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Source file src/sync/map.go

Documentation: sync

  // Copyright 2016 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 sync
  
  import (
  	"sync/atomic"
  	"unsafe"
  )
  
  // Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
  // It is safe for multiple goroutines to call a Map's methods concurrently.
  //
  // It is optimized for use in concurrent loops with keys that are
  // stable over time, and either few steady-state stores, or stores
  // localized to one goroutine per key.
  //
  // For use cases that do not share these attributes, it will likely have
  // comparable or worse performance and worse type safety than an ordinary
  // map paired with a read-write mutex.
  //
  // The zero Map is valid and empty.
  //
  // A Map must not be copied after first use.
  type Map struct {
  	mu Mutex
  
  	// read contains the portion of the map's contents that are safe for
  	// concurrent access (with or without mu held).
  	//
  	// The read field itself is always safe to load, but must only be stored with
  	// mu held.
  	//
  	// Entries stored in read may be updated concurrently without mu, but updating
  	// a previously-expunged entry requires that the entry be copied to the dirty
  	// map and unexpunged with mu held.
  	read atomic.Value // readOnly
  
  	// dirty contains the portion of the map's contents that require mu to be
  	// held. To ensure that the dirty map can be promoted to the read map quickly,
  	// it also includes all of the non-expunged entries in the read map.
  	//
  	// Expunged entries are not stored in the dirty map. An expunged entry in the
  	// clean map must be unexpunged and added to the dirty map before a new value
  	// can be stored to it.
  	//
  	// If the dirty map is nil, the next write to the map will initialize it by
  	// making a shallow copy of the clean map, omitting stale entries.
  	dirty map[interface{}]*entry
  
  	// misses counts the number of loads since the read map was last updated that
  	// needed to lock mu to determine whether the key was present.
  	//
  	// Once enough misses have occurred to cover the cost of copying the dirty
  	// map, the dirty map will be promoted to the read map (in the unamended
  	// state) and the next store to the map will make a new dirty copy.
  	misses int
  }
  
  // readOnly is an immutable struct stored atomically in the Map.read field.
  type readOnly struct {
  	m       map[interface{}]*entry
  	amended bool // true if the dirty map contains some key not in m.
  }
  
  // expunged is an arbitrary pointer that marks entries which have been deleted
  // from the dirty map.
  var expunged = unsafe.Pointer(new(interface{}))
  
  // An entry is a slot in the map corresponding to a particular key.
  type entry struct {
  	// p points to the interface{} value stored for the entry.
  	//
  	// If p == nil, the entry has been deleted and m.dirty == nil.
  	//
  	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
  	// is missing from m.dirty.
  	//
  	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
  	// != nil, in m.dirty[key].
  	//
  	// An entry can be deleted by atomic replacement with nil: when m.dirty is
  	// next created, it will atomically replace nil with expunged and leave
  	// m.dirty[key] unset.
  	//
  	// An entry's associated value can be updated by atomic replacement, provided
  	// p != expunged. If p == expunged, an entry's associated value can be updated
  	// only after first setting m.dirty[key] = e so that lookups using the dirty
  	// map find the entry.
  	p unsafe.Pointer // *interface{}
  }
  
  func newEntry(i interface{}) *entry {
  	return &entry{p: unsafe.Pointer(&i)}
  }
  
  // Load returns the value stored in the map for a key, or nil if no
  // value is present.
  // The ok result indicates whether value was found in the map.
  func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
  	read, _ := m.read.Load().(readOnly)
  	e, ok := read.m[key]
  	if !ok && read.amended {
  		m.mu.Lock()
  		// Avoid reporting a spurious miss if m.dirty got promoted while we were
  		// blocked on m.mu. (If further loads of the same key will not miss, it's
  		// not worth copying the dirty map for this key.)
  		read, _ = m.read.Load().(readOnly)
  		e, ok = read.m[key]
  		if !ok && read.amended {
  			e, ok = m.dirty[key]
  			// Regardless of whether the entry was present, record a miss: this key
  			// will take the slow path until the dirty map is promoted to the read
  			// map.
  			m.missLocked()
  		}
  		m.mu.Unlock()
  	}
  	if !ok {
  		return nil, false
  	}
  	return e.load()
  }
  
  func (e *entry) load() (value interface{}, ok bool) {
  	p := atomic.LoadPointer(&e.p)
  	if p == nil || p == expunged {
  		return nil, false
  	}
  	return *(*interface{})(p), true
  }
  
  // Store sets the value for a key.
  func (m *Map) Store(key, value interface{}) {
  	read, _ := m.read.Load().(readOnly)
  	if e, ok := read.m[key]; ok && e.tryStore(&value) {
  		return
  	}
  
  	m.mu.Lock()
  	read, _ = m.read.Load().(readOnly)
  	if e, ok := read.m[key]; ok {
  		if e.unexpungeLocked() {
  			// The entry was previously expunged, which implies that there is a
  			// non-nil dirty map and this entry is not in it.
  			m.dirty[key] = e
  		}
  		e.storeLocked(&value)
  	} else if e, ok := m.dirty[key]; ok {
  		e.storeLocked(&value)
  	} else {
  		if !read.amended {
  			// We're adding the first new key to the dirty map.
  			// Make sure it is allocated and mark the read-only map as incomplete.
  			m.dirtyLocked()
  			m.read.Store(readOnly{m: read.m, amended: true})
  		}
  		m.dirty[key] = newEntry(value)
  	}
  	m.mu.Unlock()
  }
  
