net: use strings.LastIndex to parse header

Use strings.LastIndex instead of strings.Split to reduce memory allocations.

See #3403 for details.

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

net/net.go

@@ -90,8 +90,10 @@ func RemoteAddrFromLast(r *http.Request) netip.Addr {
 // RemoteHostFromLast is *deprecated* use RemoteAddrFromLast instead
 func RemoteHostFromLast(r *http.Request) net.IP {
 	ffs := r.Header.Get("X-Forwarded-For")
-	ffa := strings.Split(ffs, ",")
-	ff := ffa[len(ffa)-1]
+	ff := ffs
+	if i := strings.LastIndex(ffs, ","); i != -1 {
+		ff = ffs[i+1:]
+	}
 	if ff != "" {
 		if ip := parse(strings.TrimSpace(ff)); ip != nil {
 			return ip

net/net_test.go

@@ -147,8 +147,11 @@ func TestRemoteHostFromLast(t *testing.T) {
 		{"no header2", "1.2.3.4", net.IPv4(1, 2, 3, 4), []string{}},
 		{"no header3", "100.200.300.400", nil, []string{}},
 		{"no header4", "127.0.0.1:8080", net.IPv4(127, 0, 0, 1), []string{}},
+		{"empty header", "127.0.0.1", net.IPv4(127, 0, 0, 1), []string{""}},
 		{"single header", "127.0.0.1", net.IPv4(172, 16, 0, 1), []string{"172.16.0.1"}},
-		{"invalid  header", "127.0.0.1", net.IPv4(127, 0, 0, 1), []string{"invalid header"}},
+		{"invalid header", "127.0.0.1", net.IPv4(127, 0, 0, 1), []string{"invalid header"}},
+		{"invalid header entry", "127.0.0.1", net.IPv4(127, 0, 0, 1), []string{"172.16.0.1", "invalid header"}},
+		{"empty header entry", "127.0.0.1", net.IPv4(127, 0, 0, 1), []string{"172.16.0.1", ""}},
 		{"multiple entries", "127.0.0.1", net.IPv4(8, 7, 6, 5), []string{"172.16.0.1", "1.2.3.4", "8.7.6.5"}},
 		{"2 entries", "127.0.0.1", net.IPv4(8, 7, 6, 5), []string{"1.2.3.4", "8.7.6.5"}},
 		{"single header2", "127.0.0.1", net.IPv4(8, 7, 6, 5), []string{"8.7.6.5"}},