wgengine/magicsock: fix buggy fast path in Conn.SetNetworkMap

Spotted by Maisem.

Fixes #6680

Change-Id: I5fdc01de8b006a1c43a2a4848f69397f54b4453a
Co-authored-By: Maisem Ali <maisem@tailscale.com>
Co-authored-By: Andrew Dunham <andrew@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2499,15 +2499,25 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 		return
 	}
 
-	if c.netMap != nil && nodesEqual(c.netMap.Peers, nm.Peers) {
-		return
+	priorNetmap := c.netMap
+	var priorDebug *tailcfg.Debug
+	if priorNetmap != nil {
+		priorDebug = priorNetmap.Debug
 	}
-
+	debugChanged := !reflect.DeepEqual(priorDebug, nm.Debug)
 	metricNumPeers.Set(int64(len(nm.Peers)))
 
-	c.logf("[v1] magicsock: got updated network map; %d peers", len(nm.Peers))
+	// Update c.netMap regardless, before the following early return.
 	c.netMap = nm
 
+	if priorNetmap != nil && nodesEqual(priorNetmap.Peers, nm.Peers) && !debugChanged {
+		// The rest of this function is all adjusting state for peers that have
+		// changed. But if the set of peers is equal and the debug flags (for
+		// silent disco) haven't changed, no need to do anything else.
+		return
+	}
+
+	c.logf("[v1] magicsock: got updated network map; %d peers", len(nm.Peers))
 	heartbeatDisabled := debugEnableSilentDisco() || (c.netMap != nil && c.netMap.Debug != nil && c.netMap.Debug.EnableSilentDisco)
 
 	// Try a pass of just upserting nodes and creating missing

wgengine/magicsock/magicsock_test.go

@@ -1836,3 +1836,21 @@ func TestRebindingUDPConn(t *testing.T) {
 	c.setConnLocked(realConn.(nettype.PacketConn), "udp4")
 	c.setConnLocked(newBlockForeverConn(), "")
 }
+
+// https://github.com/tailscale/tailscale/issues/6680: don't ignore
+// SetNetworkMap calls when there are no peers. (A too aggressive fast path was
+// previously bailing out early, thinking there were no changes since all zero
+// peers didn't change, but the netmap has non-peer info in it too we shouldn't discard)
+func TestSetNetworkMapWithNoPeers(t *testing.T) {
+	var c Conn
+	c.logf = logger.Discard
+
+	for i := 1; i <= 3; i++ {
+		nm := &netmap.NetworkMap{}
+		c.SetNetworkMap(nm)
+		t.Logf("ptr %d: %p", i, nm)
+		if c.netMap != nm {
+			t.Fatalf("call %d: didn't store netmap", i)
+		}
+	}
+}