add other session providers (#5963)

vendor/github.com/syndtr/goleveldb/leveldb/db_snapshot.go

@@ -0,0 +1,183 @@
+// Copyright (c) 2012, Suryandaru Triandana <syndtr@gmail.com>
+// All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package leveldb
+
+import (
+	"container/list"
+	"fmt"
+	"runtime"
+	"sync"
+	"sync/atomic"
+
+	"github.com/syndtr/goleveldb/leveldb/iterator"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+	"github.com/syndtr/goleveldb/leveldb/util"
+)
+
+type snapshotElement struct {
+	seq uint64
+	ref int
+	e   *list.Element
+}
+
+// Acquires a snapshot, based on latest sequence.
+func (db *DB) acquireSnapshot() *snapshotElement {
+	db.snapsMu.Lock()
+	defer db.snapsMu.Unlock()
+
+	seq := db.getSeq()
+
+	if e := db.snapsList.Back(); e != nil {
+		se := e.Value.(*snapshotElement)
+		if se.seq == seq {
+			se.ref++
+			return se
+		} else if seq < se.seq {
+			panic("leveldb: sequence number is not increasing")
+		}
+	}
+	se := &snapshotElement{seq: seq, ref: 1}
+	se.e = db.snapsList.PushBack(se)
+	return se
+}
+
+// Releases given snapshot element.
+func (db *DB) releaseSnapshot(se *snapshotElement) {
+	db.snapsMu.Lock()
+	defer db.snapsMu.Unlock()
+
+	se.ref--
+	if se.ref == 0 {
+		db.snapsList.Remove(se.e)
+		se.e = nil
+	} else if se.ref < 0 {
+		panic("leveldb: Snapshot: negative element reference")
+	}
+}
+
+// Gets minimum sequence that not being snapshotted.
+func (db *DB) minSeq() uint64 {
+	db.snapsMu.Lock()
+	defer db.snapsMu.Unlock()
+
+	if e := db.snapsList.Front(); e != nil {
+		return e.Value.(*snapshotElement).seq
+	}
+
+	return db.getSeq()
+}
+
+// Snapshot is a DB snapshot.
+type Snapshot struct {
+	db       *DB
+	elem     *snapshotElement
+	mu       sync.RWMutex
+	released bool
+}
+
+// Creates new snapshot object.
+func (db *DB) newSnapshot() *Snapshot {
+	snap := &Snapshot{
+		db:   db,
+		elem: db.acquireSnapshot(),
+	}
+	atomic.AddInt32(&db.aliveSnaps, 1)
+	runtime.SetFinalizer(snap, (*Snapshot).Release)
+	return snap
+}
+
+func (snap *Snapshot) String() string {
+	return fmt.Sprintf("leveldb.Snapshot{%d}", snap.elem.seq)
+}
+
+// Get gets the value for the given key. It returns ErrNotFound if
+// the DB does not contains the key.
+//
+// The caller should not modify the contents of the returned slice, but
+// it is safe to modify the contents of the argument after Get returns.
+func (snap *Snapshot) Get(key []byte, ro *opt.ReadOptions) (value []byte, err error) {
+	err = snap.db.ok()
+	if err != nil {
+		return
+	}
+	snap.mu.RLock()
+	defer snap.mu.RUnlock()
+	if snap.released {
+		err = ErrSnapshotReleased
+		return
+	}
+	return snap.db.get(nil, nil, key, snap.elem.seq, ro)
+}
+
+// Has returns true if the DB does contains the given key.
+//
+// It is safe to modify the contents of the argument after Get returns.
+func (snap *Snapshot) Has(key []byte, ro *opt.ReadOptions) (ret bool, err error) {
+	err = snap.db.ok()
+	if err != nil {
+		return
+	}
+	snap.mu.RLock()
+	defer snap.mu.RUnlock()
+	if snap.released {
+		err = ErrSnapshotReleased
+		return
+	}
+	return snap.db.has(nil, nil, key, snap.elem.seq, ro)
+}
+
+// NewIterator returns an iterator for the snapshot of the underlying DB.
+// The returned iterator is not safe for concurrent use, but it is safe to use
+// multiple iterators concurrently, with each in a dedicated goroutine.
+// It is also safe to use an iterator concurrently with modifying its
+// underlying DB. The resultant key/value pairs are guaranteed to be
+// consistent.
+//
+// Slice allows slicing the iterator to only contains keys in the given
+// range. A nil Range.Start is treated as a key before all keys in the
+// DB. And a nil Range.Limit is treated as a key after all keys in
+// the DB.
+//
+// The iterator must be released after use, by calling Release method.
+// Releasing the snapshot doesn't mean releasing the iterator too, the
+// iterator would be still valid until released.
+//
+// Also read Iterator documentation of the leveldb/iterator package.
+func (snap *Snapshot) NewIterator(slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
+	if err := snap.db.ok(); err != nil {
+		return iterator.NewEmptyIterator(err)
+	}
+	snap.mu.Lock()
+	defer snap.mu.Unlock()
+	if snap.released {
+		return iterator.NewEmptyIterator(ErrSnapshotReleased)
+	}
+	// Since iterator already hold version ref, it doesn't need to
+	// hold snapshot ref.
+	return snap.db.newIterator(nil, nil, snap.elem.seq, slice, ro)
+}
+
+// Release releases the snapshot. This will not release any returned
+// iterators, the iterators would still be valid until released or the
+// underlying DB is closed.
+//
+// Other methods should not be called after the snapshot has been released.
+func (snap *Snapshot) Release() {
+	snap.mu.Lock()
+	defer snap.mu.Unlock()
+
+	if !snap.released {
+		// Clear the finalizer.
+		runtime.SetFinalizer(snap, nil)
+
+		snap.released = true
+		snap.db.releaseSnapshot(snap.elem)
+		atomic.AddInt32(&snap.db.aliveSnaps, -1)
+		snap.db = nil
+		snap.elem = nil
+	}
+}