pdu: Allow PDU to Tagged Stream to write partial PDUs

[argilo]

Description

The tagged_stream_block class is a wrapper to help make Tagged Stream blocks. Unfortunately, it causes problems for the scheduler by refusing to produce any output unless the input and output buffers are long enough to contain an entire PDU:

gnuradio/gnuradio-runtime/lib/tagged_stream_block.cc

Lines 107 to 109 in 4d3eb9f

	if (d_n_input_items_reqd[i] > ninput_items[i]) {
	return 0;
	}

gnuradio/gnuradio-runtime/lib/tagged_stream_block.cc

Lines 112 to 116 in 4d3eb9f

	int min_output_size = calculate_output_stream_length(d_n_input_items_reqd);
	if (noutput_items < min_output_size) {
	set_min_noutput_items(min_output_size);
	return 0;
	}

If the scheduler observes that a block refuses to produce output and all of its upstream blocks are finished (i.e. won't produce any more output), then it concludes that the block is finished and terminates its worker thread.

For the PDU to Tagged Stream block in particular, this is problematic because the scheduler can terminate it while it still has PDUs waiting in its input queue. This causes test failures in the qa_fecapi_* tests, as noted in #6628. Failures can readily be reproduced by increasing rep to 7 in this test:

gnuradio/gr-fec/python/fec/qa_fecapi_repetition.py

Line 161 in 4d3eb9f

rep = 3

Prior to #323, the PDU to Tagged Stream block did not have this problematic behaviour; if it had PDUs in its input queue, it would always produce output. If there wasn't enough room to write out a full PDU, it would store the rest for later. Here I've rewritten the block to behave as it did before.

Related Issue

Fixes #6628.
Reverts #323.

Which blocks/areas does this affect?

• PDU to Tagged Stream

Testing Done

I verified that all tests that use the PDU to Tagged Stream block still pass, and that the qa_fecapi_repetition test passes even if rep or frame_size are increased.

Checklist

• I have read the CONTRIBUTING document.
• I have squashed my commits to have one significant change per commit.
• I have signed my commits before making this PR
• My code follows the code style of this project. See GREP1.md.
• I have updated the documentation where necessary.
• I have added tests to cover my changes, and all previous tests pass.

[argilo]

the qa_fecapi_repetition test passes even if rep or frame_size are increased

Note: Increasing them too much can still cause trouble for another block (Tagged Stream to PDU):

block_executor :error: sched: <block tagged_stream_to_pdu (9)> is requesting more input data than we can provide.

This block also inherits the problematic behaviours of tagged_stream_block.

[argilo]

I suppose a more comprehensive solution (which would fix all Tagged Stream blocks) might be to instead implement input & output PDU buffering in the tagged_stream_block class. There would be a bit more risk of performance issues in making a change that affects all Tagged Stream blocks. Let me know what you think.

[argilo]

I'll put this into draft while I explore the other option (adding buffering to tagged_stream_block) which I suspect may also solve the problem of Tagged Stream blocks being incompatible with custom buffers.