[Abacn]

GlobalWindow implements the singleton pattern via __new__ by caching and returning cls._instance. However, because Python always invokes __init__ on the instance returned by __new__ (whenever __new__ returns an instance of cls), defining GlobalWindow.__init__ to call
super().__init__(GlobalWindow._getTimestampFromProto()) caused every GlobalWindow() call to re-parse the proto timestamp constant, allocate a new Timestamp object, and re-initialize BoundedWindow._end.

Move the one-time BoundedWindow.__init__ initialization into GlobalWindow.__new__ when cls._instance is None and make GlobalWindow.__init__ a no-op.

==========

Benchmark Results (source: https://gist.github.com/Abacn/93fa3168450bf36838e70d853a77d2e3)

Benchmark	Baseline Median (sec / elem)	Post-Fix Median (sec / elem)	Speedup	Time Reduction
GlobalWindowInstantiate	5.711e-07	7.120e-08	8.02x	-87.5%
GlobalWindowsAssign	7.567e-07	2.399e-07	3.15x	-68.3%
WindowIntoWorker_GlobalWindows	4.144e-06	2.934e-06	1.41x	-29.2%
WindowIntoWorker_FixedWindows	5.970e-06	5.954e-06	1.00x	-0.3%
WriteImplBoundedPipeline	1.643e-05	1.522e-05	1.08x	-7.4%

[github-actions]

📋 Review Summary

This PR resolves a performance issue in GlobalWindow where the singleton instance was repeatedly re-initialized via BoundedWindow.__init__ and _getTimestampFromProto() on every GlobalWindow() instantiation. Moving the initialization into __new__ (when _instance is None) and making __init__ a no-op properly preserves singleton semantics and provides a nice performance boost in window assignment.

🔍 General Feedback

• The implementation is clean, Idiomatic for Python __new__-based singletons, and safely retains _end across pickle/unpickle cycles.

---

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[Abacn]

context: when benchmarking a batch FileIO write pipeline (https://github.com/Abacn/apache-beam-sstable), this path alone is found to cost 8.6% of total CPU time

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.08%. Comparing base (12bbb17) to head (f06199b).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #40910      +/-   ##
==========================================
+ Coverage   79.83%   80.08%   +0.24%     
==========================================
  Files         494      494              
  Lines       83575    83583       +8     
==========================================
+ Hits        66722    66934     +212     
+ Misses      16853    16649     -204     

Flag	Coverage Δ
python	80.08% <100.00%> (+0.24%)	⬆️

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