[MRG] feat: add PASS_CONTEXTVARS

[r1b]

Reference issue

#578

Tasks

• Unit tests added that reproduce issue or prove feature is working
• Fix or feature added
• Documentation and examples updated (if relevant)
• Unit tests passing and coverage at 100% after adding fix/feature
• Apps updated and tested (if relevant)

---

To expand on #578:

We'd like to be able to capture pynetdicom logs and filter for "all logs produced by associations in a particular calling thread". For example, if we're running a threaded storage scp we'd like to capture log output and filter for "all logs produced by a particular request thread".

Python stamps LogRecords with a threadId but pynetdicom spawns multiple threads over the course of handling an association. Some logs will be produced in the calling thread, some in the DUL thread(s) and some in the Association thread(s).

Python does not have a way to define "thread groups" nor does it maintain a reference to the calling thread in spawned threads. We can hack around those things (e.g: grouping threads by name, subclassing Thread) but in the future, pynetdicom might not use threads for concurrency at all #527. We want a forward-looking solution that will work for concurrency models that might re-use threads. We'd also like to make minimal changes to pynetdicom, so avoiding things like e.g passing logger instances around or adding additional log handlers / filtering logic there.

We think contextvars can achieve the goal here. We can attach logger filters that refer to ContextVar instances defined outside of pynetdicom and effectively hook into the logger calls inside of pynetdicom. To make this work, we need to pass along the current context to each thread spawned by pynetdicom. This is the default behavior in asyncio, so it should play nicely with a future implementation of #527.

Let me know what you think!

[codecov]

Codecov Report

Merging #582 (534c17b) into master (3ca25f6) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 534c17b differs from pull request most recent head 05ff0e2. Consider uploading reports for the commit 05ff0e2 to get more accurate results

@@            Coverage Diff            @@
##            master      #582   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         8426      8438   +12     
=========================================
+ Hits          8426      8438   +12     

Impacted Files	Coverage Δ
pynetdicom/_config.py	100.00% <100.00%> (ø)
pynetdicom/ae.py	100.00% <100.00%> (ø)
pynetdicom/association.py	100.00% <100.00%> (ø)
pynetdicom/dimse.py	100.00% <100.00%> (ø)
pynetdicom/dul.py	100.00% <100.00%> (ø)
pynetdicom/utils.py	100.00% <100.00%> (ø)

[jherman-bfly]

Some clarification would be appreciated for why make_target exists.

[scaramallion]

Could you add something to the release notes as well? And add the new config variable to docs/reference/config.rst and utils.rst?

[r1b]

@scaramallion done

[r1b]

er oops forgot release notes, one sec