reftable/writer: ensure valid range for log's update_index

Each reftable addition has an associated update_index. While writing
refs, the update_index is verified to be within the range of the
reftable writer, i.e. `writer.min_update_index <= ref.update_index` and
`writer.max_update_index => ref.update_index`.

The corresponding check for reflogs in `reftable_writer_add_log` is
however missing. Add a similar check, but only check for the upper
limit. This is because reflogs are treated a bit differently than refs.
Each reflog entry in reftable has an associated update_index and we also
allow expiring entries in the middle, which is done by simply writing a
new reflog entry with the same update_index. This means, writing reflog
entries with update_index lesser than the writer's update_index is an
expected scenario.

Add a new unit test to check for the limits and fix some of the existing
tests, which were setting arbitrary values for the update_index by
ensuring they stay within the now checked limits.

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

reftable/writer.c

@@ -412,6 +412,18 @@ int reftable_writer_add_log(struct reftable_writer *w,
 	if (log->value_type == REFTABLE_LOG_DELETION)
 		return reftable_writer_add_log_verbatim(w, log);
 
+	/*
+	 * Verify only the upper limit of the update_index. Each reflog entry
+	 * is tied to a specific update_index. Entries in the reflog can be
+	 * replaced by adding a new entry with the same update_index,
+	 * effectively canceling the old one.
+	 *
+	 * Consequently, reflog updates may include update_index values lower
+	 * than the writer's min_update_index.
+	 */
+	if (log->update_index > w->max_update_index)
+		return REFTABLE_API_ERROR;
+
 	if (!log->refname)
 		return REFTABLE_API_ERROR;
 

t/unit-tests/t-reftable-readwrite.c

@@ -90,7 +90,7 @@ static void t_log_buffer_size(void)
 	int i;
 	struct reftable_log_record
 		log = { .refname = (char *) "refs/heads/master",
-			.update_index = 0xa,
+			.update_index = update_index,
 			.value_type = REFTABLE_LOG_UPDATE,
 			.value = { .update = {
 					   .name = (char *) "Han-Wen Nienhuys",
@@ -127,7 +127,7 @@ static void t_log_overflow(void)
 	int err;
 	struct reftable_log_record log = {
 		.refname = (char *) "refs/heads/master",
-		.update_index = 0xa,
+		.update_index = update_index,
 		.value_type = REFTABLE_LOG_UPDATE,
 		.value = {
 			.update = {
@@ -151,6 +151,48 @@ static void t_log_overflow(void)
 	reftable_buf_release(&buf);
 }
 
+static void t_log_write_limits(void)
+{
+	struct reftable_write_options opts = { 0 };
+	struct reftable_buf buf = REFTABLE_BUF_INIT;
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
+	struct reftable_log_record log = {
+		.refname = (char *)"refs/head/master",
+		.update_index = 0,
+		.value_type = REFTABLE_LOG_UPDATE,
+		.value = {
+			.update = {
+				.old_hash = { 1 },
+				.new_hash = { 2 },
+				.name = (char *)"Han-Wen Nienhuys",
+				.email = (char *)"hanwen@google.com",
+				.tz_offset = 100,
+				.time = 0x5e430672,
+			},
+		},
+	};
+	int err;
+
+	reftable_writer_set_limits(w, 1, 1);
+
+	/* write with update_index (0) below set limits (1, 1) */
+	err = reftable_writer_add_log(w, &log);
+	check_int(err, ==, 0);
+
+	/* write with update_index (1) in the set limits (1, 1) */
+	log.update_index = 1;
+	err = reftable_writer_add_log(w, &log);
+	check_int(err, ==, 0);
+
+	/* write with update_index (3) above set limits (1, 1) */
+	log.update_index = 3;
+	err = reftable_writer_add_log(w, &log);
+	check_int(err, ==, REFTABLE_API_ERROR);
+
+	reftable_writer_free(w);
+	reftable_buf_release(&buf);
+}
+
 static void t_log_write_read(void)
 {
 	struct reftable_write_options opts = {
@@ -917,6 +959,7 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 	TEST(t_corrupt_table_empty(), "read-write on an empty table");
 	TEST(t_log_buffer_size(), "buffer extension for log compression");
 	TEST(t_log_overflow(), "log overflow returns expected error");
+	TEST(t_log_write_limits(), "writer limits for writing log records");
 	TEST(t_log_write_read(), "read-write on log records");
 	TEST(t_log_zlib_corruption(), "reading corrupted log record returns expected error");
 	TEST(t_table_read_api(), "read on a table");

t/unit-tests/t-reftable-stack.c

@@ -770,8 +770,12 @@ static void t_reftable_stack_tombstone(void)
 		}
 
 		logs[i].refname = xstrdup(buf);
-		/* update_index is part of the key. */
-		logs[i].update_index = 42;
+		/*
+		 * update_index is part of the key so should be constant.
+		 * The value itself should be less than the writer's upper
+		 * limit.
+		 */
+		logs[i].update_index = 1;
 		if (i % 2 == 0) {
 			logs[i].value_type = REFTABLE_LOG_UPDATE;
 			t_reftable_set_hash(logs[i].value.update.new_hash, i,