github.com/cockroachdb/pebble@v0.0.0-20231214172447-ab4952c5f87b/internal/cache/entry.go (about)

     1  // Copyright 2020 The LevelDB-Go and Pebble Authors. All rights reserved. Use
     2  // of this source code is governed by a BSD-style license that can be found in
     3  // the LICENSE file.
     4  
     5  package cache
     6  
     7  import "sync/atomic"
     8  
     9  type entryType int8
    10  
    11  const (
    12  	etTest entryType = iota
    13  	etCold
    14  	etHot
    15  )
    16  
    17  func (p entryType) String() string {
    18  	switch p {
    19  	case etTest:
    20  		return "test"
    21  	case etCold:
    22  		return "cold"
    23  	case etHot:
    24  		return "hot"
    25  	}
    26  	return "unknown"
    27  }
    28  
    29  // entry holds the metadata for a cache entry. The memory for an entry is
    30  // allocated from manually managed memory.
    31  //
    32  // Using manual memory management for entries is technically a volation of the
    33  // Cgo pointer rules:
    34  //
    35  //	https://golang.org/cmd/cgo/#hdr-Passing_pointers
    36  //
    37  // Specifically, Go pointers should not be stored in C allocated memory. The
    38  // reason for this rule is that the Go GC will not look at C allocated memory
    39  // to find pointers to Go objects. If the only reference to a Go object is
    40  // stored in C allocated memory, the object will be reclaimed. The shard field
    41  // of the entry struct points to a Go allocated object, thus the
    42  // violation. What makes this "safe" is that the Cache guarantees that there
    43  // are other pointers to the shard which will keep it alive.
    44  type entry struct {
    45  	key key
    46  	// The value associated with the entry. The entry holds a reference on the
    47  	// value which is maintained by entry.setValue().
    48  	val       *Value
    49  	blockLink struct {
    50  		next *entry
    51  		prev *entry
    52  	}
    53  	fileLink struct {
    54  		next *entry
    55  		prev *entry
    56  	}
    57  	size  int64
    58  	ptype entryType
    59  	// referenced is atomically set to indicate that this entry has been accessed
    60  	// since the last time one of the clock hands swept it.
    61  	referenced atomic.Bool
    62  	shard      *shard
    63  	// Reference count for the entry. The entry is freed when the reference count
    64  	// drops to zero.
    65  	ref refcnt
    66  }
    67  
    68  func newEntry(s *shard, key key, size int64) *entry {
    69  	e := entryAllocNew()
    70  	*e = entry{
    71  		key:   key,
    72  		size:  size,
    73  		ptype: etCold,
    74  		shard: s,
    75  	}
    76  	e.blockLink.next = e
    77  	e.blockLink.prev = e
    78  	e.fileLink.next = e
    79  	e.fileLink.prev = e
    80  	e.ref.init(1)
    81  	return e
    82  }
    83  
    84  func (e *entry) free() {
    85  	e.setValue(nil)
    86  	*e = entry{}
    87  	entryAllocFree(e)
    88  }
    89  
    90  func (e *entry) next() *entry {
    91  	if e == nil {
    92  		return nil
    93  	}
    94  	return e.blockLink.next
    95  }
    96  
    97  func (e *entry) prev() *entry {
    98  	if e == nil {
    99  		return nil
   100  	}
   101  	return e.blockLink.prev
   102  }
   103  
   104  func (e *entry) link(s *entry) {
   105  	s.blockLink.prev = e.blockLink.prev
   106  	s.blockLink.prev.blockLink.next = s
   107  	s.blockLink.next = e
   108  	s.blockLink.next.blockLink.prev = s
   109  }
   110  
   111  func (e *entry) unlink() *entry {
   112  	next := e.blockLink.next
   113  	e.blockLink.prev.blockLink.next = e.blockLink.next
   114  	e.blockLink.next.blockLink.prev = e.blockLink.prev
   115  	e.blockLink.prev = e
   116  	e.blockLink.next = e
   117  	return next
   118  }
   119  
   120  func (e *entry) linkFile(s *entry) {
   121  	s.fileLink.prev = e.fileLink.prev
   122  	s.fileLink.prev.fileLink.next = s
   123  	s.fileLink.next = e
   124  	s.fileLink.next.fileLink.prev = s
   125  }
   126  
   127  func (e *entry) unlinkFile() *entry {
   128  	next := e.fileLink.next
   129  	e.fileLink.prev.fileLink.next = e.fileLink.next
   130  	e.fileLink.next.fileLink.prev = e.fileLink.prev
   131  	e.fileLink.prev = e
   132  	e.fileLink.next = e
   133  	return next
   134  }
   135  
   136  func (e *entry) setValue(v *Value) {
   137  	if v != nil {
   138  		v.acquire()
   139  	}
   140  	old := e.val
   141  	e.val = v
   142  	old.release()
   143  }
   144  
   145  func (e *entry) peekValue() *Value {
   146  	return e.val
   147  }
   148  
   149  func (e *entry) acquireValue() *Value {
   150  	v := e.val
   151  	if v != nil {
   152  		v.acquire()
   153  	}
   154  	return v
   155  }