cmd/derpprobe,prober: add ability to restrict derpprobe to a single region

Updates #24522

Co-authored-by: Mario Minardi <mario@tailscale.com>
Signed-off-by: Percy Wegmann <percy@tailscale.com>

prober/derp_test.go

@@ -44,6 +44,19 @@ func TestDerpProber(t *testing.T) {
 					},
 				},
 			},
+			1: {
+				RegionID:   1,
+				RegionCode: "one",
+				Nodes: []*tailcfg.DERPNode{
+					{
+						Name:     "n3",
+						RegionID: 0,
+						HostName: "derpn3.tailscale.test",
+						IPv4:     "1.1.1.1",
+						IPv6:     "::1",
+					},
+				},
+			},
 		},
 	}
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -68,6 +81,7 @@ func TestDerpProber(t *testing.T) {
 		meshProbeFn:  func(_, _ string) ProbeClass { return FuncProbe(func(context.Context) error { return nil }) },
 		nodes:        make(map[string]*tailcfg.DERPNode),
 		probes:       make(map[string]*Probe),
+		regionCode:   "zero",
 	}
 	if err := dp.probeMapFn(context.Background()); err != nil {
 		t.Errorf("unexpected probeMapFn() error: %s", err)
@@ -84,9 +98,9 @@ func TestDerpProber(t *testing.T) {
 
 	// Add one more node and check that probes got created.
 	dm.Regions[0].Nodes = append(dm.Regions[0].Nodes, &tailcfg.DERPNode{
-		Name:     "n3",
+		Name:     "n4",
 		RegionID: 0,
-		HostName: "derpn3.tailscale.test",
+		HostName: "derpn4.tailscale.test",
 		IPv4:     "1.1.1.1",
 		IPv6:     "::1",
 	})
@@ -113,6 +127,19 @@ func TestDerpProber(t *testing.T) {
 	if len(dp.probes) != 4 {
 		t.Errorf("unexpected probes: %+v", dp.probes)
 	}
+
+	// Stop filtering regions.
+	dp.regionCode = ""
+	if err := dp.probeMapFn(context.Background()); err != nil {
+		t.Errorf("unexpected probeMapFn() error: %s", err)
+	}
+	if len(dp.nodes) != 2 {
+		t.Errorf("unexpected nodes: %+v", dp.nodes)
+	}
+	// 6 regular probes + 2 mesh probe
+	if len(dp.probes) != 8 {
+		t.Errorf("unexpected probes: %+v", dp.probes)
+	}
 }
 
 func TestRunDerpProbeNodePair(t *testing.T) {