wgengine/magicsock: don't change DERP home when not connected to control

This pretty much always results in an outage because peers won't
discover our new home region and thus won't be able to establish
connectivity.

Updates tailscale/corp#18095

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ic0d09133f198b528dd40c6383b16d7663d9d37a7

wgengine/magicsock/magicsock_test.go

@@ -41,9 +41,11 @@ import (
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/disco"
 	"tailscale.com/envknob"
+	"tailscale.com/health"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/connstats"
 	"tailscale.com/net/netaddr"
+	"tailscale.com/net/netcheck"
 	"tailscale.com/net/packet"
 	"tailscale.com/net/ping"
 	"tailscale.com/net/stun/stuntest"
@@ -3017,3 +3019,118 @@ func TestAddrForPingSizeLocked(t *testing.T) {
 		})
 	}
 }
+
+func TestMaybeSetNearestDERP(t *testing.T) {
+	derpMap := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			1: {
+				RegionID:   1,
+				RegionCode: "test",
+				Nodes: []*tailcfg.DERPNode{
+					{
+						Name:     "t1",
+						RegionID: 1,
+						HostName: "test-node.unused",
+						IPv4:     "127.0.0.1",
+						IPv6:     "none",
+					},
+				},
+			},
+			21: {
+				RegionID:   21,
+				RegionCode: "tor",
+				Nodes: []*tailcfg.DERPNode{
+					{
+						Name:     "21b",
+						RegionID: 21,
+						HostName: "tor.test-node.unused",
+						IPv4:     "127.0.0.1",
+						IPv6:     "none",
+					},
+				},
+			},
+			31: {
+				RegionID:   31,
+				RegionCode: "fallback",
+				Nodes: []*tailcfg.DERPNode{
+					{
+						Name:     "31b",
+						RegionID: 31,
+						HostName: "fallback.test-node.unused",
+						IPv4:     "127.0.0.1",
+						IPv6:     "none",
+					},
+				},
+			},
+		},
+	}
+
+	// Ensure that our fallback code always picks a deterministic value.
+	tstest.Replace(t, &pickDERPFallbackForTests, func() int { return 31 })
+
+	// Actually test this code path.
+	tstest.Replace(t, &checkControlHealthDuringNearestDERPInTests, true)
+
+	testCases := []struct {
+		name               string
+		old                int
+		reportDERP         int
+		connectedToControl bool
+		want               int
+	}{
+		{
+			name:               "connected_with_report_derp",
+			old:                1,
+			reportDERP:         21,
+			connectedToControl: true,
+			want:               21,
+		},
+		{
+			name:               "not_connected_with_report_derp",
+			old:                1,
+			reportDERP:         21,
+			connectedToControl: false,
+			want:               1, // no change
+		},
+		{
+			name:               "connected_no_derp",
+			old:                1,
+			reportDERP:         0,
+			connectedToControl: true,
+			want:               1, // no change
+		},
+		{
+			name:               "connected_no_derp_fallback",
+			old:                0,
+			reportDERP:         0,
+			connectedToControl: true,
+			want:               31, // deterministic fallback
+		},
+	}
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			c := newConn()
+			c.logf = t.Logf
+			c.myDerp = tt.old
+			c.derpMap = derpMap
+
+			report := &netcheck.Report{PreferredDERP: tt.reportDERP}
+
+			oldConnected := health.GetInPollNetMap()
+			if tt.connectedToControl != oldConnected {
+				if tt.connectedToControl {
+					health.GotStreamedMapResponse()
+					t.Cleanup(health.SetOutOfPollNetMap)
+				} else {
+					health.SetOutOfPollNetMap()
+					t.Cleanup(health.GotStreamedMapResponse)
+				}
+			}
+
+			got := c.maybeSetNearestDERP(report)
+			if got != tt.want {
+				t.Errorf("got new DERP region %d, want %d", got, tt.want)
+			}
+		})
+	}
+}