+/*
+ * Open the index file if needed for the given vstream.
+ *
+ * If an index file is successfully opened, the vstream will set it as its
+ * current index file.
+ *
+ * Return 0 on success, a negative value on error (-ENOENT if not ready yet).
+ *
+ * Called with rstream lock held.
+ */
+static int try_open_index(struct relay_viewer_stream *vstream,
+ struct relay_stream *rstream)
+{
+ int ret = 0;
+ const uint32_t connection_major = rstream->trace->session->major;
+ const uint32_t connection_minor = rstream->trace->session->minor;
+
+ if (vstream->index_file) {
+ goto end;
+ }
+
+ /*
+ * First time, we open the index file and at least one index is ready.
+ */
+ if (rstream->index_received_seqcount == 0) {
+ ret = -ENOENT;
+ goto end;
+ }
+ vstream->index_file = lttng_index_file_create_from_trace_chunk_read_only(
+ vstream->stream_file.trace_chunk, rstream->path_name,
+ rstream->channel_name, rstream->tracefile_size,
+ vstream->current_tracefile_id,
+ lttng_to_index_major(connection_major, connection_minor),
+ lttng_to_index_minor(connection_major, connection_minor));
+ if (!vstream->index_file) {
+ ret = -1;
+ }
+
+end:
+ return ret;
+}
+
+/*
+ * Check the status of the index for the given stream. This function
+ * updates the index structure if needed and can put (close) the vstream
+ * in the HUP situation.
+ *
+ * Return 0 means that we can proceed with the index. A value of 1 means
+ * that the index has been updated and is ready to be sent to the
+ * client. A negative value indicates an error that can't be handled.
+ *
+ * Called with rstream lock held.
+ */
+static int check_index_status(struct relay_viewer_stream *vstream,
+ struct relay_stream *rstream, struct ctf_trace *trace,
+ struct lttng_viewer_index *index)
+{
+ int ret;
+
+ if ((trace->session->connection_closed || rstream->closed)
+ && rstream->index_received_seqcount
+ == vstream->index_sent_seqcount) {
+ /*
+ * Last index sent and session connection or relay
+ * stream are closed.
+ */
+ index->status = htobe32(LTTNG_VIEWER_INDEX_HUP);
+ goto hup;
+ } else if (rstream->beacon_ts_end != -1ULL &&
+ rstream->index_received_seqcount
+ == vstream->index_sent_seqcount) {
+ /*
+ * We've received a synchronization beacon and the last index
+ * available has been sent, the index for now is inactive.
+ *
+ * In this case, we have received a beacon which allows us to
+ * inform the client of a time interval during which we can
+ * guarantee that there are no events to read (and never will
+ * be).
+ */
+ index->status = htobe32(LTTNG_VIEWER_INDEX_INACTIVE);
+ index->timestamp_end = htobe64(rstream->beacon_ts_end);
+ index->stream_id = htobe64(rstream->ctf_stream_id);
+ goto index_ready;
+ } else if (rstream->index_received_seqcount
+ == vstream->index_sent_seqcount) {
+ /*
+ * This checks whether received == sent seqcount. In
+ * this case, we have not received a beacon. Therefore,
+ * we can only ask the client to retry later.
+ */
+ index->status = htobe32(LTTNG_VIEWER_INDEX_RETRY);
+ goto index_ready;
+ } else if (!tracefile_array_seq_in_file(rstream->tfa,
+ vstream->current_tracefile_id,
+ vstream->index_sent_seqcount)) {
+ /*
+ * The next index we want to send cannot be read either
+ * because we need to perform a rotation, or due to
+ * the producer having overwritten its trace file.
+ */
+ DBG("Viewer stream %" PRIu64 " rotation",
+ vstream->stream->stream_handle);
+ ret = viewer_stream_rotate(vstream);
+ if (ret < 0) {
+ goto end;
+ } else if (ret == 1) {
+ /* EOF across entire stream. */
+ index->status = htobe32(LTTNG_VIEWER_INDEX_HUP);
+ goto hup;
+ }
+ /*
+ * If we have been pushed due to overwrite, it
+ * necessarily means there is data that can be read in
+ * the stream. If we rotated because we reached the end
+ * of a tracefile, it means the following tracefile
+ * needs to contain at least one index, else we would
+ * have already returned LTTNG_VIEWER_INDEX_RETRY to the
+ * viewer. The updated index_sent_seqcount needs to
+ * point to a readable index entry now.
+ *
+ * In the case where we "rotate" on a single file, we
+ * can end up in a case where the requested index is
+ * still unavailable.
+ */
+ if (rstream->tracefile_count == 1 &&
+ !tracefile_array_seq_in_file(
+ rstream->tfa,
+ vstream->current_tracefile_id,
+ vstream->index_sent_seqcount)) {
+ index->status = htobe32(LTTNG_VIEWER_INDEX_RETRY);
+ goto index_ready;
+ }
+ assert(tracefile_array_seq_in_file(rstream->tfa,
+ vstream->current_tracefile_id,
+ vstream->index_sent_seqcount));
+ }
+ /* ret == 0 means successful so we continue. */
+ ret = 0;
+end:
+ return ret;
+
+hup:
+ viewer_stream_put(vstream);
+index_ready:
+ return 1;
+}
+