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util/singleflight: add fork of singleflight with generics

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Forked from golang.org/x/sync/singleflight at
the x/sync repo's commit 67f06af15bc961c363a7260195bcd53487529a21

Updates golang/go#53427

Change-Id: Iec2b47b7777940017bb9b3db9bd7d93ba4a2e394
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
pull/4877/head
Brad Fitzpatrick 2 years ago committed by Brad Fitzpatrick
parent 757ecf7e80
commit 59ed846277
2 changed files with 549 additions and 0 deletions
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225
util/singleflight/singleflight.go
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// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package singleflight provides a duplicate function call suppression
// mechanism.
//
// This is a Tailscale fork of Go's singleflight package which has had several
// homes in the past:
//
// * https://github.com/golang/go/commit/61d3b2db6292581fc07a3767ec23ec94ad6100d1
// * https://github.com/golang/groupcache/tree/master/singleflight
// * https://pkg.go.dev/golang.org/x/sync/singleflight
//
// This fork adds generics.
package singleflight // import "tailscale.com/util/singleflight"
import (
"bytes"
"errors"
"fmt"
"runtime"
"runtime/debug"
"sync"
)
// errGoexit indicates the runtime.Goexit was called in
// the user given function.
var errGoexit = errors.New("runtime.Goexit was called")
// A panicError is an arbitrary value recovered from a panic
// with the stack trace during the execution of given function.
type panicError struct {
value interface{}
stack []byte
}
// Error implements error interface.
func (p *panicError) Error() string {
return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
}
func newPanicError(v interface{}) error {
stack := debug.Stack()
// The first line of the stack trace is of the form "goroutine N [status]:"
// but by the time the panic reaches Do the goroutine may no longer exist
// and its status will have changed. Trim out the misleading line.
if line := bytes.IndexByte(stack[:], '\n'); line >= 0 {
stack = stack[line+1:]
}
return &panicError{value: v, stack: stack}
}
// call is an in-flight or completed singleflight.Do call
type call[V any] struct {
wg sync.WaitGroup
// These fields are written once before the WaitGroup is done
// and are only read after the WaitGroup is done.
val V
err error
// forgotten indicates whether Forget was called with this call's key
// while the call was still in flight.
forgotten bool
// These fields are read and written with the singleflight
// mutex held before the WaitGroup is done, and are read but
// not written after the WaitGroup is done.
dups int
chans []chan<- Result[V]
}
// Group represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type Group[K comparable, V any] struct {
mu sync.Mutex // protects m
m map[K]*call[V] // lazily initialized
}
// Result holds the results of Do, so they can be passed
// on a channel.
type Result[V any] struct {
Val V
Err error
Shared bool
}
// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
// The return value shared indicates whether v was given to multiple callers.
func (g *Group[K, V]) Do(key K, fn func() (V, error)) (v V, err error, shared bool) {
g.mu.Lock()
if g.m == nil {
g.m = make(map[K]*call[V])
}
if c, ok := g.m[key]; ok {
c.dups++
g.mu.Unlock()
c.wg.Wait()
if e, ok := c.err.(*panicError); ok {
panic(e)
} else if c.err == errGoexit {
runtime.Goexit()
}
return c.val, c.err, true
}
c := new(call[V])
c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
g.doCall(c, key, fn)
return c.val, c.err, c.dups > 0
}
// DoChan is like Do but returns a channel that will receive the
// results when they are ready.
//
// The returned channel will not be closed.
func (g *Group[K, V]) DoChan(key K, fn func() (V, error)) <-chan Result[V] {
ch := make(chan Result[V], 1)
g.mu.Lock()
if g.m == nil {
g.m = make(map[K]*call[V])
}
if c, ok := g.m[key]; ok {
c.dups++
c.chans = append(c.chans, ch)
g.mu.Unlock()
return ch
}
c := &call[V]{chans: []chan<- Result[V]{ch}}
c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
go g.doCall(c, key, fn)
return ch
}
// doCall handles the single call for a key.
func (g *Group[K, V]) doCall(c *call[V], key K, fn func() (V, error)) {
normalReturn := false
recovered := false
// use double-defer to distinguish panic from runtime.Goexit,
// more details see https://golang.org/cl/134395
defer func() {
// the given function invoked runtime.Goexit
if !normalReturn && !recovered {
c.err = errGoexit
}
c.wg.Done()
g.mu.Lock()
defer g.mu.Unlock()
if !c.forgotten {
delete(g.m, key)
}
if e, ok := c.err.(*panicError); ok {
// In order to prevent the waiting channels from being blocked forever,
// needs to ensure that this panic cannot be recovered.
if len(c.chans) > 0 {
go panic(e)
select {} // Keep this goroutine around so that it will appear in the crash dump.
