feature/conn25: return expired assignments to address pools

Make it possible to remove the least recently used expired address
assignment from addrAssignments.
Before checking out a new address from the IP pools, return a handful of
expired addresses.

Updates tailscale/corp#39975

Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/addrAssignments_test.go

@@ -4,17 +4,20 @@
 package conn25
 
 import (
+	"fmt"
 	"net/netip"
 	"testing"
 	"time"
 
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 	"tailscale.com/tstest"
 )
 
 func TestAssignmentsExpire(t *testing.T) {
 	clock := tstest.NewClock(tstest.ClockOpts{Start: time.Now()})
 	assignments := addrAssignments{clock: clock}
-	as := addrs{
+	as := &addrs{
 		dst:     netip.MustParseAddr("0.0.0.1"),
 		magic:   netip.MustParseAddr("0.0.0.2"),
 		transit: netip.MustParseAddr("0.0.0.3"),
@@ -64,3 +67,83 @@ func TestAssignmentsExpire(t *testing.T) {
 		t.Fatalf("expected foundAs to expire after now")
 	}
 }
+
+func TestPopExpired(t *testing.T) {
+	clock := tstest.NewClock(tstest.ClockOpts{Start: time.Now()})
+	assignments := addrAssignments{clock: clock}
+	makeAndAddAddrs := func(n int) *addrs {
+		t.Helper()
+		as := &addrs{
+			dst:     netip.MustParseAddr(fmt.Sprintf("0.0.1.%d", n)),
+			magic:   netip.MustParseAddr(fmt.Sprintf("0.0.2.%d", n)),
+			transit: netip.MustParseAddr(fmt.Sprintf("0.0.3.%d", n)),
+			app:     "a",
+			domain:  "example.com.",
+		}
+		err := assignments.insert(as)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return as
+	}
+	// cmp.Diff addrs ignoring expiresAt
+	doDiff := func(want, got *addrs) string {
+		t.Helper()
+		return cmp.Diff(
+			want,
+			got,
+			cmp.AllowUnexported(addrs{}),
+			cmpopts.EquateComparable(netip.Addr{}),
+			cmpopts.IgnoreFields(addrs{}, "expiresAt"),
+		)
+	}
+	testAddrs := []*addrs{}
+	for i := range 2 {
+		testAddrs = append(testAddrs, makeAndAddAddrs(i+1))
+		clock.Advance(1 * time.Second)
+	}
+	if len(assignments.byMagicIP) != 2 {
+		t.Fatalf("test setup wrong")
+	}
+
+	nn := assignments.popExpired(clock.Now())
+	want := &addrs{} // invalid addr
+	if diff := doDiff(want, nn); diff != "" {
+		t.Fatalf("only expired addresses are removed: %s", diff)
+	}
+	if len(assignments.byMagicIP) != 2 {
+		t.Fatalf("nothing should have been removed")
+	}
+	if nn.isValid() {
+		t.Fatal("empty addrs should be invalid")
+	}
+
+	clock.Advance(2 * defaultExpiry) // all addrs are now expired
+
+	want = testAddrs[0]
+	nn = assignments.popExpired(clock.Now())
+	if diff := doDiff(want, nn); diff != "" {
+		t.Fatal(diff)
+	}
+	if len(assignments.byMagicIP) != 1 {
+		t.Fatalf("an assignment should have been removed")
+	}
+
+	want = testAddrs[1]
+	nn = assignments.popExpired(clock.Now())
+	if diff := doDiff(want, nn); diff != "" {
+		t.Fatal(diff)
+	}
+	if len(assignments.byMagicIP) != 0 {
+		t.Fatalf("an assignment should have been removed")
+	}
+
+	want = &addrs{}
+	nn = assignments.popExpired(clock.Now())
+	if diff := doDiff(want, nn); diff != "" {
+		t.Fatal(diff)
+	}
+	if len(assignments.byMagicIP) != 0 {
+		t.Fatalf("there should have been no change")
+	}
+}