mirror of https://go.googlesource.com/go
539 lines
16 KiB
Go
539 lines
16 KiB
Go
// Copyright 2016 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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package sync
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import (
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"sync/atomic"
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)
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// Map is like a Go map[any]any but is safe for concurrent use
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// by multiple goroutines without additional locking or coordination.
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// Loads, stores, and deletes run in amortized constant time.
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//
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// The Map type is specialized. Most code should use a plain Go map instead,
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// with separate locking or coordination, for better type safety and to make it
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// easier to maintain other invariants along with the map content.
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//
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// The Map type is optimized for two common use cases: (1) when the entry for a given
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// key is only ever written once but read many times, as in caches that only grow,
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// or (2) when multiple goroutines read, write, and overwrite entries for disjoint
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// sets of keys. In these two cases, use of a Map may significantly reduce lock
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// contention compared to a Go map paired with a separate [Mutex] or [RWMutex].
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//
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// The zero Map is empty and ready for use. A Map must not be copied after first use.
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//
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// In the terminology of the Go memory model, Map arranges that a write operation
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// “synchronizes before” any read operation that observes the effect of the write, where
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// read and write operations are defined as follows.
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// [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap],
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// and [Map.CompareAndDelete] are read operations;
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// [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations;
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// [Map.LoadOrStore] is a write operation when it returns loaded set to false;
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// [Map.CompareAndSwap] is a write operation when it returns swapped set to true;
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// and [Map.CompareAndDelete] is a write operation when it returns deleted set to true.
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type Map struct {
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mu Mutex
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// read contains the portion of the map's contents that are safe for
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// concurrent access (with or without mu held).
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//
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// The read field itself is always safe to load, but must only be stored with
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// mu held.
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//
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// Entries stored in read may be updated concurrently without mu, but updating
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// a previously-expunged entry requires that the entry be copied to the dirty
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// map and unexpunged with mu held.
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read atomic.Pointer[readOnly]
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// dirty contains the portion of the map's contents that require mu to be
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// held. To ensure that the dirty map can be promoted to the read map quickly,
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// it also includes all of the non-expunged entries in the read map.
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//
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// Expunged entries are not stored in the dirty map. An expunged entry in the
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// clean map must be unexpunged and added to the dirty map before a new value
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// can be stored to it.
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//
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// If the dirty map is nil, the next write to the map will initialize it by
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// making a shallow copy of the clean map, omitting stale entries.
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dirty map[any]*entry
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// misses counts the number of loads since the read map was last updated that
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// needed to lock mu to determine whether the key was present.
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//
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// Once enough misses have occurred to cover the cost of copying the dirty
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// map, the dirty map will be promoted to the read map (in the unamended
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// state) and the next store to the map will make a new dirty copy.
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misses int
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}
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// readOnly is an immutable struct stored atomically in the Map.read field.
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type readOnly struct {
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m map[any]*entry
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amended bool // true if the dirty map contains some key not in m.
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}
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// expunged is an arbitrary pointer that marks entries which have been deleted
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// from the dirty map.
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var expunged = new(any)
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// An entry is a slot in the map corresponding to a particular key.
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type entry struct {
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// p points to the interface{} value stored for the entry.
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//
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// If p == nil, the entry has been deleted, and either m.dirty == nil or
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// m.dirty[key] is e.
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//
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// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
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// is missing from m.dirty.
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//
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// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
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// != nil, in m.dirty[key].
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//
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// An entry can be deleted by atomic replacement with nil: when m.dirty is
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// next created, it will atomically replace nil with expunged and leave
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// m.dirty[key] unset.
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//
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// An entry's associated value can be updated by atomic replacement, provided
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// p != expunged. If p == expunged, an entry's associated value can be updated
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// only after first setting m.dirty[key] = e so that lookups using the dirty
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// map find the entry.
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p atomic.Pointer[any]
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}
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func newEntry(i any) *entry {
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e := &entry{}
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e.p.Store(&i)
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return e
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}
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func (m *Map) loadReadOnly() readOnly {
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if p := m.read.Load(); p != nil {
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return *p
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}
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return readOnly{}
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}
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// Load returns the value stored in the map for a key, or nil if no
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// value is present.
