add other session providers (#5963)

vendor/github.com/syndtr/goleveldb/leveldb/iterator/iter.go

@@ -0,0 +1,132 @@
+// 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 iterator provides interface and implementation to traverse over
+// contents of a database.
+package iterator
+
+import (
+	"errors"
+
+	"github.com/syndtr/goleveldb/leveldb/util"
+)
+
+var (
+	ErrIterReleased = errors.New("leveldb/iterator: iterator released")
+)
+
+// IteratorSeeker is the interface that wraps the 'seeks method'.
+type IteratorSeeker interface {
+	// First moves the iterator to the first key/value pair. If the iterator
+	// only contains one key/value pair then First and Last would moves
+	// to the same key/value pair.
+	// It returns whether such pair exist.
+	First() bool
+
+	// Last moves the iterator to the last key/value pair. If the iterator
+	// only contains one key/value pair then First and Last would moves
+	// to the same key/value pair.
+	// It returns whether such pair exist.
+	Last() bool
+
+	// Seek moves the iterator to the first key/value pair whose key is greater
+	// than or equal to the given key.
+	// It returns whether such pair exist.
+	//
+	// It is safe to modify the contents of the argument after Seek returns.
+	Seek(key []byte) bool
+
+	// Next moves the iterator to the next key/value pair.
+	// It returns false if the iterator is exhausted.
+	Next() bool
+
+	// Prev moves the iterator to the previous key/value pair.
+	// It returns false if the iterator is exhausted.
+	Prev() bool
+}
+
+// CommonIterator is the interface that wraps common iterator methods.
+type CommonIterator interface {
+	IteratorSeeker
+
+	// util.Releaser is the interface that wraps basic Release method.
+	// When called Release will releases any resources associated with the
+	// iterator.
+	util.Releaser
+
+	// util.ReleaseSetter is the interface that wraps the basic SetReleaser
+	// method.
+	util.ReleaseSetter
+
+	// TODO: Remove this when ready.
+	Valid() bool
+
+	// Error returns any accumulated error. Exhausting all the key/value pairs
+	// is not considered to be an error.
+	Error() error
+}
+
+// Iterator iterates over a DB's key/value pairs in key order.
+//
+// When encounter an error any 'seeks method' will return false and will
+// yield no key/value pairs. The error can be queried by calling the Error
+// method. Calling Release is still necessary.
+//
+// An iterator must be released after use, but it is not necessary to read
+// an iterator until exhaustion.
+// Also, an iterator is not necessarily safe for concurrent use, but it is
+// safe to use multiple iterators concurrently, with each in a dedicated
+// goroutine.
+type Iterator interface {
+	CommonIterator
+
+	// Key returns the key of the current key/value pair, or nil if done.
+	// The caller should not modify the contents of the returned slice, and
+	// its contents may change on the next call to any 'seeks method'.
+	Key() []byte
+
+	// Value returns the value of the current key/value pair, or nil if done.
+	// The caller should not modify the contents of the returned slice, and
+	// its contents may change on the next call to any 'seeks method'.
+	Value() []byte
+}
+
+// ErrorCallbackSetter is the interface that wraps basic SetErrorCallback
+// method.
+//
+// ErrorCallbackSetter implemented by indexed and merged iterator.
+type ErrorCallbackSetter interface {
+	// SetErrorCallback allows set an error callback of the corresponding
+	// iterator. Use nil to clear the callback.
+	SetErrorCallback(f func(err error))
+}
+
+type emptyIterator struct {
+	util.BasicReleaser
+	err error
+}
+
+func (i *emptyIterator) rErr() {
+	if i.err == nil && i.Released() {
+		i.err = ErrIterReleased
+	}
+}
+
+func (*emptyIterator) Valid() bool            { return false }
+func (i *emptyIterator) First() bool          { i.rErr(); return false }
+func (i *emptyIterator) Last() bool           { i.rErr(); return false }
+func (i *emptyIterator) Seek(key []byte) bool { i.rErr(); return false }
+func (i *emptyIterator) Next() bool           { i.rErr(); return false }
+func (i *emptyIterator) Prev() bool           { i.rErr(); return false }
+func (*emptyIterator) Key() []byte            { return nil }
+func (*emptyIterator) Value() []byte          { return nil }
+func (i *emptyIterator) Error() error         { return i.err }
+
+// NewEmptyIterator creates an empty iterator. The err parameter can be
+// nil, but if not nil the given err will be returned by Error method.
+func NewEmptyIterator(err error) Iterator {
+	return &emptyIterator{err: err}
+}