wgengine/magicsock: emit first-netmap latency for uncached resets too (#20029)

This is a refinement of #19916. Previously, we would only emit a latency log
when going from a cached netmap to an uncached one (i.e., from the control
plane). We would like to know the latency in both conditions, though, so
instead use the validity of the previous self state.

Updates #12639
Updates tailscale/projects#27

Change-Id: I6bbeb5d3162f1f98cdb3dcd244f67ef31c170957
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -3049,7 +3049,7 @@ func (c *Conn) SetNetworkMapCached(self tailcfg.NodeView, peers []tailcfg.NodeVi
 
 // setNetworkMapInternal is the shared implementation of SetNetworkMap and SetNetworkMapCached.
 func (c *Conn) setNetworkMapInternal(self tailcfg.NodeView, peers []tailcfg.NodeView, isCached bool) {
-	peersChanged := c.updateNodes(self, peers)
+	peersChanged, selfWasValid := c.updateNodes(self, peers)
 
 	relayClientEnabled := self.Valid() &&
 		!self.HasCap(tailcfg.NodeAttrDisableRelayClient) &&
@@ -3064,7 +3064,7 @@ func (c *Conn) setNetworkMapInternal(self tailcfg.NodeView, peers []tailcfg.Node
 	selfView := c.self
 	peersSnap := c.peerSnapshotLocked()
 	isClosed := c.closed
-	wasCached := c.usingCachedNetmap.Swap(isCached)
+	c.usingCachedNetmap.Store(isCached)
 	if !self.Valid() {
 		c.initializedAt = mono.Now() // the netmap is being reset
 	}
@@ -3083,11 +3083,11 @@ func (c *Conn) setNetworkMapInternal(self tailcfg.NodeView, peers []tailcfg.Node
 	if isClosed {
 		return // nothing to do here, the conn is closed and the update is no longer relevant
 	}
-	if self.Valid() && wasCached && !isCached {
-		// Reaching here, we have a "real" netmap (self is valid), and we are
-		// transitioning from cached to non-cached, so record the latency from
-		// reset.
-		c.logf("magicsock: new contact: control-netmap usec=%d", int64(mono.Since(initializedAt)/time.Microsecond))
+	if self.Valid() && !selfWasValid {
+		// Reaching here, we have our first "real" netmap (self is valid, and
+		// the previous netmap was unset or reset), so record the latency from reset.
+		c.logf("magicsock: new contact: control-netmap usec=%d cached=%v",
+			int64(mono.Since(initializedAt)/time.Microsecond), isCached)
 	}
 
 	if udpOffloadKnobsChanged {
@@ -3111,17 +3111,17 @@ func (c *Conn) setNetworkMapInternal(self tailcfg.NodeView, peers []tailcfg.Node
 }
 
 // updateNodes updates [Conn] to reflect the given self node and peers.
-// It reports whether the peer set (membership or any field) changed.
-func (c *Conn) updateNodes(self tailcfg.NodeView, peers []tailcfg.NodeView) (peersChanged bool) {
+// It reports whether the peer set (membership or any field) changed,
+// and whether the previous self node was valid.
+func (c *Conn) updateNodes(self tailcfg.NodeView, peers []tailcfg.NodeView) (peersChanged, wasValid bool) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-
 	if c.closed {
-		return false
+		return false, false
 	}
 
 	metricNumPeers.Set(int64(len(peers)))
-
+	selfWasValid := c.self.Valid()
 	c.self = self
 
 	// [debugFlags] are mutable in [Conn.SetSilentDisco] &
@@ -3146,7 +3146,7 @@ func (c *Conn) updateNodes(self tailcfg.NodeView, peers []tailcfg.NodeView) (pee
 			}
 		}
 		if allSame {
-			return false
+			return false, selfWasValid
 		}
 	}
 
@@ -3188,7 +3188,7 @@ func (c *Conn) updateNodes(self tailcfg.NodeView, peers []tailcfg.NodeView) (pee
 		}
 	}
 
-	return true
+	return true, selfWasValid
 }
 
 // upsertPeerLocked upserts a single peer's endpoint in c.peerMap. It is the