net/http/cgi: optimize internal function removeLeadingDuplicates a bit

Change-Id: I0255f24f5c5925ea4daa28a28d23606df35d4373
Reviewed-on: https://go-review.googlesource.com/15824
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/http/cgi/host.go

@@ -77,17 +77,17 @@ type Handler struct {
 //      Env: []string{"SCRIPT_FILENAME=foo.php"},
 //    }
 func removeLeadingDuplicates(env []string) (ret []string) {
-	n := len(env)
-	for i := 0; i < n; i++ {
-		e := env[i]
-		s := strings.SplitN(e, "=", 2)[0]
+	for i, e := range env {
 		found := false
-		for j := i + 1; j < n; j++ {
-			if s == strings.SplitN(env[j], "=", 2)[0] {
+		if eq := strings.IndexByte(e, '='); eq != -1 {
+			keq := e[:eq+1] // "key="
+			for _, e2 := range env[i+1:] {
+				if strings.HasPrefix(e2, keq) {
 					found = true
 					break
 				}
 			}
+		}
 		if !found {
 			ret = append(ret, e)
 		}

src/net/http/cgi/host_test.go

@@ -16,6 +16,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strconv"
 	"strings"
@@ -498,3 +499,25 @@ func TestEnvOverride(t *testing.T) {
 	}
 	runCgiTest(t, h, "GET /test.cgi HTTP/1.0\nHost: example.com\n\n", expectedMap)
 }
+
+func TestRemoveLeadingDuplicates(t *testing.T) {
+	tests := []struct {
+		env  []string
+		want []string
+	}{
+		{
+			env:  []string{"a=b", "b=c", "a=b2"},
+			want: []string{"b=c", "a=b2"},
+		},
+		{
+			env:  []string{"a=b", "b=c", "d", "e=f"},
+			want: []string{"a=b", "b=c", "d", "e=f"},
+		},
+	}
+	for _, tt := range tests {
+		got := removeLeadingDuplicates(tt.env)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("removeLeadingDuplicates(%q) = %q; want %q", tt.env, got, tt.want)
+		}
+	}
+}