cmd/tailscaled: start implementing ts_omit_netstack

Baby steps. This permits building without much of gvisor, but not all of it.

Updates #17283

Change-Id: I8433146e259918cc901fe86b4ea29be22075b32c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/gro/gro.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ts_omit_netstack
+
 // Package gro implements GRO for the receive (write) path into gVisor.
 package gro
 

wgengine/netstack/gro/netstack_disabled.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_netstack
+
+package gro
+
+func RXChecksumOffload(any) any {
+	panic("unreachable")
+}

wgengine/netstack/link_endpoint.go

@@ -187,7 +187,7 @@ func (l *linkEndpoint) injectInbound(p *packet.Parsed) {
 	l.mu.RLock()
 	d := l.dispatcher
 	l.mu.RUnlock()
-	if d == nil {
+	if d == nil || !buildfeatures.HasNetstack {
 		return
 	}
 	pkt := gro.RXChecksumOffload(p)

wgengine/netstack/netstack.go

@@ -578,9 +578,16 @@ func (ns *Impl) decrementInFlightTCPForward(tei stack.TransportEndpointID, remot
 	}
 }
 
+// LocalBackend is a fake name for *ipnlocal.LocalBackend to avoid an import cycle.
+type LocalBackend = any
+
 // Start sets up all the handlers so netstack can start working. Implements
 // wgengine.FakeImpl.
-func (ns *Impl) Start(lb *ipnlocal.LocalBackend) error {
+func (ns *Impl) Start(b LocalBackend) error {
+	if b == nil {
+		panic("nil LocalBackend interface")
+	}
+	lb := b.(*ipnlocal.LocalBackend)
 	if lb == nil {
 		panic("nil LocalBackend")
 	}