net/http: export Header.Clone, reduce its allocations, use it everywhere

Fixes #29915

Change-Id: I6e6edf4f9a0e062211f74d120ae1a242bce1b274
Reviewed-on: https://go-review.googlesource.com/c/go/+/173658
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ingo Oeser <nightlyone@googlemail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

1 files changed, 1 insertions, 11 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index 0e0731b08f..3c522b2af4 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -132,16 +132,6 @@ func copyHeader(dst, src http.Header) {
 	}
 }
 
-func cloneHeader(h http.Header) http.Header {
-	h2 := make(http.Header, len(h))
-	for k, vv := range h {
-		vv2 := make([]string, len(vv))
-		copy(vv2, vv)
-		h2[k] = vv2
-	}
-	return h2
-}
-
 // Hop-by-hop headers. These are removed when sent to the backend.
 // As of RFC 7230, hop-by-hop headers are required to appear in the
 // Connection header field. These are the headers defined by the
@@ -211,7 +201,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
 
-	outreq.Header = cloneHeader(req.Header)
+	outreq.Header = req.Header.Clone()
 
 	p.Director(outreq)
 	outreq.Close = false