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