fix: Data leak in ThreadingIntegration between threads

[antonpirker]

It is possible to leak data from started threads into the main thread via the scopes. (Because the same scope object from the main thread could be changed in the started thread.)
This change always makes a fork (copy) of the scopes of the main thread before it propagates those scopes into the started thread.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.55%. Comparing base (706d2d2) to head (ca8aa30).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4281      +/-   ##
==========================================
+ Coverage   79.52%   79.55%   +0.03%     
==========================================
  Files         142      142              
  Lines       15905    15917      +12     
  Branches     2721     2722       +1     
==========================================
+ Hits        12648    12663      +15     
  Misses       2390     2390              
+ Partials      867      864       -3     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/threading.py	89.33% <100.00%> (+2.03%)	⬆️

... and 4 files with indirect coverage changes

[szokeasaurusrex]

lgtm, but check with @sl0thentr0py first (I was unclear about the outcome from the slack thread discussion)

[sentrivana]

Nice! Please check comments

[sentrivana]

Since this is happening in Thread.start, we might log this even if a thread is started outside of Django -- but I don't think we have a way of detecting that. And since this is warnings.warn message, it should only be printed once and not spam on every Thread.start, so hopefully this is fine.

[antonpirker]

the problem with the unawaited future is only happening if old python, django and channels are used in combination. on a vanilla python project on old python versions the threading works fine.

[sentrivana]

Yeah, my point was more that someone might have Django/channels installed in their venv but uses threads in an unrelated script that has nothing to do with their Django app and they would get this warning too. But as said, I don't see a way around this

[antonpirker]

now I get it. yes that is true. but as you said, the warning is only emitted once, so I think it is ok.