net/http: make copyValues append instead of calling Add

This results in a performance boost:

name          old time/op    new time/op    delta
CopyValues-4    3.46µs ± 3%    1.53µs ± 3%  -55.85%  (p=0.000 n=18+19)

name          old alloc/op   new alloc/op   delta
CopyValues-4    1.52kB ± 0%    0.74kB ± 0%  -51.58%  (p=0.000 n=20+20)

name          old allocs/op  new allocs/op  delta
CopyValues-4      24.0 ± 0%      11.0 ± 0%  -54.17%  (p=0.000 n=20+20)

Fixes #33744.

Change-Id: Ibc653fb076a9a6aaa775fcc9ca720fb90e68cf96
Reviewed-on: https://go-review.googlesource.com/c/go/+/191057
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/http/http_test.go

@@ -9,6 +9,7 @@ package http
 import (
 	"bytes"
 	"internal/testenv"
+	"net/url"
 	"os/exec"
 	"reflect"
 	"testing"
@@ -109,3 +110,28 @@ func TestCmdGoNoHTTPServer(t *testing.T) {
 		}
 	}
 }
+
+var valuesCount int
+
+func BenchmarkCopyValues(b *testing.B) {
+	b.ReportAllocs()
+	src := url.Values{
+		"a": {"1", "2", "3", "4", "5"},
+		"b": {"2", "2", "3", "4", "5"},
+		"c": {"3", "2", "3", "4", "5"},
+		"d": {"4", "2", "3", "4", "5"},
+		"e": {"1", "1", "2", "3", "4", "5", "6", "7", "abcdef", "l", "a", "b", "c", "d", "z"},
+		"j": {"1", "2"},
+		"m": nil,
+	}
+	for i := 0; i < b.N; i++ {
+		dst := url.Values{"a": {"b"}, "b": {"2"}, "c": {"3"}, "d": {"4"}, "j": nil, "m": {"x"}}
+		copyValues(dst, src)
+		if valuesCount = len(dst["a"]); valuesCount != 6 {
+			b.Fatalf(`%d items in dst["a"] but expected 6`, valuesCount)
+		}
+	}
+	if valuesCount == 0 {
+		b.Fatal("Benchmark wasn't run")
+	}
+}

src/net/http/request.go

@@ -1165,9 +1165,7 @@ func (l *maxBytesReader) Close() error {
 
 func copyValues(dst, src url.Values) {
 	for k, vs := range src {
-		for _, value := range vs {
-			dst.Add(k, value)
-		}
+		dst[k] = append(dst[k], vs...)
 	}
 }