ipn/ipnlocal: add back a watchdog after earlier removal from engine

Commit 2b338dd6a8 removed watchdogEngine because it was weird
(so many methods) and increasingly unnecessary after we'd cleaned up
and simplified so much of the locking.

This adds back a watchdog, but an easier to maintain one that's more
idiomatic.

Updates #19759

Change-Id: I86c458473e126c0809f37696446ce7acf4cc4eb9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager.go

@@ -131,6 +131,16 @@ func (m *Manager) Resolver() *resolver.Resolver {
 	return m.resolver
 }
 
+// ProbeLocks acquires and releases the manager's internal mutexes.
+func (m *Manager) ProbeLocks() {
+	m.mu.Lock()
+	m.mu.Unlock()
+
+	if r := m.Resolver(); r != nil {
+		r.ProbeLocks()
+	}
+}
+
 // RecompileDNSConfig recompiles the last attempted DNS configuration, which has
 // the side effect of re-querying the OS's interface nameservers.  This should be used
 // on platforms where the interface nameservers can change.  Darwin, for example,

net/dns/resolver/forwarder.go

@@ -347,6 +347,11 @@ type forwarder struct {
 	acceptDNS bool
 }
 
+func (f *forwarder) probeLocks() {
+	f.mu.Lock()
+	f.mu.Unlock()
+}
+
 func newForwarder(logf logger.Logf, netMon *netmon.Monitor, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, health *health.Tracker, knobs *controlknobs.Knobs) *forwarder {
 	if !buildfeatures.HasDNS {
 		return nil

net/dns/resolver/tsdns.go

@@ -266,6 +266,16 @@ func New(logf logger.Logf, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, h
 
 func (r *Resolver) TestOnlySetHook(hook func(Config)) { r.saveConfigForTests = hook }
 
+// ProbeLocks acquires and releases the resolver's internal mutexes.
+func (r *Resolver) ProbeLocks() {
+	r.mu.Lock()
+	r.mu.Unlock()
+
+	if r.forwarder != nil {
+		r.forwarder.probeLocks()
+	}
+}
+
 func (r *Resolver) SetConfig(cfg Config) error {
 	if !buildfeatures.HasDNS {
 		return nil

net/netmon/netmon.go

@@ -418,6 +418,12 @@ func (m *Monitor) InterfaceState() *State {
 	return m.ifState
 }
 
+// ProbeLocks acquires and releases the monitor's internal mutex.
+func (m *Monitor) ProbeLocks() {
+	m.mu.Lock()
+	m.mu.Unlock()
+}
+
 func (m *Monitor) interfaceStateUncached() (*State, error) {
 	return getState(tsIfProps.tsIfName())
 }

net/tsdial/tsdial.go

@@ -136,6 +136,12 @@ func (d *Dialer) TUNName() string {
 	return d.tunName
 }
 
+// ProbeLocks acquires and releases the dialer's internal mutex.
+func (d *Dialer) ProbeLocks() {
+	d.mu.Lock()
+	d.mu.Unlock()
+}
+
 // SetExitDNSDoH sets (or clears) the exit node DNS DoH server base URL to use.
 // The doh URL should contain the scheme, authority, and path, but without
 // a '?' and/or query parameters.

net/tstun/wrap.go

@@ -945,6 +945,15 @@ func (t *Wrapper) IdleDuration() time.Duration {
 	return mono.Since(t.lastActivityAtomic.LoadAtomic())
 }
 
+// ProbeLocks acquires and releases Wrapper's internal mutexes.
+func (t *Wrapper) ProbeLocks() {
+	t.bufferConsumedMu.Lock()
+	t.bufferConsumedMu.Unlock()
+
+	t.outboundMu.Lock()
+	t.outboundMu.Unlock()
+}
+
 func (t *Wrapper) awaitStart() {
 	for {
 		select {