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

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
+}