http: implement slab allocation for HTTP header parsing

[mertcanaltin]

fix: // TODO(bnoordhuis) Use slab allocation, O(n) allocs is bad.

before:

http/bench-parser-fragmented.js n=100000 len=8 frags=2: 951,629.0928924936
http/bench-parser-fragmented.js n=100000 len=16 frags=2: 677,295.1203426437
http/bench-parser-fragmented.js n=100000 len=8 frags=4: 759,881.7857984888
http/bench-parser-fragmented.js n=100000 len=16 frags=4: 509,278.74034996366
http/bench-parser-fragmented.js n=100000 len=8 frags=8: 680,706.0381048893
http/bench-parser-fragmented.js n=100000 len=16 frags=8: 466,898.1840452692

after:
52% faster HTTP header parsing

http/bench-parser-fragmented.js n=100000 len=8 frags=2: 1,231,534.676937666
http/bench-parser-fragmented.js n=100000 len=16 frags=2: 890,580.6675901335
http/bench-parser-fragmented.js n=100000 len=8 frags=4: 1,117,723.8010822271
http/bench-parser-fragmented.js n=100000 len=16 frags=4: 789,398.1180369954
http/bench-parser-fragmented.js n=100000 len=8 frags=8: 869,642.0937523921
http/bench-parser-fragmented.js n=100000 len=16 frags=8: 711,293.8345939534

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[Copilot]

Pull request overview

This pull request implements slab allocation for HTTP header parsing to optimize performance by reducing heap allocations. The implementation addresses a TODO comment about O(n) allocations being inefficient and achieves a reported 52% performance improvement in HTTP header parsing benchmarks.

Changes:

• Added a 128-byte fixed-size buffer (slab) to each StringPtr instance for caching small strings
• Modified Save() and Update() methods to use slab allocation before falling back to heap allocation
• Added using_slab_ flag to track slab usage state alongside existing on_heap_ flag

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[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.52%. Comparing base (daeafc0) to head (9115556).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_http_parser.cc	90.90%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #61375   +/-   ##
=======================================
  Coverage   88.51%   88.52%           
=======================================
  Files         704      704           
  Lines      208776   208790   +14     
  Branches    40301    40301           
=======================================
+ Hits       184803   184832   +29     
+ Misses      15966    15925   -41     
- Partials     8007     8033   +26     

Files with missing lines	Coverage Δ
src/node_http_parser.cc	84.94% <90.90%> (+0.14%)	⬆️

... and 44 files with indirect coverage changes

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

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[Copilot]

The memory calculation is slightly inaccurate. 66 × 128 = 8,448 bytes, which is approximately 8.25KB, not 8KB. Consider updating to '~8.4KB' or '~8.5KB' for better accuracy.

Suggested change

	// Memory impact: ~8KB per parser (66 StringPtr × 128 bytes).
	// Memory impact: ~8.4KB per parser (66 StringPtr × 128 bytes).

[jasnell]

Thank you overly pedantic AI code review. The ~8KB is accurate enough here.

[mertcanaltin]

Yes, I agree.