Merge branch 'ps/update-ref-batch-flush' into maint

"git update-ref --stdin" failed to flush its output as needed,
which potentially led the conversation to a deadlock.

* ps/update-ref-batch-flush:
  t1400: avoid SIGPIPE race condition on fifo
  update-ref: fix streaming of status updates

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);
 }
 

t/t1400-update-ref.sh

@@ -1598,6 +1598,40 @@ 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 &&
+	exec 8<out &&
+	test_when_finished "exec 9>&-" &&
+	test_when_finished "exec 8<&-" &&
+
+	echo "start" >&9 &&
+	echo "start: ok" >expected &&
+	read line <&8 &&
+	echo "$line" >actual &&
+	test_cmp expected actual &&
+
+	echo "create refs/heads/flush $A" >&9 &&
+
+	echo prepare >&9 &&
+	echo "prepare: ok" >expected &&
+	read line <&8 &&
+	echo "$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 <&8 &&
+	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 &&