syncs: fix flaky test by deleting the code it tested (Watch)

Fixes #11766

Change-Id: Id5a875aab23eb1b48a57dc379d0cdd42412fd18b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

syncs/watchdog.go

@@ -1,97 +0,0 @@
-// Copyright (c) Tailscale Inc & AUTHORS
-// SPDX-License-Identifier: BSD-3-Clause
-
-package syncs
-
-import (
-	"context"
-	"sync"
-	"time"
-)
-
-// Watch monitors mu for contention.
-// On first call, and at every tick, Watch locks and unlocks mu.
-// (Tick should be large to avoid adding contention to mu.)
-// Max is the maximum length of time Watch will wait to acquire the lock.
-// The time required to lock mu is sent on the returned channel.
-// Watch exits when ctx is done, and closes the returned channel.
-func Watch(ctx context.Context, mu sync.Locker, tick, max time.Duration) chan time.Duration {
-	// Set up the return channel.
-	c := make(chan time.Duration)
-	var (
-		closemu sync.Mutex
-		closed  bool
-	)
-	sendc := func(d time.Duration) {
-		closemu.Lock()
-		defer closemu.Unlock()
-		if closed {
-			// Drop values written after c is closed.
-			return
-		}
-		select {
-		case c <- d:
-		case <-ctx.Done():
-		}
-	}
-	closec := func() {
-		closemu.Lock()
-		defer closemu.Unlock()
-		close(c)
-		closed = true
-	}
-
-	// check locks the mutex and writes how long it took to c.
-	// check returns ~immediately.
-	check := func() {
-		// Start a race between two goroutines.
-		// One locks the mutex; the other times out.
-		// Ensure that only one of the two gets to write its result.
-		// Since the common case is that locking the mutex is fast,
-		// let the timeout goroutine exit early when that happens.
-		var sendonce sync.Once
-		done := make(chan bool)
-		go func() {
-			start := time.Now()
-			mu.Lock()
-			mu.Unlock()
-			elapsed := time.Since(start)
-			if elapsed > max {
-				elapsed = max
-			}
-			close(done)
-			sendonce.Do(func() { sendc(elapsed) })
-		}()
-		go func() {
-			select {
-			case <-time.After(max):
-				// the other goroutine may not have sent a value
-				sendonce.Do(func() { sendc(max) })
-			case <-done:
-				// the other goroutine sent a value
-			}
-		}()
-	}
-
-	// Check once at startup.
-	// This is mainly to make testing easier.
-	check()
-
-	// Start the watchdog goroutine.
-	// It checks the mutex every tick, until ctx is done.
-	go func() {
-		ticker := time.NewTicker(tick)
-		for {
-			select {
-			case <-ctx.Done():
-				closec()
-				ticker.Stop()
-				return
-			case <-ticker.C:
-				check()
-			}
-		}
-	}()
-
-	return c
-}

syncs/watchdog_test.go

@@ -1,77 +0,0 @@
-// Copyright (c) Tailscale Inc & AUTHORS
-// SPDX-License-Identifier: BSD-3-Clause
-
-package syncs
-
-import (
-	"context"
-	"runtime"
-	"sync"
-	"testing"
-	"time"
-)
-
-// Time-based tests are fundamentally flaky.
-// We use exaggerated durations in the hopes of minimizing such issues.
-
-func TestWatchUncontended(t *testing.T) {
-	mu := new(sync.Mutex)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-	// Once an hour, and now, check whether we can lock mu in under an hour.
-	tick := time.Hour
-	max := time.Hour
-	c := Watch(ctx, mu, tick, max)
-	d := <-c
-	if d == max {
-		t.Errorf("uncontended mutex did not lock in under %v", max)
-	}
-}
-
-func TestWatchContended(t *testing.T) {
-	mu := new(sync.Mutex)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-	// Every hour, and now, check whether we can lock mu in under a millisecond,
-	// which is enough time for an uncontended mutex by several orders of magnitude.
-	tick := time.Hour
-	max := time.Millisecond
-	mu.Lock()
-	defer mu.Unlock()
-	c := Watch(ctx, mu, tick, max)
-	d := <-c
-	if d != max {
-		t.Errorf("contended mutex locked in under %v", max)
-	}
-}
-
-func TestWatchMultipleValues(t *testing.T) {
-	mu := new(sync.Mutex)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel() // not necessary, but keep vet happy
-	// Check the mutex every millisecond.
-	// The goal is to see that we get a sufficient number of values out of the channel.
-	tick := time.Millisecond
-	max := time.Millisecond
-	c := Watch(ctx, mu, tick, max)
-	start := time.Now()
-	n := 0
-	for d := range c {
-		n++
-		if d == max {
-			t.Errorf("uncontended mutex did not lock in under %v", max)
-		}
-		if n == 10 {
-			cancel()
-		}
-	}
-	// See https://github.com/golang/go/issues/44343 - on Windows the Go runtime timer resolution is currently too coarse. Allow longer in that case.
-	want := 100 * time.Millisecond
-	if runtime.GOOS == "windows" {
-		want = 500 * time.Millisecond
-	}
-
-	if elapsed := time.Since(start); elapsed > want {
-		t.Errorf("expected 1 event per millisecond, got only %v events in %v", n, elapsed)
-	}
-}