net/http: avoid deferred unlock in ServeMux.shouldRedirect
CL 96575 introduced concurrency protection for
ServeMux.shouldRedirect with a read lock and deferred unlock.
However, the change produced a noticeable regression.
Instead add the suffix "RLocked" to the function name to
declare that we should hold the read lock as a pre-requisite
before calling it, hence avoiding the defer altogether.
Benchmarks:
name old time/op new time/op delta
ServeMux-8 63.3µs ± 0% 54.6µs ± 0% -13.74% (p=0.000 n=9+9)
ServeMux_SkipServe-8 41.4µs ± 2% 32.7µs ± 1% -21.05% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
ServeMux-8 17.3kB ± 0% 17.3kB ± 0% ~ (all equal)
ServeMux_SkipServe-8 0.00B 0.00B ~ (all equal)
name old allocs/op new allocs/op delta
ServeMux-8 360 ± 0% 360 ± 0% ~ (all equal)
ServeMux_SkipServe-8 0.00 0.00 ~ (all equal)