net/http: re-simplify HTTP/1.x status line writing

It used to be simple, and then it got complicated for speed (to reduce
allocations, mostly), but that involved a mutex and hurt multi-core
performance, contending on the mutex.

A change was sent to try to improve that mutex contention in
https://go-review.googlesource.com/c/42110/2/src/net/http/server.go
but that introduced its own allocations (the string->interface{}
boxing for the sync.Map key), which runs counter to the whole point of
that statusLine function: to remove allocations.

Instead, make the code simple again and not have a mutex. It's a bit
slower for the single-core case, but nobody with a single-user HTTP
server cares about 50 nanoseconds:

name                  old time/op    new time/op    delta
ResponseStatusLine      37.5ns ± 2%    87.1ns ± 2%  +132.42%          (p=0.029 n=4+4)
ResponseStatusLine-2    63.1ns ± 1%    43.1ns ±12%   -31.67%          (p=0.029 n=4+4)
ResponseStatusLine-4    53.8ns ± 8%    40.2ns ± 2%   -25.29%          (p=0.029 n=4+4)

name                  old alloc/op   new alloc/op   delta
ResponseStatusLine      0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)
ResponseStatusLine-2    0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)
ResponseStatusLine-4    0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)

name                  old allocs/op  new allocs/op  delta
ResponseStatusLine       0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)
ResponseStatusLine-2     0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)
ResponseStatusLine-4     0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)

(Note the code could be even simpler with fmt.Fprintf, but that is
 relatively slow and involves a bunch of allocations getting arguments
 into interface{} for the call)

Change-Id: I1fa119132dbbf97a8e7204ce3e0707d433060da2
Reviewed-on: https://go-review.googlesource.com/42133
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

1 files changed, 13 insertions, 13 deletions

diff --git a/src/net/http/export_test.go b/src/net/http/export_test.go
index 596171f5f0..98fb0834dd 100644
--- a/src/net/http/export_test.go
+++ b/src/net/http/export_test.go
@@ -17,17 +17,19 @@ import (
 )
 
 var (
-	DefaultUserAgent             = defaultUserAgent
-	NewLoggingConn               = newLoggingConn
-	ExportAppendTime             = appendTime
-	ExportRefererForURL          = refererForURL
-	ExportServerNewConn          = (*Server).newConn
-	ExportCloseWriteAndWait      = (*conn).closeWriteAndWait
-	ExportErrRequestCanceled     = errRequestCanceled
-	ExportErrRequestCanceledConn = errRequestCanceledConn
-	ExportServeFile              = serveFile
-	ExportScanETag               = scanETag
-	ExportHttp2ConfigureServer   = http2ConfigureServer
+	DefaultUserAgent                  = defaultUserAgent
+	NewLoggingConn                    = newLoggingConn
+	ExportAppendTime                  = appendTime
+	ExportRefererForURL               = refererForURL
+	ExportServerNewConn               = (*Server).newConn
+	ExportCloseWriteAndWait           = (*conn).closeWriteAndWait
+	ExportErrRequestCanceled          = errRequestCanceled
+	ExportErrRequestCanceledConn      = errRequestCanceledConn
+	ExportServeFile                   = serveFile
+	ExportScanETag                    = scanETag
+	ExportHttp2ConfigureServer        = http2ConfigureServer
+	Export_shouldCopyHeaderOnRedirect = shouldCopyHeaderOnRedirect
+	Export_writeStatusLine            = writeStatusLine
 )
 
 func init() {
@@ -188,8 +190,6 @@ func ExportHttp2ConfigureTransport(t *Transport) error {
 	return nil
 }
 
-var Export_shouldCopyHeaderOnRedirect = shouldCopyHeaderOnRedirect
-
 func (s *Server) ExportAllConnsIdle() bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()