wgengine/magicsock: remove endpoint parameter from handlePingLocked

We can reply to a ping without knowing which exact node it's from.  As
long as it's in our netmap, it's safe to reply. If there's more than
one node with that discokey, it doesn't matter who we're relpying to.

Updates #3088

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

wgengine/magicsock/magicsock.go

@@ -158,6 +158,18 @@ func (m *peerMap) forEachDiscoEndpoint(f func(ep *endpoint)) {
 	}
 }
 
+// forEachEndpointWithDiscoKey invokes f on every endpoint in m
+// that has the provided DiscoKey.
+func (m *peerMap) forEachEndpointWithDiscoKey(dk tailcfg.DiscoKey, f func(ep *endpoint)) {
+	// TODO(bradfitz): once byDiscoKey is a set of endpoints, then range
+	// over that instead.
+	for _, pi := range m.byNodeKey {
+		if pi.ep != nil && pi.ep.discoKey == dk {
+			f(pi.ep)
+		}
+	}
+}
+
 // upsertDiscoEndpoint stores endpoint in the peerInfo for
 // ep.publicKey, and updates indexes. m must already have a
 // tailcfg.Node for ep.publicKey.
@@ -1689,6 +1701,12 @@ const (
 	discoVerboseLog
 )
 
+// sendDiscoMessage sends discovery message m to dstDisco at dst.
+//
+// If dst is a DERP IP:port, then dstKey must be non-zero.
+//
+// The dstKey should only be non-zero if the dstDisco key
+// unambiguously maps to exactly one peer.
 func (c *Conn) sendDiscoMessage(dst netaddr.IPPort, dstKey tailcfg.NodeKey, dstDisco tailcfg.DiscoKey, m disco.Message, logLevel discoLogLevel) (sent bool, err error) {
 	c.mu.Lock()
 	if c.closed {
@@ -1710,7 +1728,11 @@ func (c *Conn) sendDiscoMessage(dst netaddr.IPPort, dstKey tailcfg.NodeKey, dstD
 	sent, err = c.sendAddr(dst, key.Public(dstKey), pkt)
 	if sent {
 		if logLevel == discoLog || (logLevel == discoVerboseLog && debugDisco) {
-			c.logf("[v1] magicsock: disco: %v->%v (%v, %v) sent %v", c.discoShort, dstDisco.ShortString(), dstKey.ShortString(), derpStr(dst.String()), disco.MessageSummary(m))
+			node := "?"
+			if !dstKey.IsZero() {
+				node = dstKey.ShortString()
+			}
+			c.logf("[v1] magicsock: disco: %v->%v (%v, %v) sent %v", c.discoShort, dstDisco.ShortString(), node, derpStr(dst.String()), disco.MessageSummary(m))
 		}
 	} else if err == nil {
 		// Can't send. (e.g. no IPv6 locally)
@@ -1836,7 +1858,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort, derpNodeSrc ta
 
 	switch dm := dm.(type) {
 	case *disco.Ping:
-		c.handlePingLocked(dm, ep, src, di, derpNodeSrc)
+		c.handlePingLocked(dm, src, di, derpNodeSrc)
 	case *disco.Pong:
 		ep.handlePongConnLocked(dm, src)
 	case *disco.CallMeMaybe:
@@ -1864,21 +1886,57 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort, derpNodeSrc ta
 
 // di is the discoInfo of the source of the ping.
 // derpNodeSrc is non-zero if the ping arrived via DERP.
-func (c *Conn) handlePingLocked(dm *disco.Ping, de *endpoint, src netaddr.IPPort, di *discoInfo, derpNodeSrc tailcfg.NodeKey) {
+func (c *Conn) handlePingLocked(dm *disco.Ping, src netaddr.IPPort, di *discoInfo, derpNodeSrc tailcfg.NodeKey) {
 	likelyHeartBeat := src == di.lastPingFrom && time.Since(di.lastPingTime) < 5*time.Second
 	di.lastPingFrom = src
 	di.lastPingTime = time.Now()
-	if !likelyHeartBeat || debugDisco {
-		c.logf("[v1] magicsock: disco: %v<-%v (%v, %v)  got ping tx=%x", c.discoShort, di.discoShort, de.publicKey.ShortString(), src, dm.TxID[:6])
-	}
+
+	// If we got a ping over DERP, then derpNodeSrc is non-zero and we reply
+	// over DERP (in which case ipDst is also a DERP address).
+	// But if the ping was over UDP (ipDst is not a DERP address), then dstKey
+	// will be zero here, but that's fine: sendDiscoMessage only requires
+	// a dstKey if the dst ip:port is DERP.
+	dstKey := derpNodeSrc
 
 	// Remember this route if not present.
 	c.setAddrToDiscoLocked(src, di.discoKey)
-	de.addCandidateEndpoint(src)
+	var numNodes int
+	if !derpNodeSrc.IsZero() {
+		if ep, ok := c.peerMap.endpointForNodeKey(derpNodeSrc); ok {
+			ep.addCandidateEndpoint(src)
+			numNodes = 1
+		}
+	} else {
+		c.peerMap.forEachEndpointWithDiscoKey(di.discoKey, func(ep *endpoint) {
+			ep.addCandidateEndpoint(src)
+			numNodes++
+			if numNodes == 1 && dstKey.IsZero() {
+				dstKey = ep.publicKey
+			}
+		})
+		if numNodes > 1 {
+			// Zero it out if it's ambiguous, so sendDiscoMessage logging
+			// isn't confusing.
+			dstKey = tailcfg.NodeKey{}
+		}
+	}
+
+	if numNodes == 0 {
+		c.logf("[unexpected] got disco ping from %v/%v for node not in peers", src, derpNodeSrc)
+		return
+	}
+
+	if !likelyHeartBeat || debugDisco {
+		pingNodeSrcStr := dstKey.ShortString()
+		if numNodes > 1 {
+			pingNodeSrcStr = "[one-of-multi]"
+		}
+		c.logf("[v1] magicsock: disco: %v<-%v (%v, %v)  got ping tx=%x", c.discoShort, di.discoShort, pingNodeSrcStr, src, dm.TxID[:6])
+	}
 
 	ipDst := src
 	discoDest := di.discoKey
-	go c.sendDiscoMessage(ipDst, de.publicKey, discoDest, &disco.Pong{
+	go c.sendDiscoMessage(ipDst, dstKey, discoDest, &disco.Pong{
 		TxID: dm.TxID,
 		Src:  src,
 	}, discoVerboseLog)