+	cat >stdin <<-EOF &&
+	commit
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" commit >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction handles empty abort' '
+	cat >stdin <<-EOF &&
+	start
+	prepare
+	abort
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start prepare abort >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction exits on multiple aborts' '
+	cat >stdin <<-EOF &&
+	abort
+	abort
+	EOF
+	test_must_fail git update-ref --stdin <stdin >actual 2>err &&
+	printf "%s: ok\n" abort >expect &&
+	test_cmp expect actual &&
+	grep "fatal: transaction is closed" err
+'
+
+test_expect_success 'transaction exits on start after prepare' '
+	cat >stdin <<-EOF &&
+	prepare
+	start
+	EOF
+	test_must_fail git update-ref --stdin <stdin 2>err >actual &&
+	printf "%s: ok\n" prepare >expect &&
+	test_cmp expect actual &&
+	grep "fatal: prepared transactions can only be closed" err
+'
+
+test_expect_success 'transaction handles empty abort with missing prepare' '
+	cat >stdin <<-EOF &&
+	start
+	abort
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start abort >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction handles sole abort' '
+	cat >stdin <<-EOF &&
+	abort
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" abort >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction can handle commit' '
+	cat >stdin <<-EOF &&
+	start
+	create $a HEAD
+	commit
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start commit >expect &&
+	test_cmp expect actual &&
+	git rev-parse HEAD >expect &&
+	git rev-parse $a >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'transaction can handle abort' '
+	cat >stdin <<-EOF &&
+	start
+	create $b HEAD
+	abort
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start abort >expect &&
+	test_cmp expect actual &&
+	test_path_is_missing .git/$b
+'
+
+test_expect_success 'transaction aborts by default' '
+	cat >stdin <<-EOF &&
+	start
+	create $b HEAD
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start >expect &&
+	test_cmp expect actual &&
+	test_path_is_missing .git/$b
+'
+
+test_expect_success 'transaction with prepare aborts by default' '
+	cat >stdin <<-EOF &&
+	start
+	create $b HEAD
+	prepare
+	EOF
+	git update-ref --stdin <stdin >actual &&
+	printf "%s: ok\n" start prepare >expect &&
+	test_cmp expect actual &&
+	test_path_is_missing .git/$b
+'
+
 test_done