wgengine/magicsock: add envknob to toggle UDP relay feature (#16396)

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/debugknobs.go

@@ -62,6 +62,12 @@ var (
 	//
 	//lint:ignore U1000 used on Linux/Darwin only
 	debugPMTUD = envknob.RegisterBool("TS_DEBUG_PMTUD")
+	// debugAssumeUDPRelayCapable forces magicsock to assume that all peers are
+	// UDP relay capable clients and servers. This will eventually be replaced
+	// by a [tailcfg.CapabilityVersion] comparison. It enables early testing of
+	// the UDP relay feature before we have established related
+	// [tailcfg.CapabilityVersion]'s.
+	debugAssumeUDPRelayCapable = envknob.RegisterBool("TS_DEBUG_ASSUME_UDP_RELAY_CAPABLE")
 	// Hey you! Adding a new debugknob? Make sure to stub it out in the
 	// debugknobs_stubs.go file too.
 )

wgengine/magicsock/debugknobs_stubs.go

@@ -31,3 +31,4 @@ func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }
 func pretendpoints() []netip.AddrPort  { return []netip.AddrPort{} }
+func debugAssumeUDPRelayCapable() bool { return false }

wgengine/magicsock/magicsock.go

@@ -2592,12 +2592,12 @@ func (c *Conn) SetProbeUDPLifetime(v bool) {
 
 func capVerIsRelayCapable(version tailcfg.CapabilityVersion) bool {
 	// TODO(jwhited): implement once capVer is bumped
-	return version == math.MinInt32
+	return version == math.MinInt32 || debugAssumeUDPRelayCapable()
 }
 
 func capVerIsRelayServerCapable(version tailcfg.CapabilityVersion) bool {
-	// TODO(jwhited): implement once capVer is bumped
-	return version == math.MinInt32
+	// TODO(jwhited): implement once capVer is bumped & update Test_peerAPIIfCandidateRelayServer
+	return version == math.MinInt32 || debugAssumeUDPRelayCapable()
 }
 
 // onFilterUpdate is called when a [FilterUpdate] is received over the