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// The ok result indicates whether value was found in the map.
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func (m *Map) Load(key any) (value any, ok bool) {
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read := m.loadReadOnly()
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e, ok := read.m[key]
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if !ok && read.amended {
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m.mu.Lock()
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// Avoid reporting a spurious miss if m.dirty got promoted while we were
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// blocked on m.mu. (If further loads of the same key will not miss, it's
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// not worth copying the dirty map for this key.)
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read = m.loadReadOnly()
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e, ok = read.m[key]
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if !ok && read.amended {
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e, ok = m.dirty[key]
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// Regardless of whether the entry was present, record a miss: this key
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// will take the slow path until the dirty map is promoted to the read
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// map.
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m.missLocked()
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}
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m.mu.Unlock()
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}
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if !ok {
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return nil, false
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}
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return e.load()
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}
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func (e *entry) load() (value any, ok bool) {
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p := e.p.Load()
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if p == nil || p == expunged {
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return nil, false
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}
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return *p, true
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}
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// Store sets the value for a key.
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func (m *Map) Store(key, value any) {
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_, _ = m.Swap(key, value)
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}
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// Clear deletes all the entries, resulting in an empty Map.
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func (m *Map) Clear() {
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read := m.loadReadOnly()
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if len(read.m) == 0 && !read.amended {
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// Avoid allocating a new readOnly when the map is already clear.
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return
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}
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m.mu.Lock()
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defer m.mu.Unlock()
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read = m.loadReadOnly()
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if len(read.m) > 0 || read.amended {
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m.read.Store(&readOnly{})
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}
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clear(m.dirty)
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// Don't immediately promote the newly-cleared dirty map on the next operation.
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m.misses = 0
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}
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// tryCompareAndSwap compare the entry with the given old value and swaps
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// it with a new value if the entry is equal to the old value, and the entry
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// has not been expunged.
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//
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// If the entry is expunged, tryCompareAndSwap returns false and leaves
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// the entry unchanged.
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func (e *entry) tryCompareAndSwap(old, new any) bool {
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p := e.p.Load()
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if p == nil || p == expunged || *p != old {
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return false
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}
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// Copy the interface after the first load to make this method more amenable
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// to escape analysis: if the comparison fails from the start, we shouldn't
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// bother heap-allocating an interface value to store.
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nc := new
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for {
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if e.p.CompareAndSwap(p, &nc) {
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return true
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}
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p = e.p.Load()
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if p == nil || p == expunged || *p != old {
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return false
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}
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}
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}
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// unexpungeLocked ensures that the entry is not marked as expunged.
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//
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// If the entry was previously expunged, it must be added to the dirty map
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// before m.mu is unlocked.
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func (e *entry) unexpungeLocked() (wasExpunged bool) {
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return e.p.CompareAndSwap(expunged, nil)
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}
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// swapLocked unconditionally swaps a value into the entry.
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//
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// The entry must be known not to be expunged.
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func (e *entry) swapLocked(i *any) *any {
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return e.p.Swap(i)
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}
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// LoadOrStore returns the existing value for the key if present.
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// Otherwise, it stores and returns the given value.
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// The loaded result is true if the value was loaded, false if stored.
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func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) {
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// Avoid locking if it's a clean hit.
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read := m.loadReadOnly()
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if e, ok := read.m[key]; ok {
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actual, loaded, ok := e.tryLoadOrStore(value)
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if ok {
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return actual, loaded
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}
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}
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m.mu.Lock()
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read = m.loadReadOnly()
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if e, ok := read.m[key]; ok {
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if e.unexpungeLocked() {
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m.dirty[key] = e
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}
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actual, loaded, _ = e.tryLoadOrStore(value)
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} else if e, ok := m.dirty[key]; ok {
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actual, loaded, _ = e.tryLoadOrStore(value)
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m.missLocked()
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} else {
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if !read.amended {
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// We're adding the first new key to the dirty map.
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// Make sure it is allocated and mark the read-only map as incomplete.
