[PATCH BlueZ 1/2] profiles/midi: Fix running status across channels

[RasmusML <rasmusml0501@gmail.com>]

From: RasmusML <rasmusml@hotmail.dk>

When two MIDI messages of the same type but on different channels end
up in the same BLE-MIDI packet, read_ev_others() writes the second
status byte without the timestampLow byte that must precede every
status byte. A receiver then reads that status byte as the timestamp
and applies the following data bytes to the previous message's channel
through running status, e.g.:

  sent:    ch1 NoteOff 60, ch2 NoteOff 64, ch3 CC123, ch4 CC123
  packet:  B5 94 81 3C 00 82 40 00 94 B3 7B 00 B4 7B 00
  decoded: ch1 NoteOff 60, ch1 NoteOff 64, ch3 CC123, ch3 CC123

This happens because rstatus holds the ALSA event type, which carries
no channel, so the writer assumes running status while
snd_midi_event_decode() correctly emits a new status byte.

Fix this by decoding each message into a local buffer first and
appending timestampLow whenever the decoder actually wrote a status
byte. If the message does not fit, flush the packet and, if it relied
on running status, decode it again with a reset decoder so the new
packet starts with a full status byte. This replaces the -ENOMEM retry
path.

Closes: https://github.com/bluez/bluez/issues/2630
Assisted-by: Claude:claude-opus-5-5
---
 profiles/midi/libmidi.c | 59 +++++++++++++++++++++++------------------
 1 file changed, 33 insertions(+), 26 deletions(-)

diff --git a/profiles/midi/libmidi.c b/profiles/midi/libmidi.c
index ef33f7dfd..349f5f7b2 100644
--- a/profiles/midi/libmidi.c
+++ b/profiles/midi/libmidi.c
@@ -191,40 +191,47 @@ static void read_ev_sysex(struct midi_write_parser *parser, const snd_seq_event_
 static void read_ev_others(struct midi_write_parser *parser, const snd_seq_event_t *ev,
                            midi_read_ev_cb write_cb, void *user_data)
 {
-	int length;
+	uint8_t msg[MIDI_MSG_MAX_SIZE];
+	long length;
+	bool status;
 
-	/* check for running status */
-	if (parser->rstatus != ev->type) {
+	/* running status only ever continues the same event type */
+	if (parser->rstatus != ev->type)
 		snd_midi_event_reset_decode(parser->midi_ev);
-		append_timestamp_low(parser);
-	}
 
-	/* each midi message has timestampLow byte to follow */
-	length = snd_midi_event_decode(parser->midi_ev,
-	                               parser->midi_stream.data +
-	                               parser->midi_stream.len,
-	                               parser_get_available_size(parser),
-	                               ev);
-
-	if (length == -ENOMEM) {
-		/* remove previously added timestampLow */
-		if (parser->rstatus != ev->type)
-			parser->midi_stream.len--;
+	length = snd_midi_event_decode(parser->midi_ev, msg, sizeof(msg), ev);
+	if (length <= 0)
+		return;
+
+	/*
+	 * The decoder leaves the status byte out only when it repeats the
+	 * previous one, channel included, and every status byte it does write
+	 * needs a timestampLow before it. Decide that from the decoded message,
+	 * not from the event type: two note-ons on different channels share a
+	 * type but not a status byte.
+	 */
+	status = msg[0] & 0x80;
+
+	if (parser_get_available_size(parser) < (size_t) length + status) {
 		write_cb(parser, user_data);
-		/* cleanup state for next packet */
-		snd_midi_event_reset_decode(parser->midi_ev);
 		midi_write_reset(parser);
 		append_timestamp_high_maybe(parser);
-		append_timestamp_low(parser);
-		length = snd_midi_event_decode(parser->midi_ev,
-		                               parser->midi_stream.data +
-		                               parser->midi_stream.len,
-		                               parser_get_available_size(parser),
-		                               ev);
+
+		/* a new packet cannot continue running status */
+		if (!status) {
+			snd_midi_event_reset_decode(parser->midi_ev);
+			length = snd_midi_event_decode(parser->midi_ev, msg,
+							sizeof(msg), ev);
+			if (length <= 0)
+				return;
+			status = true;
+		}
 	}
 
-	if (length > 0)
-		parser->midi_stream.len += length;
+	if (status)
+		append_timestamp_low(parser);
+
+	buffer_append_data(&parser->midi_stream, msg, length);
 }
 
 void midi_read_ev(struct midi_write_parser *parser, const snd_seq_event_t *ev,
-- 
2.43.0