[HTTP]Feat: pool for http headers path

[pulkit4tech]

• Description and context for reviewers: one partner, one stranger

Problem

Every outbound RPC call allocates a fresh http.Header map in
createRequest → ToHTTPHeaders. This map is used only to build
and send request headers; after roundTrip returns it is garbage.
In high-throughput services this is a steady stream of short-lived
allocations that pressure the GC.

Solution

Add a package-level sync.Pool of pre-sized http.Header maps
(capacity 16 — enough for YARPC core headers + typical application
headers without growing the backing hash table). Each call() borrows
a map from the pool, passes it through createRequest →
ToHTTPHeaders, then clears and returns it via defer after
roundTrip completes.

net/http serialises request headers to the wire inside roundTrip;
no code path in call() reads the request header map after that
returns, so pool reuse is safe.

Changes

File	Change
transport/http/outbound.go	headerPool (sync.Pool), _headerPoolInitSize = 16; call() borrows/returns map via defer; createRequest accepts pre-allocated http.Header
transport/http/outbound_test.go	Update createRequest call to pass header map
transport/http/createrequest_bench_test.go	Update createRequest calls to pass header map

Benchmarks

End-to-end, -benchmem -benchtime=3s, AMD EPYC 7B13, Go 1.26.1:

BenchmarkHeaderAlloc_EndToEnd_NoAppHeaders 141 → 139 allocs/op (-2) 14302 → 13894 B/op (-408 B)
BenchmarkHeaderAlloc_EndToEnd_10AppHeaders 201 → 195 allocs/op (-6) 21355 → 19052 B/op (-2303 B)

RELEASE NOTES: Pool http.Header maps on the outbound hot path, saving allocations and memory per RPC call

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.22%. Comparing base (78bf565) to head (144d489).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2486      +/-   ##
==========================================
- Coverage   85.23%   85.22%   -0.01%     
==========================================
  Files         281      281              
  Lines       16773    16784      +11     
==========================================
+ Hits        14296    14305       +9     
- Misses       2015     2016       +1     
- Partials      462      463       +1     

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[rabbbit]

what about upgrading yarpc to a modern go? :)

[pulkit4tech]

sure, will do it as follow-up :) Thanks!