} else {
panic(e)
}
} else if c.err == errGoexit {
// Already in the process of goexit, no need to call again
} else {
// Normal return
for _, ch := range c.chans {
ch <- Result[V]{c.val, c.err, c.dups > 0}
}
}
}()
func() {
defer func() {
if !normalReturn {
// Ideally, we would wait to take a stack trace until we've determined
// whether this is a panic or a runtime.Goexit.
//
// Unfortunately, the only way we can distinguish the two is to see
// whether the recover stopped the goroutine from terminating, and by
// the time we know that, the part of the stack trace relevant to the
// panic has been discarded.
if r := recover(); r != nil {
c.err = newPanicError(r)
}
}
}()
c.val, c.err = fn()
normalReturn = true
}()
if !normalReturn {
recovered = true
}
}
// Forget tells the singleflight to forget about a key. Future calls
// to Do for this key will call the function rather than waiting for
// an earlier call to complete.
func (g *Group[K, V]) Forget(key K) {
g.mu.Lock()
if c, ok := g.m[key]; ok {
c.forgotten = true
}
delete(g.m, key)
g.mu.Unlock()
}

324
util/singleflight/singleflight_test.go
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// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package singleflight
import (
"bytes"
"errors"
"fmt"
"os"
"os/exec"
"runtime"
"runtime/debug"
"strings"
"sync"
"sync/atomic"
"testing"
"time"
)
func TestDo(t *testing.T) {
var g Group[string, any]
v, err, _ := g.Do("key", func() (interface{}, error) {
return "bar", nil
})
if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
t.Errorf("Do = %v; want %v", got, want)
}
if err != nil {
t.Errorf("Do error = %v", err)
}
}
func TestDoErr(t *testing.T) {
var g Group[string, any]
someErr := errors.New("Some error")
v, err, _ := g.Do("key", func() (interface{}, error) {
return nil, someErr
})
if err != someErr {
t.Errorf("Do error = %v; want someErr %v", err, someErr)
}
if v != nil {
t.Errorf("unexpected non-nil value %#v", v)
}
}
func TestDoDupSuppress(t *testing.T) {
var g Group[string, any]
var wg1, wg2 sync.WaitGroup
c := make(chan string, 1)
var calls int32
fn := func() (interface{}, error) {
if atomic.AddInt32(&calls, 1) == 1 {
// First invocation.
wg1.Done()
}
v := <-c
c <- v // pump; make available for any future calls
time.Sleep(10 * time.Millisecond) // let more goroutines enter Do
return v, nil
}
const n = 10
wg1.Add(1)
for i := 0; i < n; i++ {
wg1.Add(1)
wg2.Add(1)
go func() {
defer wg2.Done()
wg1.Done()
v, err, _ := g.Do("key", fn)
if err != nil {
t.Errorf("Do error: %v", err)
return
}
if s, _ := v.(string); s != "bar" {
t.Errorf("Do = %T %v; want %q", v, v, "bar")
}
}()
}
wg1.Wait()
// At least one goroutine is in fn now and all of them have at
// least reached the line before the Do.
c <- "bar"
wg2.Wait()
if got := atomic.LoadInt32(&calls); got <= 0 || got >= n {
t.Errorf("number of calls = %d; want over 0 and less than %d", got, n)
}
}
// Test that singleflight behaves correctly after Forget called.
// See https://github.com/golang/go/issues/31420
func TestForget(t *testing.T) {
var g Group[string, any]
var (
firstStarted = make(chan struct{})
unblockFirst = make(chan struct{})
firstFinished = make(chan struct{})
)
go func() {
g.Do("key", func() (i interface{}, e error) {
close(firstStarted)
<-unblockFirst
close(firstFinished)
return
})
}()
<-firstStarted
g.Forget("key")
unblockSecond := make(chan struct{})
secondResult := g.DoChan("key", func() (i interface{}, e error) {
<-unblockSecond
return 2, nil
})
close(unblockFirst)
<-firstFinished
thirdResult := g.DoChan("key", func() (i interface{}, e error) {
return 3, nil
})
close(unblockSecond)
<-secondResult
r := <-thirdResult
if r.Val != 2 {
t.Errorf("We should receive result produced by second call, expected: 2, got %d", r.Val)
}
}
func TestDoChan(t *testing.T) {
var g Group[string, any]
ch := g.DoChan("key", func() (interface{}, error) {
return "bar", nil
})
res := <-ch
v := res.Val
err := res.Err
if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
t.Errorf("Do = %v; want %v", got, want)
}
if err != nil {
t.Errorf("Do error = %v", err)
}
}
// Test singleflight behaves correctly after Do panic.