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m.dirtyLocked()
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m.read.Store(&readOnly{m: read.m, amended: true})
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}
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m.dirty[key] = newEntry(value)
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actual, loaded = value, false
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}
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m.mu.Unlock()
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return actual, loaded
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}
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// tryLoadOrStore atomically loads or stores a value if the entry is not
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// expunged.
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//
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// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
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// returns with ok==false.
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func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) {
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p := e.p.Load()
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if p == expunged {
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return nil, false, false
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}
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if p != nil {
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return *p, true, true
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}
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// Copy the interface after the first load to make this method more amenable
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// to escape analysis: if we hit the "load" path or the entry is expunged, we
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// shouldn't bother heap-allocating.
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ic := i
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for {
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if e.p.CompareAndSwap(nil, &ic) {
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return i, false, true
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}
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p = e.p.Load()
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if p == expunged {
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return nil, false, false
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}
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if p != nil {
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return *p, true, true
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}
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}
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}
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// LoadAndDelete deletes the value for a key, returning the previous value if any.
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// The loaded result reports whether the key was present.
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func (m *Map) LoadAndDelete(key any) (value any, loaded bool) {
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read := m.loadReadOnly()
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e, ok := read.m[key]
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if !ok && read.amended {
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m.mu.Lock()
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read = m.loadReadOnly()
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e, ok = read.m[key]
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if !ok && read.amended {
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e, ok = m.dirty[key]
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delete(m.dirty, key)
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// Regardless of whether the entry was present, record a miss: this key
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// will take the slow path until the dirty map is promoted to the read
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// map.
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m.missLocked()
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}
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m.mu.Unlock()
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}
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if ok {
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return e.delete()
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}
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return nil, false
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}
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// Delete deletes the value for a key.
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func (m *Map) Delete(key any) {
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m.LoadAndDelete(key)
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}
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func (e *entry) delete() (value any, ok bool) {
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for {
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p := e.p.Load()
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if p == nil || p == expunged {
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return nil, false
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}
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if e.p.CompareAndSwap(p, nil) {
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return *p, true
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}
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}
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}
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// trySwap swaps a value if the entry has not been expunged.
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//
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// If the entry is expunged, trySwap returns false and leaves the entry
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// unchanged.
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func (e *entry) trySwap(i *any) (*any, bool) {
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for {
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p := e.p.Load()
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if p == expunged {
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return nil, false
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}
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if e.p.CompareAndSwap(p, i) {
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return p, true
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}
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}
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}
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// Swap swaps the value for a key and returns the previous value if any.
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// The loaded result reports whether the key was present.
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func (m *Map) Swap(key, value any) (previous any, loaded bool) {
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read := m.loadReadOnly()
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if e, ok := read.m[key]; ok {
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if v, ok := e.trySwap(&value); ok {
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if v == nil {
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return nil, false
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}
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return *v, true
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}
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}
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m.mu.Lock()
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read = m.loadReadOnly()
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if e, ok := read.m[key]; ok {
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if e.unexpungeLocked() {
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// The entry was previously expunged, which implies that there is a
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// non-nil dirty map and this entry is not in it.
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m.dirty[key] = e
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}
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if v := e.swapLocked(&value); v != nil {
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loaded = true
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previous = *v
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}
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} else if e, ok := m.dirty[key]; ok {
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if v := e.swapLocked(&value); v != nil {
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loaded = true
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previous = *v
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}
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} else {
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if !read.amended {
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// We're adding the first new key to the dirty map.
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// Make sure it is allocated and mark the read-only map as incomplete.
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m.dirtyLocked()
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m.read.Store(&readOnly{m: read.m, amended: true})
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}
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m.dirty[key] = newEntry(value)
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}
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m.mu.Unlock()
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return previous, loaded
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}
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// CompareAndSwap swaps the old and new values for key
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// if the value stored in the map is equal to old.
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// The old value must be of a comparable type.
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func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) {
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read := m.loadReadOnly()
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if e, ok := read.m[key]; ok {
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return e.tryCompareAndSwap(old, new)
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} else if !read.amended {
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return false // No existing value for key.
