http: improve performance for end() with known-length string

[pimterry]

When you call end(data) on an outgoing HTTP request, we previously wrote the data, and then ran _send('', ...) just to trigger flushing the headers, which resulted in an extra zero-byte write call and bonus nextTick before 'finish'.

When the length is known and content-length is used (not chunking, so we don't need a terminator) we can skip this completely: prep the content-length header explicitly, and combine the finish into the existing write step.

This drops a nextTick, and goes from 2 write calls to 1 for the very common case of response.end(data) with a string, and for buffer data goes from 3 writes to 2.

In practice, in the tiny benchmark here which I think represents a very common use case (small API responses) I see this boosting HTTP RPS by up to 9% for writes up to 1KB or so. For larger responses the response write time starts to outweigh this, but I still see 3% RPS boost at 16KB. Needs autocannon to hit the rates required, you can test yourself with:

node benchmark/compare.js --old ./node-main --new ./node-pr --runs 20 --set benchmarker=autocannon --filter end-string http

Added a new test confirming the specific details and edge cases this touches, and updated one existing test which specifically asserted that end(data) triggered two writes, including the empty one.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net
• @nodejs/performance

[mcollina]

lgtm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (15940ef) to head (659d6b9).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65466      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         751      751              
  Lines      252298   252333      +35     
  Branches    47446    47449       +3     
==========================================
+ Hits       227436   227451      +15     
- Misses      16175    16186      +11     
- Partials     8687     8696       +9     

Files with missing lines	Coverage Δ
lib/_http_outgoing.js	97.76% <100.00%> (-0.02%)	⬇️

... and 34 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76211/