all: make client use server-provided DERP map, add DERP region support

Instead of hard-coding the DERP map (except for cmd/tailscale netcheck
for now), get it from the control server at runtime.

And make the DERP map support multiple nodes per region with clients
picking the first one that's available. (The server will balance the
order presented to clients for load balancing)

This deletes the stunner package, merging it into the netcheck package
instead, to minimize all the config hooks that would've been
required.

Also fix some test flakes & races.

Fixes #387 (Don't hard-code the DERP map)
Updates #388 (Add DERP region support)
Fixes #399 (wgengine: flaky tests)

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

cmd/tailscale/netcheck.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"log"
 	"sort"
+	"time"
 
 	"github.com/peterbourgon/ff/v2/ffcli"
 	"tailscale.com/derp/derpmap"
@@ -26,12 +27,12 @@ var netcheckCmd = &ffcli.Command{
 
 func runNetcheck(ctx context.Context, args []string) error {
 	c := &netcheck.Client{
-		DERP:     derpmap.Prod(),
 		Logf:     logger.WithPrefix(log.Printf, "netcheck: "),
 		DNSCache: dnscache.Get(),
 	}
 
-	report, err := c.GetReport(ctx)
+	dm := derpmap.Prod()
+	report, err := c.GetReport(ctx, dm)
 	if err != nil {
 		log.Fatalf("netcheck: %v", err)
 	}
@@ -55,18 +56,23 @@ func runNetcheck(ctx context.Context, args []string) error {
 	// When DERP latency checking failed,
 	// magicsock will try to pick the DERP server that
 	// most of your other nodes are also using
-	if len(report.DERPLatency) == 0 {
+	if len(report.RegionLatency) == 0 {
 		fmt.Printf("\t* Nearest DERP: unknown (no response to latency probes)\n")
 	} else {
-		fmt.Printf("\t* Nearest DERP: %v (%v)\n", report.PreferredDERP, c.DERP.LocationOfID(report.PreferredDERP))
+		fmt.Printf("\t* Nearest DERP: %v (%v)\n", report.PreferredDERP, dm.Regions[report.PreferredDERP].RegionCode)
 		fmt.Printf("\t* DERP latency:\n")
-		var ss []string
-		for s := range report.DERPLatency {
-			ss = append(ss, s)
+		var rids []int
+		for rid := range dm.Regions {
+			rids = append(rids, rid)
 		}
-		sort.Strings(ss)
-		for _, s := range ss {
-			fmt.Printf("\t\t- %s = %v\n", s, report.DERPLatency[s])
+		sort.Ints(rids)
+		for _, rid := range rids {
+			d, ok := report.RegionLatency[rid]
+			var latency string
+			if ok {
+				latency = d.Round(time.Millisecond / 10).String()
+			}
+			fmt.Printf("\t\t- %v, %3s = %s\n", rid, dm.Regions[rid].RegionCode, latency)
 		}
 	}
 	return nil