+static ssize_t
+relay_unpack_rotate_streams_header(const struct lttng_buffer_view *payload,
+ struct lttcomm_relayd_rotate_streams *_rotate_streams)
+{
+ struct lttcomm_relayd_rotate_streams rotate_streams;
+ /*
+ * Set to the smallest version (packed) of `lttcomm_relayd_rotate_streams`.
+ * This is the smallest version of this structure, but it can be larger;
+ * this variable is updated once the proper size of the structure is known.
+ *
+ * See comment at the declaration of this structure for more information.
+ */
+ ssize_t header_len = sizeof(struct lttcomm_relayd_rotate_streams_packed);
+ size_t expected_payload_size_no_padding, expected_payload_size_3_bytes_padding,
+ expected_payload_size_7_bytes_padding;
+
+ if (payload->size < header_len) {
+ ERR("Unexpected payload size in \"relay_rotate_session_stream\": expected >= %zu bytes, got %zu bytes",
+ header_len,
+ payload->size);
+ goto error;
+ }
+
+ /*
+ * Some versions incorrectly omitted the LTTNG_PACKED annotation on the
+ * `new_chunk_id` optional field of struct lttcomm_relayd_rotate_streams.
+ *
+ * We start by "unpacking" `stream_count` to figure out the padding length
+ * emited by our peer.
+ */
+ {
+ decltype(rotate_streams.stream_count) stream_count;
+
+ memcpy(&stream_count, payload->data, sizeof(stream_count));
+ rotate_streams.stream_count = be32toh(stream_count);
+ }
+
+ rotate_streams.new_chunk_id = LTTNG_OPTIONAL_INIT_UNSET;
+
+ /*
+ * Payload size expected given the possible padding lengths in
+ * `struct lttcomm_relayd_rotate_streams`.
+ */
+ expected_payload_size_no_padding =
+ (rotate_streams.stream_count * sizeof(*rotate_streams.rotation_positions)) +
+ sizeof(lttcomm_relayd_rotate_streams_packed);
+ expected_payload_size_3_bytes_padding =
+ (rotate_streams.stream_count * sizeof(*rotate_streams.rotation_positions)) +
+ sizeof(lttcomm_relayd_rotate_streams_3_bytes_padding);
+ expected_payload_size_7_bytes_padding =
+ (rotate_streams.stream_count * sizeof(*rotate_streams.rotation_positions)) +
+ sizeof(lttcomm_relayd_rotate_streams_7_bytes_padding);
+
+ if (payload->size == expected_payload_size_no_padding) {
+ struct lttcomm_relayd_rotate_streams_packed packed_rotate_streams;
+
+ /*
+ * This handles cases where someone might build with
+ * -fpack-struct or any other toolchain that wouldn't produce
+ * padding to align `value`.
+ */
+ DBG("Received `struct lttcomm_relayd_rotate_streams` with no padding");
+
+ header_len = sizeof(packed_rotate_streams);
+ memcpy(&packed_rotate_streams, payload->data, header_len);
+
+ /* Unpack the packed structure to the natively-packed version. */
+ _rotate_streams->new_chunk_id = (typeof(_rotate_streams->new_chunk_id)){
+ .is_set = !!packed_rotate_streams.new_chunk_id.is_set,
+ .value = be64toh(packed_rotate_streams.new_chunk_id.value),
+ };
+ _rotate_streams->stream_count = be32toh(packed_rotate_streams.stream_count);
+ } else if (payload->size == expected_payload_size_3_bytes_padding) {
+ struct lttcomm_relayd_rotate_streams_3_bytes_padding padded_rotate_streams;
+
+ DBG("Received `struct lttcomm_relayd_rotate_streams` with 3 bytes of padding (4-byte aligned peer)");
+
+ header_len = sizeof(padded_rotate_streams);
+ memcpy(&padded_rotate_streams, payload->data, header_len);
+
+ /* Unpack the 3-byte padded structure to the natively-packed version. */
+ _rotate_streams->new_chunk_id = (typeof(_rotate_streams->new_chunk_id)){
+ .is_set = !!padded_rotate_streams.new_chunk_id.is_set,
+ .value = be64toh(padded_rotate_streams.new_chunk_id.value),
+ };
+ _rotate_streams->stream_count = be32toh(padded_rotate_streams.stream_count);
+ } else if (payload->size == expected_payload_size_7_bytes_padding) {
+ struct lttcomm_relayd_rotate_streams_7_bytes_padding padded_rotate_streams;
+
+ DBG("Received `struct lttcomm_relayd_rotate_streams` with 7 bytes of padding (8-byte aligned peer)");
+
+ header_len = sizeof(padded_rotate_streams);
+ memcpy(&padded_rotate_streams, payload->data, header_len);
+
+ /* Unpack the 7-byte padded structure to the natively-packed version. */
+ _rotate_streams->new_chunk_id = (typeof(_rotate_streams->new_chunk_id)){
+ .is_set = !!padded_rotate_streams.new_chunk_id.is_set,
+ .value = be64toh(padded_rotate_streams.new_chunk_id.value),
+ };
+ _rotate_streams->stream_count = be32toh(padded_rotate_streams.stream_count);
+
+ header_len = sizeof(padded_rotate_streams);
+ } else {
+ ERR("Unexpected payload size in \"relay_rotate_session_stream\": expected %zu, %zu or %zu bytes, got %zu bytes",
+ expected_payload_size_no_padding,
+ expected_payload_size_3_bytes_padding,
+ expected_payload_size_7_bytes_padding,
+ payload->size);
+ goto error;
+ }
+
+ return header_len;
+error:
+ return -1;
+}
+