prober: support probing DERP regions with nodes in mixed VPCs

Updates tailscale/corp#24232

Change-Id: Ief50a337b61b5791f03d48d7274f4e9c25e5656a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

prober/derp.go

@@ -172,6 +172,15 @@ func (d *derpProber) probeMapFn(ctx context.Context) error {
 			}
 
 			for _, to := range region.Nodes {
+				if !inSameVPC(server, to) {
+					// This pair of nodes in the region are in different VPCs; don't expect
+					// them to be meshed. In the general case, the DERPMap data structure and client
+					// don't support such a region configuration. This is a special case for
+					// Tailscale's probers being able to monitor a meta DERP map that's a superset
+					// of multiple independent DERP maps.
+					continue
+				}
+
 				if d.meshInterval > 0 {
 					n := fmt.Sprintf("derp/%s/%s/%s/mesh", region.RegionCode, server.Name, to.Name)
 					wantProbes[n] = true
@@ -204,6 +213,24 @@ func (d *derpProber) probeMapFn(ctx context.Context) error {
 	return nil
 }
 
+func inSameVPC(a, b *tailcfg.DERPNode) bool {
+	return vpcName(a.Name) == vpcName(b.Name)
+}
+
+// vpcName returns the name of a VPC that a DERP Node is running
+// in as a function of its DERPNode.Name.
+//
+// This is a naming convention for the Tailscale-run DERP nodes only
+// and is not a thing that's supported by clients. See the comments
+// at the caller of this.
+func vpcName(n string) string {
+	_, name, ok := strings.Cut(n, "-vpc")
+	if !ok || name == "" {
+		return "default"
+	}
+	return name
+}
+
 // probeMesh returs a probe class that sends a test packet through a pair of DERP
 // servers (or just one server, if 'from' and 'to' are the same). 'from' and 'to'
 // are expected to be names (DERPNode.Name) of two DERP servers in the same region.

prober/derp_test.go

@@ -21,7 +21,7 @@
 	"tailscale.com/types/key"
 )
 
-func TestDerpProber(t *testing.T) {
+func TestDERPProber(t *testing.T) {
 	dm := &tailcfg.DERPMap{
 		Regions: map[int]*tailcfg.DERPRegion{
 			0: {
@@ -210,3 +210,19 @@ func Test_packetsForSize(t *testing.T) {
 		})
 	}
 }
+
+func TestInSameVPC(t *testing.T) {
+	tests := []struct {
+		a, b string
+		want bool
+	}{
+		{"foo", "bar", true},
+		{"foo-vpc1", "bar-vpc1", true},
+		{"foo-vpc1", "bar-vpc2", false},
+	}
+	for _, tt := range tests {
+		if got := inSameVPC(&tailcfg.DERPNode{Name: tt.a}, &tailcfg.DERPNode{Name: tt.b}); got != tt.want {
+			t.Errorf("inSameVPC(%q, %q) = %v; want %v", tt.a, tt.b, got, tt.want)
+		}
+	}
+}