tailcfg: add DERPRegion.NoMeasureNoHome, deprecate+document Avoid [cap 115]

Fixes tailscale/corp#24697

Change-Id: Ib81994b5ded3dc87a1eef079eb268906a2acb3f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/captivedetection/endpoints.go

@@ -89,7 +89,7 @@ func availableEndpoints(derpMap *tailcfg.DERPMap, preferredDERPRegionID int, log
 	// Use the DERP IPs as captive portal detection endpoints. Using IPs is better than hostnames
 	// because they do not depend on DNS resolution.
 	for _, region := range derpMap.Regions {
-		if region.Avoid {
+		if region.Avoid || region.NoMeasureNoHome {
 			continue
 		}
 		for _, node := range region.Nodes {

net/netcheck/netcheck.go

@@ -387,6 +387,9 @@ type probePlan map[string][]probe
 func sortRegions(dm *tailcfg.DERPMap, last *Report, preferredDERP int) (prev []*tailcfg.DERPRegion) {
 	prev = make([]*tailcfg.DERPRegion, 0, len(dm.Regions))
 	for _, reg := range dm.Regions {
+		if reg.NoMeasureNoHome {
+			continue
+		}
 		// include an otherwise avoid region if it is the current preferred region
 		if reg.Avoid && reg.RegionID != preferredDERP {
 			continue
@@ -533,7 +536,7 @@ func makeProbePlanInitial(dm *tailcfg.DERPMap, ifState *netmon.State) (plan prob
 	plan = make(probePlan)
 
 	for _, reg := range dm.Regions {
-		if len(reg.Nodes) == 0 {
+		if reg.NoMeasureNoHome || len(reg.Nodes) == 0 {
 			continue
 		}
 

net/netcheck/netcheck_test.go

@@ -455,7 +455,7 @@ func TestMakeProbePlan(t *testing.T) {
 	basicMap := &tailcfg.DERPMap{
 		Regions: map[int]*tailcfg.DERPRegion{},
 	}
-	for rid := 1; rid <= 5; rid++ {
+	for rid := 1; rid <= 6; rid++ {
 		var nodes []*tailcfg.DERPNode
 		for nid := 0; nid < rid; nid++ {
 			nodes = append(nodes, &tailcfg.DERPNode{
@@ -467,8 +467,9 @@ func TestMakeProbePlan(t *testing.T) {
 			})
 		}
 		basicMap.Regions[rid] = &tailcfg.DERPRegion{
-			RegionID: rid,
-			Nodes:    nodes,
+			RegionID:        rid,
+			Nodes:           nodes,
+			NoMeasureNoHome: rid == 6,
 		}
 	}