When executing git-update-ref(1) with the `--stdin` flag, then the user
can queue updates and, since e48cf33b61 (update-ref: implement
interactive transaction handling, 2020-04-02), interactively drive the
transaction's state via a set of transactional verbs. This interactivity
is somewhat broken though: while the caller can use these verbs to drive
the transaction's state, the status messages which confirm that a verb
has been processed is not flushed. The caller may thus be left hanging
waiting for the acknowledgement.

Fix the bug by flushing stdout after writing the status update. Add a
test which catches this bug.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/update-ref.c  | 14 ++++++++++----
 t/t1400-update-ref.sh | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 6029a80544..a84e7b47a2 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -302,6 +302,12 @@ static void parse_cmd_verify(struct ref_transaction *transaction,
 	strbuf_release(&err);
 }
 
+static void report_ok(const char *command)
+{
+	fprintf(stdout, "%s: ok\n", command);
+	fflush(stdout);
+}
+
 static void parse_cmd_option(struct ref_transaction *transaction,
 			     const char *next, const char *end)
 {
@@ -317,7 +323,7 @@ static void parse_cmd_start(struct ref_transaction *transaction,
 {
 	if (*next != line_termination)
 		die("start: extra input: %s", next);
-	puts("start: ok");
+	report_ok("start");
 }
 
 static void parse_cmd_prepare(struct ref_transaction *transaction,
@@ -328,7 +334,7 @@ static void parse_cmd_prepare(struct ref_transaction *transaction,
 		die("prepare: extra input: %s", next);
 	if (ref_transaction_prepare(transaction, &error))
 		die("prepare: %s", error.buf);
-	puts("prepare: ok");
+	report_ok("prepare");
 }
 
 static void parse_cmd_abort(struct ref_transaction *transaction,
@@ -339,7 +345,7 @@ static void parse_cmd_abort(struct ref_transaction *transaction,
 		die("abort: extra input: %s", next);
 	if (ref_transaction_abort(transaction, &error))
 		die("abort: %s", error.buf);
-	puts("abort: ok");
+	report_ok("abort");
 }
 
 static void parse_cmd_commit(struct ref_transaction *transaction,
@@ -350,7 +356,7 @@ static void parse_cmd_commit(struct ref_transaction *transaction,
 		die("commit: extra input: %s", next);
 	if (ref_transaction_commit(transaction, &error))
 		die("commit: %s", error.buf);
-	puts("commit: ok");
+	report_ok("commit");
 	ref_transaction_free(transaction);
 }
 
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index 4506cd435b..1e754e258f 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -1598,6 +1598,38 @@ test_expect_success 'transaction cannot restart ongoing transaction' '
 	test_must_fail git show-ref --verify refs/heads/restart
 '
 
+test_expect_success PIPE 'transaction flushes status updates' '
+	mkfifo in out &&
+	(git update-ref --stdin <in >out &) &&
+
+	exec 9>in &&
+	test_when_finished "exec 9>&-" &&
+
+	echo "start" >&9 &&
+	echo "start: ok" >expected &&
+	read line <out &&
+	echo "$line" >actual &&
+	test_cmp expected actual &&
+
+	echo "create refs/heads/flush $A" >&9 &&
+
+	echo prepare >&9 &&
+	echo "prepare: ok" >expected &&
+	read line <out &&
+	echo "$line" >actual &&
+	test_cmp expected actual &&
+
+	# This must now fail given that we have locked the ref.
+	test_must_fail git update-ref refs/heads/flush $B 2>stderr &&
+	grep "fatal: update_ref failed for ref ${SQ}refs/heads/flush${SQ}: cannot lock ref" stderr &&
+
+	echo commit >&9 &&
+	echo "commit: ok" >expected &&
+	read line <out &&
+	echo "$line" >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'directory not created deleting packed ref' '
 	git branch d1/d2/r1 HEAD &&
 	git pack-refs --all &&
-- 
2.33.0