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Diffstat (limited to '')
-rw-r--r-- | dependencies/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/syncmap/pre_go19.go | 371 |
1 files changed, 371 insertions, 0 deletions
diff --git a/dependencies/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/syncmap/pre_go19.go b/dependencies/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/syncmap/pre_go19.go new file mode 100644 index 0000000..5bba413 --- /dev/null +++ b/dependencies/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/syncmap/pre_go19.go @@ -0,0 +1,371 @@ +// 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. + +//go:build !go1.9 +// +build !go1.9 + +package syncmap + +import ( + "sync" + "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. +// +// The zero Map is valid and empty. +// +// A Map must not be copied after first use. +type Map struct { + mu sync.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 +} |