cmd/tailscale, magicsock: add debug command to flip DERP homes

For testing netmap patchification server-side.

Updates #1909

Change-Id: Ib1d784bd97b8d4a31e48374b4567404aae5280cc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2715,6 +2715,33 @@ func (c *Conn) getPinger() *ping.Pinger {
 	})
 }
 
+// DebugPickNewDERP picks a new DERP random home temporarily (even if just for
+// seconds) and reports it to control. It exists to test DERP home changes and
+// netmap deltas, etc. It serves no useful user purpose.
+func (c *Conn) DebugPickNewDERP() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	dm := c.derpMap
+	if dm == nil {
+		return errors.New("no derpmap")
+	}
+	if c.netInfoLast == nil {
+		return errors.New("no netinfo")
+	}
+	for _, r := range dm.Regions {
+		if r.RegionID == c.myDerp {
+			continue
+		}
+		c.logf("magicsock: [debug] switching derp home to random %v (%v)", r.RegionID, r.RegionCode)
+		go c.setNearestDERP(r.RegionID)
+		ni2 := c.netInfoLast.Clone()
+		ni2.PreferredDERP = r.RegionID
+		c.callNetInfoCallbackLocked(ni2)
+		return nil
+	}
+	return errors.New("too few regions")
+}
+
 // portableTrySetSocketBuffer sets SO_SNDBUF and SO_RECVBUF on pconn to socketBufferSize,
 // logging an error if it occurs.
 func portableTrySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {