[Abacn]

WriteImpl._expand_bounded previously applied WindowInto(GlobalWindows()) unconditionally to all input records before writing bundles, even when the input PCollection was already using the default global windowing strategy. In typical batch pipelines, this added an unnecessary per-element WindowInto pass across the entire dataset (~22% overhead in bounded write benchmarks).

Check windowing.is_default() at pipeline expansion time and only apply WindowInto(GlobalWindows()) when the input PCollection is not already in the default global window.

========

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

Benchmark	Baseline Median (sec / elem)	This change (sec / elem)	Speedup	Time Reduction
WriteImplBoundedPipeline	1.643e-05	1.281e-05	1.28x	-22.0%

[Abacn]

context: when benchmarking a batch FileIO write pipeline (https://github.com/Abacn/apache-beam-sstable), this path alone is found to cost 10% of total CPU time

While #40910 is a more generic fix for global window instantiate, for FileIO write batch, we can skip this overhead altogether and save 10% CPU time

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.08%. Comparing base (12bbb17) to head (6d26e8c).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/io/iobase.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #40911      +/-   ##
==========================================
+ Coverage   79.83%   80.08%   +0.25%     
==========================================
  Files         494      494              
  Lines       83575    83586      +11     
==========================================
+ Hits        66722    66944     +222     
+ Misses      16853    16642     -211     

Flag	Coverage Δ
python	80.08% <75.00%> (+0.25%)	⬆️

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[github-actions]

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