prober: add a DERP probe manager based on derpprobe

This ensures that each DERP server is probed individually (TLS and STUN)
and also manages per-region mesh probing. Actual probing code has been
copied from cmd/derpprobe.

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

prober/derp_test.go

@@ -0,0 +1,106 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package prober
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"tailscale.com/tailcfg"
+)
+
+func TestDerpProber(t *testing.T) {
+	dm := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			0: {
+				RegionID:   0,
+				RegionCode: "zero",
+				Nodes: []*tailcfg.DERPNode{
+					{
+						Name:     "n1",
+						RegionID: 0,
+						HostName: "derpn1.tailscale.test",
+						IPv4:     "1.1.1.1",
+						IPv6:     "::1",
+					},
+					{
+						Name:     "n2",
+						RegionID: 0,
+						HostName: "derpn2.tailscale.test",
+						IPv4:     "1.1.1.1",
+						IPv6:     "::1",
+					},
+				},
+			},
+		},
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		resp, err := json.Marshal(dm)
+		if err != nil {
+			t.Fatal(err)
+		}
+		w.Write(resp)
+	}))
+	defer srv.Close()
+
+	clk := newFakeTime()
+	p := newForTest(clk.Now, clk.NewTicker)
+	dp := &derpProber{
+		p:           p,
+		derpMapURL:  srv.URL,
+		tlsProbeFn:  func(_ string) ProbeFunc { return func(context.Context) error { return nil } },
+		udpProbeFn:  func(_ string, _ int) ProbeFunc { return func(context.Context) error { return nil } },
+		meshProbeFn: func(_, _ string) ProbeFunc { return func(context.Context) error { return nil } },
+		nodes:       make(map[string]*tailcfg.DERPNode),
+		probes:      make(map[string]*Probe),
+	}
+	if err := dp.ProbeMap(context.Background()); err != nil {
+		t.Errorf("unexpected ProbeMap() error: %s", err)
+	}
+	if len(dp.nodes) != 2 || dp.nodes["derpn1.tailscale.test"] == nil || dp.nodes["derpn2.tailscale.test"] == nil {
+		t.Errorf("unexpected nodes: %+v", dp.nodes)
+	}
+	// Probes expected for two nodes:
+	// - 3 regular probes per node (TLS, UDPv4, UDPv6)
+	// - 4 mesh probes (N1->N2, N1->N1, N2->N1, N2->N2)
+	if len(dp.probes) != 10 {
+		t.Errorf("unexpected probes: %+v", dp.probes)
+	}
+
+	// Add one more node and check that probes got created.
+	dm.Regions[0].Nodes = append(dm.Regions[0].Nodes, &tailcfg.DERPNode{
+		Name:     "n3",
+		RegionID: 0,
+		HostName: "derpn3.tailscale.test",
+		IPv4:     "1.1.1.1",
+		IPv6:     "::1",
+	})
+	if err := dp.ProbeMap(context.Background()); err != nil {
+		t.Errorf("unexpected ProbeMap() error: %s", err)
+	}
+	if len(dp.nodes) != 3 {
+		t.Errorf("unexpected nodes: %+v", dp.nodes)
+	}
+	// 9 regular probes + 9 mesh probes
+	if len(dp.probes) != 18 {
+		t.Errorf("unexpected probes: %+v", dp.probes)
+	}
+
+	// Remove 2 nodes and check that probes have been destroyed.
+	dm.Regions[0].Nodes = dm.Regions[0].Nodes[:1]
+	if err := dp.ProbeMap(context.Background()); err != nil {
+		t.Errorf("unexpected ProbeMap() error: %s", err)
+	}
+	if len(dp.nodes) != 1 {
+		t.Errorf("unexpected nodes: %+v", dp.nodes)
+	}
+	// 3 regular probes + 1 mesh probe
+	if len(dp.probes) != 4 {
+		t.Errorf("unexpected probes: %+v", dp.probes)
+	}
+}