[Discuss-gnuradio] stream_mux tag propagation

[Merlin Chlosta]

Hi all,

I stumbled across the stream_mux tag propagation. In my understanding, tags are 
associated with one item and should be associated with the same item after 
processing, if possible. The stream_mux block however does not propagate tags 
like that:

Say we mux two tagged streams A and B, while A has packet_len_a=2 and B 
packet_len_b=3.

A: a1 a2 a1 a2 a1 …
---^-----^-----^---    Tag packet_len_a=2
B: b1 b2 b3 b1 b2 …
---^--------^------    Tag packet_len_b=3

packet_len tags are associated with a1 / b1 (denoted with ^).
When muxed with the scheme (2, 3), the muxed stream looks like that:

a1 a2 b1 b2 b3 a1 a2 b1 …
^-----^-----^-----^------   Tag packet_len_a=2
^--------^--------^------   Tag packet_len_b=3

The tags got associated to different items! "packet_len=2" is associated with 
every second item, "packet_len=3" with every third item, instead of a1 / b1.

I would assume a correct tag propagation to look like this:
a1 a2 b1 b2 b3 a1 a2 b1 …
^--------------^---------   Tag packet_len_a=2
------^--------------^---   Tag packet_len_b=3

Is that a bug in stream_mux? It means that the streams cannot be demuxed by 
looking at the tags.

A basic python flowgraph is attached, it prints tags and their offsets after 
stream_mux processing.

test.py
Description: Text Data