control/controlclient,health,ipn/ipnlocal,health: fix deadlock by deleting health reporting

A recent change (009d702adf) introduced a deadlock where the
/machine/update-health network request to report the client's health
status update to the control plane was moved to being synchronous
within the eventbus's pump machinery.

I started to instead make the health reporting be async, but then we
realized in the three years since we added that, it's barely been used
and doesn't pay for itself, for how many HTTP requests it makes.

Instead, delete it all and replace it with a c2n handler, which
provides much more helpful information.

Fixes tailscale/corp#32952

Change-Id: I9e8a5458269ebfdda1c752d7bbb8af2780d71b04
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/auto.go

@@ -12,7 +12,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"tailscale.com/health"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstime"
@@ -23,7 +22,6 @@ import (
 	"tailscale.com/types/structs"
 	"tailscale.com/util/backoff"
 	"tailscale.com/util/clientmetric"
-	"tailscale.com/util/eventbus"
 	"tailscale.com/util/execqueue"
 )
 
@@ -123,8 +121,6 @@ type Auto struct {
 	observerQueue execqueue.ExecQueue
 	shutdownFn    func() // to be called prior to shutdown or nil
 
-	eventSubs eventbus.Monitor
-
 	mu sync.Mutex // mutex guards the following fields
 
 	wantLoggedIn bool   // whether the user wants to be logged in per last method call
@@ -195,10 +191,6 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 		shutdownFn: opts.Shutdown,
 	}
 
-	// Set up eventbus client and subscriber
-	ec := opts.Bus.Client("controlClient.Auto")
-	c.eventSubs = ec.Monitor(c.consumeEventbusTopics(ec))
-
 	c.authCtx, c.authCancel = context.WithCancel(context.Background())
 	c.authCtx = sockstats.WithSockStats(c.authCtx, sockstats.LabelControlClientAuto, opts.Logf)
 
@@ -208,27 +200,6 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 	return c, nil
 }
 
-// consumeEventbusTopics consumes events from all relevant
-// [eventbus.Subscriber]'s and passes them to their related handler. Events are
-// always handled in the order they are received, i.e. the next event is not
-// read until the previous event's handler has returned. It returns when the
-// [eventbus.Client] is closed.
-func (c *Auto) consumeEventbusTopics(ec *eventbus.Client) func(*eventbus.Client) {
-	healthChangeSub := eventbus.Subscribe[health.Change](ec)
-	return func(cli *eventbus.Client) {
-		for {
-			select {
-			case <-cli.Done():
-				return
-			case change := <-healthChangeSub.Events():
-				if change.WarnableChanged {
-					c.direct.ReportWarnableChange(change.Warnable, change.UnhealthyState)
-				}
-			}
-		}
-	}
-}
-
 // SetPaused controls whether HTTP activity should be paused.
 //
 // The client can be paused and unpaused repeatedly, unlike Start and Shutdown, which can only be used once.
@@ -782,8 +753,6 @@ func (c *Auto) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 }
 
 func (c *Auto) Shutdown() {
-	c.eventSubs.Close()
-
 	c.mu.Lock()
 	if c.closed {
 		c.mu.Unlock()