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}
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m.mu.Lock()
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defer m.mu.Unlock()
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read = m.loadReadOnly()
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swapped = false
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if e, ok := read.m[key]; ok {
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swapped = e.tryCompareAndSwap(old, new)
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} else if e, ok := m.dirty[key]; ok {
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swapped = e.tryCompareAndSwap(old, new)
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// We needed to lock mu in order to load the entry for key,
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// and the operation didn't change the set of keys in the map
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// (so it would be made more efficient by promoting the dirty
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// map to read-only).
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// Count it as a miss so that we will eventually switch to the
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// more efficient steady state.
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m.missLocked()
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}
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return swapped
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}
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// CompareAndDelete deletes the entry for key if its value is equal to old.
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// The old value must be of a comparable type.
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//
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// If there is no current value for key in the map, CompareAndDelete
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// returns false (even if the old value is the nil interface value).
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func (m *Map) CompareAndDelete(key, old any) (deleted bool) {
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read := m.loadReadOnly()
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e, ok := read.m[key]
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if !ok && read.amended {
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m.mu.Lock()
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read = m.loadReadOnly()
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e, ok = read.m[key]
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if !ok && read.amended {
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e, ok = m.dirty[key]
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// Don't delete key from m.dirty: we still need to do the “compare” part
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// of the operation. The entry will eventually be expunged when the
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// dirty map is promoted to the read map.
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//
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// Regardless of whether the entry was present, record a miss: this key
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// will take the slow path until the dirty map is promoted to the read
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// map.
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m.missLocked()
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}
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m.mu.Unlock()
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}
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for ok {
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p := e.p.Load()
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if p == nil || p == expunged || *p != old {
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return false
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}
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if e.p.CompareAndSwap(p, nil) {
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return true
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}
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}
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return false
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}
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// Range calls f sequentially for each key and value present in the map.
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// If f returns false, range stops the iteration.
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//
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// Range does not necessarily correspond to any consistent snapshot of the Map's
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// contents: no key will be visited more than once, but if the value for any key
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// is stored or deleted concurrently (including by f), Range may reflect any
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// mapping for that key from any point during the Range call. Range does not
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// block other methods on the receiver; even f itself may call any method on m.
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//
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// Range may be O(N) with the number of elements in the map even if f returns
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// false after a constant number of calls.
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func (m *Map) Range(f func(key, value any) bool) {
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// We need to be able to iterate over all of the keys that were already
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// present at the start of the call to Range.
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// If read.amended is false, then read.m satisfies that property without
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// requiring us to hold m.mu for a long time.
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read := m.loadReadOnly()
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if read.amended {
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// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
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// (assuming the caller does not break out early), so a call to Range
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// amortizes an entire copy of the map: we can promote the dirty copy
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// immediately!
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m.mu.Lock()
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read = m.loadReadOnly()
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if read.amended {
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read = readOnly{m: m.dirty}
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copyRead := read
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m.read.Store(©Read)
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m.dirty = nil
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m.misses = 0
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}
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m.mu.Unlock()
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}
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for k, e := range read.m {
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v, ok := e.load()
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if !ok {
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continue
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}
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if !f(k, v) {
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break
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}
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}
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}
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func (m *Map) missLocked() {
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m.misses++
|
|
if m.misses < len(m.dirty) {
|
|
return
|
|
}
|
|
m.read.Store(&readOnly{m: m.dirty})
|
|
m.dirty = nil
|
|
m.misses = 0
|
|
}
|
|
|
|
func (m *Map) dirtyLocked() {
|
|
if m.dirty != nil {
|
|
return
|
|
}
|
|
|
|
read := m.loadReadOnly()
|
|
m.dirty = make(map[any]*entry, len(read.m))
|
|
for k, e := range read.m {
|
|
if !e.tryExpungeLocked() {
|
|
m.dirty[k] = e
|
|
}
|
|
}
|
|
}
|
|
|
|
func (e *entry) tryExpungeLocked() (isExpunged bool) {
|
|
p := e.p.Load()
|
|
for p == nil {
|
|
if e.p.CompareAndSwap(nil, expunged) {
|
|
return true
|
|
}
|
|
p = e.p.Load()
|
|
}
|
|
return p == expunged
|
|
}
|