bgzf threads not configurable

[eseiler]

Platform

• SeqAn version: master
• Operating system: Linux 5.8.0-rc5+ [INTERNAL] Renamed sequence file headers. seqan3#1 SMP Fri Jul 17 19:07:45 CEST 2020 ppc64le ppc64le ppc64le GNU/Linux
• Compiler: all

Description

https://github.com/seqan/seqan3/blob/d37ba83a333992a551ec835097e564d8b9a00604/include/seqan3/contrib/stream/bgzf_stream_util.hpp#L41

We always use all threads when using bgzf stuff.
In conjunction with seqan/seqan3#2278, it makes the nightly situation worse.

How to repeat the problem

Run any executable making use of bgzf

Expected behaviour

I want to be able to not use all threads.
I think we had a similar problem in seqan2 and solved it via some definition?

Actual behaviour

Uses all threads.

[h-2]

Workaround: You should be able to just set the global variable to a different value to change the behaviour globally.
Fix: I have changed the code in the bgzf handling to expose the thread number as a constructor argument, but this would need to be tied into the configuration system of I/O which is still undecided (AFAIK).

edit: I fixed it locally I meant. Hope to have time soon to make a PR.

[eseiler]

This was addressed in seqan/seqan3#2493 which added an example on how to configure the threads.

See here for some thoughts: seqan/seqan3#2455 (comment)
We do not want a global variable, but it's also not easy to elegantly include this setting.

[marehr]

For now, it is documented, but we want to have a finer control per file-instance. I'll move this issue to product_backlog, to remind us in the design.

[eseiler]

seqan/seqan3#2752