syncs: fix flaky test by deleting the code it tested (Watch)
Fixes #11766 Change-Id: Id5a875aab23eb1b48a57dc379d0cdd42412fd18b Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Brad Fitzpatrick
Brad Fitzpatrick
parent
63b3c82587
commit
c07aa2cfed
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@ -1,97 +0,0 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package syncs
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import (
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"context"
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"sync"
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"time"
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)
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// Watch monitors mu for contention.
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// On first call, and at every tick, Watch locks and unlocks mu.
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// (Tick should be large to avoid adding contention to mu.)
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// Max is the maximum length of time Watch will wait to acquire the lock.
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// The time required to lock mu is sent on the returned channel.
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// Watch exits when ctx is done, and closes the returned channel.
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func Watch(ctx context.Context, mu sync.Locker, tick, max time.Duration) chan time.Duration {
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// Set up the return channel.
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c := make(chan time.Duration)
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var (
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closemu sync.Mutex
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closed bool
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)
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sendc := func(d time.Duration) {
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closemu.Lock()
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defer closemu.Unlock()
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if closed {
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// Drop values written after c is closed.
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return
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}
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select {
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case c <- d:
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case <-ctx.Done():
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}
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}
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closec := func() {
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closemu.Lock()
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defer closemu.Unlock()
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close(c)
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closed = true
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}
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// check locks the mutex and writes how long it took to c.
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// check returns ~immediately.
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check := func() {
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// Start a race between two goroutines.
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// One locks the mutex; the other times out.
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// Ensure that only one of the two gets to write its result.
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// Since the common case is that locking the mutex is fast,
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// let the timeout goroutine exit early when that happens.
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var sendonce sync.Once
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done := make(chan bool)
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go func() {
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start := time.Now()
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mu.Lock()
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mu.Unlock()
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elapsed := time.Since(start)
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if elapsed > max {
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elapsed = max
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}
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close(done)
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sendonce.Do(func() { sendc(elapsed) })
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}()
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go func() {
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select {
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case <-time.After(max):
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// the other goroutine may not have sent a value
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sendonce.Do(func() { sendc(max) })
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case <-done:
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// the other goroutine sent a value
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}
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}()
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}
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// Check once at startup.
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// This is mainly to make testing easier.
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check()
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// Start the watchdog goroutine.
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// It checks the mutex every tick, until ctx is done.
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go func() {
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ticker := time.NewTicker(tick)
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for {
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select {
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case <-ctx.Done():
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closec()
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ticker.Stop()
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return
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case <-ticker.C:
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check()
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}
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}
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}()
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return c
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}
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@ -1,77 +0,0 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package syncs
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import (
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"context"
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"runtime"
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"sync"
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"testing"
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"time"
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)
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// Time-based tests are fundamentally flaky.
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// We use exaggerated durations in the hopes of minimizing such issues.
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func TestWatchUncontended(t *testing.T) {
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mu := new(sync.Mutex)
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ctx, cancel := context.WithCancel(context.Background())
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defer cancel()
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// Once an hour, and now, check whether we can lock mu in under an hour.
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tick := time.Hour
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max := time.Hour
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c := Watch(ctx, mu, tick, max)
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d := <-c
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if d == max {
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t.Errorf("uncontended mutex did not lock in under %v", max)
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}
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}
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func TestWatchContended(t *testing.T) {
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mu := new(sync.Mutex)
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ctx, cancel := context.WithCancel(context.Background())
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defer cancel()
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// Every hour, and now, check whether we can lock mu in under a millisecond,
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// which is enough time for an uncontended mutex by several orders of magnitude.
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tick := time.Hour
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max := time.Millisecond
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mu.Lock()
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defer mu.Unlock()
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c := Watch(ctx, mu, tick, max)
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d := <-c
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if d != max {
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t.Errorf("contended mutex locked in under %v", max)
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}
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}
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func TestWatchMultipleValues(t *testing.T) {
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mu := new(sync.Mutex)
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ctx, cancel := context.WithCancel(context.Background())
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defer cancel() // not necessary, but keep vet happy
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// Check the mutex every millisecond.
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// The goal is to see that we get a sufficient number of values out of the channel.
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tick := time.Millisecond
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max := time.Millisecond
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c := Watch(ctx, mu, tick, max)
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start := time.Now()
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n := 0
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for d := range c {
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n++
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if d == max {
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t.Errorf("uncontended mutex did not lock in under %v", max)
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}
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if n == 10 {
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cancel()
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}
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}
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// See https://github.com/golang/go/issues/44343 - on Windows the Go runtime timer resolution is currently too coarse. Allow longer in that case.
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want := 100 * time.Millisecond
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if runtime.GOOS == "windows" {
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want = 500 * time.Millisecond
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}
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if elapsed := time.Since(start); elapsed > want {
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t.Errorf("expected 1 event per millisecond, got only %v events in %v", n, elapsed)
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}
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}
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