addHTTPVarsToReplacer ran SetVar(ctx, "uuid", new(requestID)) for every
request during replacer setup, allocating a *requestID unconditionally even
though it is only ever read by the {http.request.uuid} placeholder. The UUID
value itself was already generated lazily (requestID.String caches on first
call), but the container was not.
Allocate the *requestID on first access instead: the uuid placeholder now
creates and stores it in the request's vars table when absent, so repeated
references within a request still share one instance (and thus one UUID),
while requests that never reference the UUID pay nothing.
Add a benchmark for the per-request replacer setup and a regression test for
the uuid placeholder, which previously had no coverage.
Benchmark (BenchmarkAddHTTPVarsToReplacer, linux/arm64, count=6), common path
where the UUID is never referenced:
before: ~700 ns/op 352 B/op 9 allocs/op
after: ~690 ns/op 336 B/op 8 allocs/op
One 16-byte allocation removed per request; a GC-pressure win rather than a
latency one.
63f4e387c3