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// 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.
package singleflight // import "golang.org/x/sync/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 (p *panicError) Unwrap() error {
err, ok := p.value.(error)
if !ok {
return nil
}
return err
}
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 struct {
wg sync.WaitGroup
// These fields are written once before the WaitGroup is done
// and are only read after the WaitGroup is done.
val interface{}
err error
// 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
}
// Group represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type Group struct {
mu sync.Mutex // protects m
m map[string]*call // lazily initialized
}
// Result holds the results of Do, so they can be passed
// on a channel.
type Result struct {
Val interface{}
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) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
g.mu.Lock()
if g.m == nil {
g.m = make(map[string]*call)
}
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)
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) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
ch := make(chan Result, 1)
g.mu.Lock()
if g.m == nil {
g.m = make(map[string]*call)
}
if c, ok := g.m[key]; ok {
c.dups++
c.chans = append(c.chans, ch)
g.mu.Unlock()
return ch
}
c := &call{chans: []chan<- Result{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) doCall(c *call, key string, fn func() (interface{}, 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
}
g.mu.Lock()
defer g.mu.Unlock()
c.wg.Done()
if g.m[key] == c {
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{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) Forget(key string) {
g.mu.Lock()
delete(g.m, key)
g.mu.Unlock()
}