[WIP] Initial commit

vendor/github.com/cockroachdb/pebble/internal/cache/entry.go

@@ -0,0 +1,155 @@
+// Copyright 2020 The LevelDB-Go and Pebble 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 cache
+
+import "sync/atomic"
+
+type entryType int8
+
+const (
+	etTest entryType = iota
+	etCold
+	etHot
+)
+
+func (p entryType) String() string {
+	switch p {
+	case etTest:
+		return "test"
+	case etCold:
+		return "cold"
+	case etHot:
+		return "hot"
+	}
+	return "unknown"
+}
+
+// entry holds the metadata for a cache entry. The memory for an entry is
+// allocated from manually managed memory.
+//
+// Using manual memory management for entries is technically a volation of the
+// Cgo pointer rules:
+//
+//	https://golang.org/cmd/cgo/#hdr-Passing_pointers
+//
+// Specifically, Go pointers should not be stored in C allocated memory. The
+// reason for this rule is that the Go GC will not look at C allocated memory
+// to find pointers to Go objects. If the only reference to a Go object is
+// stored in C allocated memory, the object will be reclaimed. The shard field
+// of the entry struct points to a Go allocated object, thus the
+// violation. What makes this "safe" is that the Cache guarantees that there
+// are other pointers to the shard which will keep it alive.
+type entry struct {
+	key key
+	// The value associated with the entry. The entry holds a reference on the
+	// value which is maintained by entry.setValue().
+	val       *Value
+	blockLink struct {
+		next *entry
+		prev *entry
+	}
+	fileLink struct {
+		next *entry
+		prev *entry
+	}
+	size  int64
+	ptype entryType
+	// referenced is atomically set to indicate that this entry has been accessed
+	// since the last time one of the clock hands swept it.
+	referenced atomic.Bool
+	shard      *shard
+	// Reference count for the entry. The entry is freed when the reference count
+	// drops to zero.
+	ref refcnt
+}
+
+func newEntry(s *shard, key key, size int64) *entry {
+	e := entryAllocNew()
+	*e = entry{
+		key:   key,
+		size:  size,
+		ptype: etCold,
+		shard: s,
+	}
+	e.blockLink.next = e
+	e.blockLink.prev = e
+	e.fileLink.next = e
+	e.fileLink.prev = e
+	e.ref.init(1)
+	return e
+}
+
+func (e *entry) free() {
+	e.setValue(nil)
+	*e = entry{}
+	entryAllocFree(e)
+}
+
+func (e *entry) next() *entry {
+	if e == nil {
+		return nil
+	}
+	return e.blockLink.next
+}
+
+func (e *entry) prev() *entry {
+	if e == nil {
+		return nil
+	}
+	return e.blockLink.prev
+}
+
+func (e *entry) link(s *entry) {
+	s.blockLink.prev = e.blockLink.prev
+	s.blockLink.prev.blockLink.next = s
+	s.blockLink.next = e
+	s.blockLink.next.blockLink.prev = s
+}
+
+func (e *entry) unlink() *entry {
+	next := e.blockLink.next
+	e.blockLink.prev.blockLink.next = e.blockLink.next
+	e.blockLink.next.blockLink.prev = e.blockLink.prev
+	e.blockLink.prev = e
+	e.blockLink.next = e
+	return next
+}
+
+func (e *entry) linkFile(s *entry) {
+	s.fileLink.prev = e.fileLink.prev
+	s.fileLink.prev.fileLink.next = s
+	s.fileLink.next = e
+	s.fileLink.next.fileLink.prev = s
+}
+
+func (e *entry) unlinkFile() *entry {
+	next := e.fileLink.next
+	e.fileLink.prev.fileLink.next = e.fileLink.next
+	e.fileLink.next.fileLink.prev = e.fileLink.prev
+	e.fileLink.prev = e
+	e.fileLink.next = e
+	return next
+}
+
+func (e *entry) setValue(v *Value) {
+	if v != nil {
+		v.acquire()
+	}
+	old := e.val
+	e.val = v
+	old.release()
+}
+
+func (e *entry) peekValue() *Value {
+	return e.val
+}
+
+func (e *entry) acquireValue() *Value {
+	v := e.val
+	if v != nil {
+		v.acquire()
+	}
+	return v
+}