// See https://github.com/golang/go/issues/41133
func TestPanicDo(t *testing.T) {
var g Group[string, any]
fn := func() (interface{}, error) {
panic("invalid memory address or nil pointer dereference")
}
const n = 5
waited := int32(n)
panicCount := int32(0)
done := make(chan struct{})
for i := 0; i < n; i++ {
go func() {
defer func() {
if err := recover(); err != nil {
t.Logf("Got panic: %v\n%s", err, debug.Stack())
atomic.AddInt32(&panicCount, 1)
}
if atomic.AddInt32(&waited, -1) == 0 {
close(done)
}
}()
g.Do("key", fn)
}()
}
select {
case <-done:
if panicCount != n {
t.Errorf("Expect %d panic, but got %d", n, panicCount)
}
case <-time.After(time.Second):
t.Fatalf("Do hangs")
}
}
func TestGoexitDo(t *testing.T) {
var g Group[string, any]
fn := func() (interface{}, error) {
runtime.Goexit()
return nil, nil
}
const n = 5
waited := int32(n)
done := make(chan struct{})
for i := 0; i < n; i++ {
go func() {
var err error
defer func() {
if err != nil {
t.Errorf("Error should be nil, but got: %v", err)
}
if atomic.AddInt32(&waited, -1) == 0 {
close(done)
}
}()
_, err, _ = g.Do("key", fn)
}()
}
select {
case <-done:
case <-time.After(time.Second):
t.Fatalf("Do hangs")
}
}
func TestPanicDoChan(t *testing.T) {
if runtime.GOOS == "js" {
t.Skipf("js does not support exec")
}
if os.Getenv("TEST_PANIC_DOCHAN") != "" {
defer func() {
recover()
}()
g := new(Group[string, any])
ch := g.DoChan("", func() (interface{}, error) {
panic("Panicking in DoChan")
})
<-ch
t.Fatalf("DoChan unexpectedly returned")
}
t.Parallel()
cmd := exec.Command(os.Args[0], "-test.run="+t.Name(), "-test.v")
cmd.Env = append(os.Environ(), "TEST_PANIC_DOCHAN=1")
out := new(bytes.Buffer)
cmd.Stdout = out
cmd.Stderr = out
if err := cmd.Start(); err != nil {
t.Fatal(err)
}
err := cmd.Wait()
t.Logf("%s:\n%s", strings.Join(cmd.Args, " "), out)
if err == nil {
t.Errorf("Test subprocess passed; want a crash due to panic in DoChan")
}
if bytes.Contains(out.Bytes(), []byte("DoChan unexpectedly")) {
t.Errorf("Test subprocess failed with an unexpected failure mode.")
}
if !bytes.Contains(out.Bytes(), []byte("Panicking in DoChan")) {
t.Errorf("Test subprocess failed, but the crash isn't caused by panicking in DoChan")
}
}
func TestPanicDoSharedByDoChan(t *testing.T) {
if runtime.GOOS == "js" {
t.Skipf("js does not support exec")
}
if os.Getenv("TEST_PANIC_DOCHAN") != "" {
blocked := make(chan struct{})
unblock := make(chan struct{})
g := new(Group[string, any])
go func() {
defer func() {
recover()
}()
g.Do("", func() (interface{}, error) {
close(blocked)
<-unblock
panic("Panicking in Do")
})
}()
<-blocked
ch := g.DoChan("", func() (interface{}, error) {
panic("DoChan unexpectedly executed callback")
})
close(unblock)
<-ch
t.Fatalf("DoChan unexpectedly returned")
}
t.Parallel()
cmd := exec.Command(os.Args[0], "-test.run="+t.Name(), "-test.v")
cmd.Env = append(os.Environ(), "TEST_PANIC_DOCHAN=1")
out := new(bytes.Buffer)
cmd.Stdout = out
cmd.Stderr = out
if err := cmd.Start(); err != nil {
t.Fatal(err)
}
err := cmd.Wait()
t.Logf("%s:\n%s", strings.Join(cmd.Args, " "), out)
if err == nil {
t.Errorf("Test subprocess passed; want a crash due to panic in Do shared by DoChan")
}
if bytes.Contains(out.Bytes(), []byte("DoChan unexpectedly")) {
t.Errorf("Test subprocess failed with an unexpected failure mode.")
}
if !bytes.Contains(out.Bytes(), []byte("Panicking in Do")) {
t.Errorf("Test subprocess failed, but the crash isn't caused by panicking in Do")
}
}
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