wgengine: print in-flight operations on watchdog trigger (#5447)

In addition to printing goroutine stacks, explicitly track all in-flight
operations and print them when the watchdog triggers (along with the
time they were started at). This should make debugging watchdog failures
easier, since we can look at the longest-running operation(s) first.

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

wgengine/watchdog.go

@@ -8,10 +8,12 @@
 package wgengine
 
 import (
+	"fmt"
 	"log"
 	"net/netip"
 	"runtime/pprof"
 	"strings"
+	"sync"
 	"time"
 
 	"tailscale.com/envknob"
@@ -38,21 +40,49 @@ func NewWatchdog(e Engine) Engine {
 		return e
 	}
 	return &watchdogEngine{
-		wrap:    e,
-		logf:    log.Printf,
-		fatalf:  log.Fatalf,
-		maxWait: 45 * time.Second,
+		wrap:     e,
+		logf:     log.Printf,
+		fatalf:   log.Fatalf,
+		maxWait:  45 * time.Second,
+		inFlight: make(map[inFlightKey]time.Time),
 	}
 }
 
+type inFlightKey struct {
+	op  string
+	ctr uint64
+}
+
 type watchdogEngine struct {
 	wrap    Engine
 	logf    func(format string, args ...any)
 	fatalf  func(format string, args ...any)
 	maxWait time.Duration
+
+	// Track the start time(s) of in-flight operations
+	inFlightMu  sync.Mutex
+	inFlight    map[inFlightKey]time.Time
+	inFlightCtr uint64
 }
 
 func (e *watchdogEngine) watchdogErr(name string, fn func() error) error {
+	// Track all in-flight operations so we can print more useful error
+	// messages on watchdog failure
+	e.inFlightMu.Lock()
+	key := inFlightKey{
+		op:  name,
+		ctr: e.inFlightCtr,
+	}
+	e.inFlightCtr++
+	e.inFlight[key] = time.Now()
+	e.inFlightMu.Unlock()
+
+	defer func() {
+		e.inFlightMu.Lock()
+		defer e.inFlightMu.Unlock()
+		delete(e.inFlight, key)
+	}()
+
 	errCh := make(chan error)
 	go func() {
 		errCh <- fn()
@@ -66,6 +96,22 @@ func (e *watchdogEngine) watchdogErr(name string, fn func() error) error {
 		buf := new(strings.Builder)
 		pprof.Lookup("goroutine").WriteTo(buf, 1)
 		e.logf("wgengine watchdog stacks:\n%s", buf.String())
+
+		// Collect the list of in-flight operations for debugging.
+		var (
+			b   []byte
+			now = time.Now()
+		)
+		e.inFlightMu.Lock()
+		for k, t := range e.inFlight {
+			dur := now.Sub(t).Round(time.Millisecond)
+			b = fmt.Appendf(b, "in-flight[%d]: name=%s duration=%v start=%s\n", k.ctr, k.op, dur, t.Format(time.RFC3339Nano))
+		}
+		e.inFlightMu.Unlock()
+
+		// Print everything as a single string to avoid log
+		// rate limits.
+		e.logf("wgengine watchdog in-flight:\n%s", b)
 		e.fatalf("wgengine: watchdog timeout on %s", name)
 		return nil
 	}

wgengine/watchdog_test.go

@@ -6,6 +6,7 @@ package wgengine
 
 import (
 	"fmt"
+	"regexp"
 	"runtime"
 	"strings"
 	"testing"
@@ -68,9 +69,15 @@ func TestWatchdog(t *testing.T) {
 
 		select {
 		case <-fatalCalled:
-			if !strings.Contains(logBuf.String(), "goroutine profile: total ") {
-				t.Errorf("fatal called without watchdog stacks, got: %s", logBuf.String())
+			logs := logBuf.String()
+			if !strings.Contains(logs, "goroutine profile: total ") {
+				t.Errorf("fatal called without watchdog stacks, got: %s", logs)
 			}
+			re := regexp.MustCompile(`(?m)^\s*in-flight\[\d+\]: name=RequestStatus duration=.* start=.*$`)
+			if !re.MatchString(logs) {
+				t.Errorf("fatal called without in-flight list, got: %s", logs)
+			}
+
 			// expected
 		case <-time.After(3 * time.Second):
 			t.Fatalf("watchdog failed to fire")