wgengine/filter: preallocate some hot slices in MatchesFromFilterRules (#4672)

Profiling identified this as a fairly hot path for growing a slice.

Given this is only used in control & when a new packet filter is received, this shouldnt be hot in the client.

wgengine/filter/filter_test.go

@@ -780,6 +780,7 @@ func TestMatchesFromFilterRules(t *testing.T) {
 					Srcs: []netaddr.IPPrefix{
 						netaddr.MustParseIPPrefix("100.64.1.1/32"),
 					},
+					Caps: []CapMatch{},
 				},
 			},
 		},
@@ -809,6 +810,7 @@ func TestMatchesFromFilterRules(t *testing.T) {
 					Srcs: []netaddr.IPPrefix{
 						netaddr.MustParseIPPrefix("100.64.1.1/32"),
 					},
+					Caps: []CapMatch{},
 				},
 			},
 		},
@@ -819,8 +821,11 @@ func TestMatchesFromFilterRules(t *testing.T) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			if !reflect.DeepEqual(got, tt.want) {
+
-				t.Errorf("wrong\n got: %v\nwant: %v\n", got, tt.want)
+			compareIP := cmp.Comparer(func(a, b netaddr.IP) bool { return a == b })
+			compareIPPrefix := cmp.Comparer(func(a, b netaddr.IPPrefix) bool { return a == b })
+			if diff := cmp.Diff(got, tt.want, compareIP, compareIPPrefix); diff != "" {
+				t.Errorf("wrong (-got+want)\n%s", diff)
 			}
 		})
 	}

wgengine/filter/tailcfg.go

@@ -28,7 +28,14 @@ func MatchesFromFilterRules(pf []tailcfg.FilterRule) ([]Match, error) {
 	var erracc error
 
 	for _, r := range pf {
-		m := Match{}
+		// Profiling determined that this function was spending a lot
+		// of time in runtime.growslice. As such, we attempt to
+		// pre-allocate some slices. Multipliers were chosen arbitrarily.
+		m := Match{
+			Srcs: make([]netaddr.IPPrefix, 0, len(r.SrcIPs)),
+			Dsts: make([]NetPortRange, 0, 2*len(r.DstPorts)),
+			Caps: make([]CapMatch, 0, 3*len(r.CapGrant)),
+		}
 
 		if len(r.IPProto) == 0 {
 			m.IPProto = append([]ipproto.Proto(nil), defaultProtos...)