Initial open fd monitoring w/ cell write lock (SYN-10120)

[vEpiphyte]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 80.88235% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.70%. Comparing base (27398a7) to head (a4c3288).

Files with missing lines	Patch %	Lines
synapse/lib/platforms/darwin.py	0.00%	13 Missing ⚠️

❌ Your patch check has failed because the patch coverage (80.88%) is below the target coverage (97.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4684      +/-   ##
==========================================
- Coverage   97.72%   97.70%   -0.02%     
==========================================
  Files         296      296              
  Lines       62151    62209      +58     
==========================================
+ Hits        60736    60782      +46     
- Misses       1415     1427      +12     

Flag	Coverage Δ
linux	97.62% <80.88%> (-0.03%)	⬇️
linux_replay	93.37% <80.88%> (-0.02%)	⬇️

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[MichaelSquires]

We should disable/not start the FD monitoring if the limit is already set to unlimited.

[vEpiphyte]

@MichaelSquires that would be safe if we assume that no one will ever change the soft limit of the process while it is running. If we do decide to bail in that case we should include in docs that monitoring isn't done if the soft limit is set to unlimited.