wgengine/magicsock: rename debugPMTUD() to debugEnablePMTUD()

Make the debugknob variable name for enabling peer path MTU discovery match the
env variable name.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/debugknobs.go

@@ -49,8 +49,11 @@ var (
 	// debugRingBufferMaxSizeBytes overrides the default size of the endpoint
 	// history ringbuffer.
 	debugRingBufferMaxSizeBytes = envknob.RegisterInt("TS_DEBUG_MAGICSOCK_RING_BUFFER_MAX_SIZE_BYTES")
-	// debugPMTUD enables path MTU discovery. Currently only sets the Don't Fragment sockopt.
-	debugPMTUD = envknob.RegisterBool("TS_DEBUG_ENABLE_PMTUD")
+	// debugEnablePMTUD enables the peer MTU feature, which does path MTU
+	// discovery on UDP connections between peers. Currently (2023-09-05)
+	// this only turns on the don't fragment bit for the magicsock UDP
+	// sockets.
+	debugEnablePMTUD = envknob.RegisterBool("TS_DEBUG_ENABLE_PMTUD")
 	// Hey you! Adding a new debugknob? Make sure to stub it out in the debugknob_stubs.go
 	// file too.
 )

wgengine/magicsock/debugknobs_stubs.go

@@ -20,7 +20,7 @@ func debugAlwaysDERP() bool            { return false }
 func debugUseDERPHTTP() bool           { return false }
 func debugEnableSilentDisco() bool     { return false }
 func debugSendCallMeUnknownPeer() bool { return false }
-func debugPMTUD() bool                 { return false }
+func debugEnablePMTUD() bool           { return false }
 func debugUseDERPAddr() string         { return "" }
 func debugUseDerpRouteEnv() string     { return "" }
 func debugUseDerpRoute() opt.Bool      { return "" }

wgengine/magicsock/peermtu_darwin.go

@@ -31,5 +31,5 @@ func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
 }
 
 func CanPMTUD() bool {
-	return debugPMTUD() // only if the envknob is for now.
+	return debugEnablePMTUD() // only if the envknob is for now.
 }

wgengine/magicsock/peermtu_linux.go

@@ -30,5 +30,5 @@ func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
 }
 
 func CanPMTUD() bool {
-	return debugPMTUD() // only if the envknob is enabled, for now.
+	return debugEnablePMTUD() // only if the envknob is enabled, for now.
 }