magicsock: mute noisy expected peer mtu related error (#10870)

wgengine/magicsock/magicsock.go

@@ -1317,7 +1317,7 @@ func (c *Conn) sendDiscoMessage(dst netip.AddrPort, dstKey key.NodePublic, dstDi
 	} else if err == nil {
 		// Can't send. (e.g. no IPv6 locally)
 	} else {
-		if !c.networkDown() {
+		if !c.networkDown() && pmtuShouldLogDiscoTxErr(m, err) {
 			c.logf("magicsock: disco: failed to send %v to %v: %v", disco.MessageSummary(m), dst, err)
 		}
 	}

wgengine/magicsock/peermtu.go

@@ -5,7 +5,13 @@
 
 package magicsock
 
-import "tailscale.com/net/tstun"
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/disco"
+	"tailscale.com/net/tstun"
+)
 
 // Peer path MTU routines shared by platforms that implement it.
 
@@ -110,3 +116,15 @@ func (c *Conn) UpdatePMTUD() {
 	c.peerMTUEnabled.Store(newStatus)
 	c.resetEndpointStates()
 }
+
+var errEMSGSIZE error = unix.EMSGSIZE
+
+func pmtuShouldLogDiscoTxErr(m disco.Message, err error) bool {
+	// Large disco.Ping packets used to probe path MTU may result in
+	// an EMSGSIZE error fairly regularly which can pollute logs.
+	p, ok := m.(*disco.Ping)
+	if !ok || p.Padding == 0 || !errors.Is(err, errEMSGSIZE) || debugPMTUD() {
+		return true
+	}
+	return false
+}

wgengine/magicsock/peermtu_stubs.go

@@ -5,6 +5,8 @@
 
 package magicsock
 
+import "tailscale.com/disco"
+
 func (c *Conn) DontFragSetting() (bool, error) {
 	return false, nil
 }
@@ -19,3 +21,7 @@ func (c *Conn) PeerMTUEnabled() bool {
 
 func (c *Conn) UpdatePMTUD() {
 }
+
+func pmtuShouldLogDiscoTxErr(m disco.Message, err error) bool {
+	return true
+}