  // tryStore stores a value if the entry has not been expunged.
  //
  // If the entry is expunged, tryStore returns false and leaves the entry
  // unchanged.
  func (e *entry) tryStore(i *interface{}) bool {
  	p := atomic.LoadPointer(&e.p)
  	if p == expunged {
  		return false
  	}
  	for {
  		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
  			return true
  		}
  		p = atomic.LoadPointer(&e.p)
  		if p == expunged {
  			return false
  		}
  	}
  }
  
  // unexpungeLocked ensures that the entry is not marked as expunged.
  //
  // If the entry was previously expunged, it must be added to the dirty map
  // before m.mu is unlocked.
  func (e *entry) unexpungeLocked() (wasExpunged bool) {
  	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
  }
  
  // storeLocked unconditionally stores a value to the entry.
  //
  // The entry must be known not to be expunged.
  func (e *entry) storeLocked(i *interface{}) {
  	atomic.StorePointer(&e.p, unsafe.Pointer(i))
  }
  
  // LoadOrStore returns the existing value for the key if present.
  // Otherwise, it stores and returns the given value.
  // The loaded result is true if the value was loaded, false if stored.
  func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
  	// Avoid locking if it's a clean hit.
  	read, _ := m.read.Load().(readOnly)
  	if e, ok := read.m[key]; ok {
  		actual, loaded, ok := e.tryLoadOrStore(value)
  		if ok {
  			return actual, loaded
  		}
  	}
  
  	m.mu.Lock()
  	read, _ = m.read.Load().(readOnly)
  	if e, ok := read.m[key]; ok {
  		if e.unexpungeLocked() {
  			m.dirty[key] = e
  		}
  		actual, loaded, _ = e.tryLoadOrStore(value)
  	} else if e, ok := m.dirty[key]; ok {
  		actual, loaded, _ = e.tryLoadOrStore(value)
  		m.missLocked()
  	} else {
  		if !read.amended {
  			// We're adding the first new key to the dirty map.
  			// Make sure it is allocated and mark the read-only map as incomplete.
  			m.dirtyLocked()
  			m.read.Store(readOnly{m: read.m, amended: true})
  		}
  		m.dirty[key] = newEntry(value)
  		actual, loaded = value, false
  	}
  	m.mu.Unlock()
  
  	return actual, loaded
  }
  
  // tryLoadOrStore atomically loads or stores a value if the entry is not
  // expunged.
  //
  // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
  // returns with ok==false.
  func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
  	p := atomic.LoadPointer(&e.p)
  	if p == expunged {
  		return nil, false, false
  	}
  	if p != nil {
  		return *(*interface{})(p), true, true
  	}
  
  	// Copy the interface after the first load to make this method more amenable
  	// to escape analysis: if we hit the "load" path or the entry is expunged, we
  	// shouldn't bother heap-allocating.
  	ic := i
  	for {
  		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
  			return i, false, true
  		}
  		p = atomic.LoadPointer(&e.p)
  		if p == expunged {
  			return nil, false, false
  		}
  		if p != nil {
  			return *(*interface{})(p), true, true
  		}
  	}
  }
  
  // Delete deletes the value for a key.
  func (m *Map) Delete(key interface{}) {
  	read, _ := m.read.Load().(readOnly)
  	e, ok := read.m[key]
  	if !ok && read.amended {
  		m.mu.Lock()
  		read, _ = m.read.Load().(readOnly)
  		e, ok = read.m[key]
  		if !ok && read.amended {
  			delete(m.dirty, key)
  		}
  		m.mu.Unlock()
  	}
  	if ok {
  		e.delete()
  	}
  }
  
  func (e *entry) delete() (hadValue bool) {
  	for {
  		p := atomic.LoadPointer(&e.p)
  		if p == nil || p == expunged {
  			return false
  		}
  		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
  			return true
  		}
  	}
  }
  
  // Range calls f sequentially for each key and value present in the map.
  // If f returns false, range stops the iteration.
  //
  // Range does not necessarily correspond to any consistent snapshot of the Map's
  // contents: no key will be visited more than once, but if the value for any key
  // is stored or deleted concurrently, Range may reflect any mapping for that key
  // from any point during the Range call.
  //
  // Range may be O(N) with the number of elements in the map even if f returns
  // false after a constant number of calls.
  func (m *Map) Range(f func(key, value interface{}) bool) {
  	// We need to be able to iterate over all of the keys that were already
  	// present at the start of the call to Range.
  	// If read.amended is false, then read.m satisfies that property without
  	// requiring us to hold m.mu for a long time.
  	read, _ := m.read.Load().(readOnly)
  	if read.amended {
  		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
  		// (assuming the caller does not break out early), so a call to Range
  		// amortizes an entire copy of the map: we can promote the dirty copy
  		// immediately!
  		m.mu.Lock()
  		read, _ = m.read.Load().(readOnly)
  		if read.amended {
  			read = readOnly{m: m.dirty}
  			m.read.Store(read)
  			m.dirty = nil
  			m.misses = 0
  		}
  		m.mu.Unlock()
  	}
  
  	for k, e := range read.m {
  		v, ok := e.load()
  		if !ok {
  			continue
  		}
  		if !f(k, v) {
  			break
  		}
  	}
  }
  
  func (m *Map) missLocked() {
  	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.read.Load().(readOnly)
  	m.dirty = make(map[interface{}]*entry, len(read.m))
  	for k, e := range read.m {
  		if !e.tryExpungeLocked() {
  			m.dirty[k] = e
  		}
  	}
  }
  
  func (e *entry) tryExpungeLocked() (isExpunged bool) {
  	p := atomic.LoadPointer(&e.p)
  	for p == nil {
  		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
  			return true
  		}
  		p = atomic.LoadPointer(&e.p)
  	}
  	return p == expunged
  }
  

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