ipn/ipnstate: add home DERP to tailscale status JSON

Fixes #15625

Change-Id: Ic20dad2dab4ac52c666057845bdc3cf5c0ffcd8f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnstate/ipnstate.go

@@ -53,6 +53,9 @@ type Status struct {
 	// If nil, an exit node is not in use.
 	ExitNodeStatus *ExitNodeStatus `json:"ExitNodeStatus,omitempty"`
 
+	// DERPHomeRegionID is the current home DERP region ID.
+	DERPHomeRegionID int
+
 	// Health contains health check problems.
 	// Empty means everything is good. (or at least that no known
 	// problems are detected)

wgengine/magicsock/magicsock.go

@@ -2892,11 +2892,15 @@ func (c *Conn) UpdateStatus(sb *ipnstate.StatusBuilder) {
 		})
 	}
 
-	c.foreachActiveDerpSortedLocked(func(node int, ad activeDerp) {
-		// TODO(bradfitz): add a method to ipnstate.StatusBuilder
-		// to include all the DERP connections we have open
-		// and add it here. See the other caller of foreachActiveDerpSortedLocked.
+	sb.MutateStatus(func(s *ipnstate.Status) {
+		s.DERPHomeRegionID = c.myDerp
+		c.foreachActiveDerpSortedLocked(func(node int, ad activeDerp) {
+			// TODO(bradfitz): add a method to ipnstate.StatusBuilder
+			// to include all the DERP connections we have open
+			// and add it here. See the other caller of foreachActiveDerpSortedLocked.
+		})
 	})
+
 }
 
 // SetStatistics specifies a per-connection statistics aggregator.