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			156 lines
		
	
	
		
			3.3 KiB
		
	
	
	
		
			Go
		
	
	
	
	
	
			
		
		
	
	
			156 lines
		
	
	
		
			3.3 KiB
		
	
	
	
		
			Go
		
	
	
	
	
	
// Copyright 2020 The LevelDB-Go and Pebble Authors. All rights reserved. Use
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// of this source code is governed by a BSD-style license that can be found in
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// the LICENSE file.
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package cache
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import "sync/atomic"
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type entryType int8
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const (
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	etTest entryType = iota
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	etCold
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	etHot
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)
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func (p entryType) String() string {
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	switch p {
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	case etTest:
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		return "test"
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	case etCold:
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		return "cold"
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	case etHot:
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		return "hot"
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	}
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	return "unknown"
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}
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// entry holds the metadata for a cache entry. The memory for an entry is
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// allocated from manually managed memory.
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//
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// Using manual memory management for entries is technically a volation of the
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// Cgo pointer rules:
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//
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//	https://golang.org/cmd/cgo/#hdr-Passing_pointers
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//
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// Specifically, Go pointers should not be stored in C allocated memory. The
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// reason for this rule is that the Go GC will not look at C allocated memory
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// to find pointers to Go objects. If the only reference to a Go object is
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// stored in C allocated memory, the object will be reclaimed. The shard field
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// of the entry struct points to a Go allocated object, thus the
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// violation. What makes this "safe" is that the Cache guarantees that there
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// are other pointers to the shard which will keep it alive.
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type entry struct {
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	key key
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	// The value associated with the entry. The entry holds a reference on the
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	// value which is maintained by entry.setValue().
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	val       *Value
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	blockLink struct {
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		next *entry
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		prev *entry
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	}
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	fileLink struct {
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		next *entry
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		prev *entry
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	}
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	size  int64
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	ptype entryType
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	// referenced is atomically set to indicate that this entry has been accessed
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	// since the last time one of the clock hands swept it.
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	referenced atomic.Bool
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	shard      *shard
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	// Reference count for the entry. The entry is freed when the reference count
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	// drops to zero.
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	ref refcnt
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}
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func newEntry(s *shard, key key, size int64) *entry {
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	e := entryAllocNew()
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	*e = entry{
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		key:   key,
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		size:  size,
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		ptype: etCold,
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		shard: s,
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	}
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	e.blockLink.next = e
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	e.blockLink.prev = e
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	e.fileLink.next = e
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	e.fileLink.prev = e
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	e.ref.init(1)
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	return e
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}
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func (e *entry) free() {
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	e.setValue(nil)
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	*e = entry{}
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	entryAllocFree(e)
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}
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func (e *entry) next() *entry {
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	if e == nil {
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		return nil
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	}
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	return e.blockLink.next
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}
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func (e *entry) prev() *entry {
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	if e == nil {
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		return nil
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	}
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	return e.blockLink.prev
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}
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func (e *entry) link(s *entry) {
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	s.blockLink.prev = e.blockLink.prev
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	s.blockLink.prev.blockLink.next = s
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	s.blockLink.next = e
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	s.blockLink.next.blockLink.prev = s
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}
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func (e *entry) unlink() *entry {
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	next := e.blockLink.next
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	e.blockLink.prev.blockLink.next = e.blockLink.next
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	e.blockLink.next.blockLink.prev = e.blockLink.prev
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	e.blockLink.prev = e
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	e.blockLink.next = e
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	return next
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}
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func (e *entry) linkFile(s *entry) {
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	s.fileLink.prev = e.fileLink.prev
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	s.fileLink.prev.fileLink.next = s
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	s.fileLink.next = e
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	s.fileLink.next.fileLink.prev = s
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}
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func (e *entry) unlinkFile() *entry {
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	next := e.fileLink.next
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	e.fileLink.prev.fileLink.next = e.fileLink.next
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	e.fileLink.next.fileLink.prev = e.fileLink.prev
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	e.fileLink.prev = e
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	e.fileLink.next = e
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	return next
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}
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func (e *entry) setValue(v *Value) {
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	if v != nil {
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		v.acquire()
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	}
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	old := e.val
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	e.val = v
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	old.release()
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}
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func (e *entry) peekValue() *Value {
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	return e.val
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}
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func (e *entry) acquireValue() *Value {
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	v := e.val
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	if v != nil {
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		v.acquire()
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	}
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	return v
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}
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