Remove allocations of lists before use (#1989)

* policy: remove allocs before appends in acls

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* notifier: make batcher tests stable/non-flaky

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* {db,derp,mapper}: dont allocate until append

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

---------

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/notifier/notifier_test.go

@@ -3,6 +3,7 @@ package notifier
 import (
 	"context"
 	"net/netip"
+	"sort"
 	"testing"
 	"time"
 
@@ -221,6 +222,11 @@ func TestBatcher(t *testing.T) {
 					// We will call flush manually for the tests,
 					// so do not run the worker.
 					BatchChangeDelay: time.Hour,
+
+					// Since we do not load the config, we wont get the
+					// default, so set it manually so we dont time out
+					// and have flakes.
+					NotifierSendTimeout: time.Second,
 				},
 			})
 
@@ -241,6 +247,16 @@ func TestBatcher(t *testing.T) {
 				got = append(got, out)
 			}
 
+			// Make the inner order stable for comparison.
+			for _, u := range got {
+				sort.Slice(u.ChangeNodes, func(i, j int) bool {
+					return u.ChangeNodes[i] < u.ChangeNodes[j]
+				})
+				sort.Slice(u.ChangePatches, func(i, j int) bool {
+					return u.ChangePatches[i].NodeID < u.ChangePatches[j].NodeID
+				})
+			}
+
 			if diff := cmp.Diff(tt.want, got, util.Comparers...); diff != "" {
 				t.Errorf("batcher() unexpected result (-want +got):\n%s", diff)
 			}