all: use buildfeatures.HasCapture const in a handful of places

Help out the linker's dead code elimination.

Updates #12614

Change-Id: I6c13cb44d3250bf1e3a01ad393c637da4613affb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1026,6 +1026,9 @@ func (b *LocalBackend) onHealthChange(change health.Change) {
 // GetOrSetCaptureSink returns the current packet capture sink, creating it
 // with the provided newSink function if it does not already exist.
 func (b *LocalBackend) GetOrSetCaptureSink(newSink func() packet.CaptureSink) packet.CaptureSink {
+	if !buildfeatures.HasCapture {
+		return nil
+	}
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
@@ -1039,6 +1042,9 @@ func (b *LocalBackend) GetOrSetCaptureSink(newSink func() packet.CaptureSink) pa
 }
 
 func (b *LocalBackend) ClearCaptureSink() {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	// Shut down & uninstall the sink if there are no longer
 	// any outputs on it.
 	b.mu.Lock()

net/tstun/wrap.go

@@ -24,6 +24,7 @@ import (
 	"go4.org/mem"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 	"tailscale.com/disco"
+	"tailscale.com/feature/buildfeatures"
 	tsmetrics "tailscale.com/metrics"
 	"tailscale.com/net/connstats"
 	"tailscale.com/net/packet"
@@ -1491,5 +1492,8 @@ var (
 )
 
 func (t *Wrapper) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	t.captureHook.Store(cb)
 }

wgengine/magicsock/magicsock.go

@@ -890,6 +890,9 @@ func deregisterMetrics(m *metrics) {
 // can be called with a nil argument to uninstall the capture
 // hook.
 func (c *Conn) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	c.captureHook.Store(cb)
 }
 

wgengine/userspace.go

@@ -23,6 +23,7 @@ import (
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/drive"
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
@@ -1652,6 +1653,9 @@ var (
 )
 
 func (e *userspaceEngine) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	e.tundev.InstallCaptureHook(cb)
 	e.magicConn.InstallCaptureHook(cb)
 }

wgengine/watchdog.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
 	"tailscale.com/net/packet"
@@ -163,6 +164,9 @@ func (e *watchdogEngine) Done() <-chan struct{} {
 }
 
 func (e *watchdogEngine) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	e.wrap.InstallCaptureHook(cb)
 }