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

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codecov]

Codecov Report

❌ Patch coverage is 88.70968% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.51%. Comparing base (daeafc0) to head (3d643fe).
⚠️ Report is 76 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_http_parser.cc	88.70%	1 Missing and 6 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #61375    +/-   ##
========================================
  Coverage   88.51%   88.51%            
========================================
  Files         704      704            
  Lines      208776   209024   +248     
  Branches    40301    40361    +60     
========================================
+ Hits       184803   185028   +225     
+ Misses      15966    15963     -3     
- Partials     8007     8033    +26     

Files with missing lines	Coverage Δ
src/node_http_parser.cc	84.72% <88.70%> (-0.08%)	⬇️

... and 74 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Copilot]

Pull request overview

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

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[addaleax]

This overall seems like we're re-inventing MaybeStackBuffer here – I think we could just re-use that?

[mertcanaltin]

Thanks! , I've refactored this to use MaybeStackBuffer as the backing store for a allocator.

[mertcanaltin]

new benchmark result :

asis (main):

./out/Release/node benchmark/http/bench-parser.js

http/bench-parser.js n=100000 len=4: 1,728,678.1850683796
http/bench-parser.js n=100000 len=8: 1,395,311.5272280464
http/bench-parser.js n=100000 len=16: 981,492.7277523204
http/bench-parser.js n=100000 len=32: 641,907.2347759664

./out/Release/node benchmark/http/bench-parser-fragmented.js

http/bench-parser-fragmented.js n=100000 frags=2 len=8: 1,092,175.0170709684
http/bench-parser-fragmented.js n=100000 frags=4 len=8: 883,046.0674095292
http/bench-parser-fragmented.js n=100000 frags=8 len=8: 772,164.5013385202
http/bench-parser-fragmented.js n=100000 frags=2 len=16: 766,964.5365185371
http/bench-parser-fragmented.js n=100000 frags=4 len=16: 640,379.4448008832
http/bench-parser-fragmented.js n=100000 frags=8 len=16: 520,572.0305757982

new:

./out/Release/node benchmark/http/bench-parser.js

http/bench-parser.js n=100000 len=4: 1,764,556.6527588428
http/bench-parser.js n=100000 len=8: 1,500,760.6980788389
http/bench-parser.js n=100000 len=16: 1,095,013.3922327897
http/bench-parser.js n=100000 len=32: 646,285.1635436564

./out/Release/node benchmark/http/bench-parser-fragmented.js

http/bench-parser-fragmented.js n=100000 frags=2 len=8: 1,361,774.7618279774
http/bench-parser-fragmented.js n=100000 frags=4 len=8: 1,263,148.5873261988
http/bench-parser-fragmented.js n=100000 frags=8 len=8: 1,056,199.0301452405
http/bench-parser-fragmented.js n=100000 frags=2 len=16: 1,021,757.915898309
http/bench-parser-fragmented.js n=100000 frags=4 len=16: 923,235.2934387976
http/bench-parser-fragmented.js n=100000 frags=8 len=16: 826,609.3826364499

[mertcanaltin]

uh sorry, I'm forget send benchmark file

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70923/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70931/