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285 lines
5.4 KiB
285 lines
5.4 KiB
package clist
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/*
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The purpose of CList is to provide a goroutine-safe linked-list.
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This list can be traversed concurrently by any number of goroutines.
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However, removed CElements cannot be added back.
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NOTE: Not all methods of container/list are (yet) implemented.
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NOTE: Removed elements need to DetachPrev or DetachNext consistently
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to ensure garbage collection of removed elements.
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*/
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import (
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"sync"
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"sync/atomic"
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"unsafe"
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)
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// CElement is an element of a linked-list
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// Traversal from a CElement are goroutine-safe.
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type CElement struct {
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prev unsafe.Pointer
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prevWg *sync.WaitGroup
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next unsafe.Pointer
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nextWg *sync.WaitGroup
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removed uint32
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Value interface{}
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}
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// Blocking implementation of Next().
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// May return nil iff CElement was tail and got removed.
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func (e *CElement) NextWait() *CElement {
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for {
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e.nextWg.Wait()
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next := e.Next()
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if next == nil {
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if e.Removed() {
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return nil
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} else {
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continue
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}
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} else {
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return next
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}
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}
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}
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// Blocking implementation of Prev().
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// May return nil iff CElement was head and got removed.
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func (e *CElement) PrevWait() *CElement {
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for {
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e.prevWg.Wait()
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prev := e.Prev()
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if prev == nil {
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if e.Removed() {
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return nil
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} else {
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continue
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}
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} else {
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return prev
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}
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}
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}
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// Nonblocking, may return nil if at the end.
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func (e *CElement) Next() *CElement {
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return (*CElement)(atomic.LoadPointer(&e.next))
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}
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// Nonblocking, may return nil if at the end.
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func (e *CElement) Prev() *CElement {
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return (*CElement)(atomic.LoadPointer(&e.prev))
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}
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func (e *CElement) Removed() bool {
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return atomic.LoadUint32(&(e.removed)) > 0
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}
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func (e *CElement) DetachNext() {
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if !e.Removed() {
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panic("DetachNext() must be called after Remove(e)")
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}
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atomic.StorePointer(&e.next, nil)
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}
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func (e *CElement) DetachPrev() {
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if !e.Removed() {
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panic("DetachPrev() must be called after Remove(e)")
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}
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atomic.StorePointer(&e.prev, nil)
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}
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func (e *CElement) setNextAtomic(next *CElement) {
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for {
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oldNext := atomic.LoadPointer(&e.next)
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if !atomic.CompareAndSwapPointer(&(e.next), oldNext, unsafe.Pointer(next)) {
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continue
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}
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if next == nil && oldNext != nil { // We for-loop in NextWait() so race is ok
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e.nextWg.Add(1)
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}
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if next != nil && oldNext == nil {
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e.nextWg.Done()
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}
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return
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}
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}
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func (e *CElement) setPrevAtomic(prev *CElement) {
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for {
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oldPrev := atomic.LoadPointer(&e.prev)
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if !atomic.CompareAndSwapPointer(&(e.prev), oldPrev, unsafe.Pointer(prev)) {
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continue
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}
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if prev == nil && oldPrev != nil { // We for-loop in PrevWait() so race is ok
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e.prevWg.Add(1)
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}
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if prev != nil && oldPrev == nil {
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e.prevWg.Done()
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}
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return
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}
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}
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func (e *CElement) setRemovedAtomic() {
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atomic.StoreUint32(&(e.removed), 1)
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}
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//--------------------------------------------------------------------------------
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// CList represents a linked list.
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// The zero value for CList is an empty list ready to use.
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// Operations are goroutine-safe.
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type CList struct {
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mtx sync.Mutex
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wg *sync.WaitGroup
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head *CElement // first element
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tail *CElement // last element
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len int // list length
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}
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func (l *CList) Init() *CList {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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l.wg = waitGroup1()
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l.head = nil
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l.tail = nil
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l.len = 0
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return l
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}
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func New() *CList { return new(CList).Init() }
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func (l *CList) Len() int {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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return l.len
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}
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func (l *CList) Front() *CElement {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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return l.head
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}
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func (l *CList) FrontWait() *CElement {
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for {
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l.mtx.Lock()
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head := l.head
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wg := l.wg
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l.mtx.Unlock()
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if head == nil {
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wg.Wait()
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} else {
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return head
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}
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}
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}
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func (l *CList) Back() *CElement {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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return l.tail
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}
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func (l *CList) BackWait() *CElement {
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for {
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l.mtx.Lock()
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tail := l.tail
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wg := l.wg
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l.mtx.Unlock()
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if tail == nil {
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wg.Wait()
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} else {
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return tail
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}
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}
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}
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func (l *CList) PushBack(v interface{}) *CElement {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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// Construct a new element
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e := &CElement{
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prev: nil,
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prevWg: waitGroup1(),
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next: nil,
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nextWg: waitGroup1(),
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Value: v,
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}
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// Release waiters on FrontWait/BackWait maybe
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if l.len == 0 {
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l.wg.Done()
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}
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l.len += 1
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// Modify the tail
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if l.tail == nil {
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l.head = e
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l.tail = e
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} else {
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l.tail.setNextAtomic(e)
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e.setPrevAtomic(l.tail)
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l.tail = e
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}
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return e
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}
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// CONTRACT: Caller must call e.DetachPrev() and/or e.DetachNext() to avoid memory leaks.
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// NOTE: As per the contract of CList, removed elements cannot be added back.
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func (l *CList) Remove(e *CElement) interface{} {
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l.mtx.Lock()
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defer l.mtx.Unlock()
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prev := e.Prev()
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next := e.Next()
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if l.head == nil || l.tail == nil {
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panic("Remove(e) on empty CList")
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}
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if prev == nil && l.head != e {
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panic("Remove(e) with false head")
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}
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if next == nil && l.tail != e {
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panic("Remove(e) with false tail")
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}
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// If we're removing the only item, make CList FrontWait/BackWait wait.
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if l.len == 1 {
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l.wg.Add(1)
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}
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l.len -= 1
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// Connect next/prev and set head/tail
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if prev == nil {
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l.head = next
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} else {
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prev.setNextAtomic(next)
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}
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if next == nil {
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l.tail = prev
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} else {
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next.setPrevAtomic(prev)
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}
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// Set .Done() on e, otherwise waiters will wait forever.
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e.setRemovedAtomic()
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if prev == nil {
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e.prevWg.Done()
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}
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if next == nil {
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e.nextWg.Done()
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}
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return e.Value
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}
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func waitGroup1() (wg *sync.WaitGroup) {
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wg = &sync.WaitGroup{}
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wg.Add(1)
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return
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}
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