Add an option to create MemDB iterators with no Mutex for the CacheKV…

…Store (#187)

* Add an option to create MemDB iterators with no Mutex for the CacheKVStore

* Update changelog

.gitignore

@@ -11,4 +11,5 @@
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
 
-.idea
+.idea
+vendor/*

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Pending
+
+* Add an option for creating a mem db with no mutex
+
 ## 0.6.4
 
 **2021-02-09**

memdb.go

@@ -190,3 +190,19 @@ func (db *MemDB) ReverseIterator(start, end []byte) (Iterator, error) {
 	}
 	return newMemDBIterator(db, start, end, true), nil
 }
+
+// IteratorNoMtx makes an iterator with no mutex.
+func (db *MemDB) IteratorNoMtx(start, end []byte) (Iterator, error) {
+	if (start != nil && len(start) == 0) || (end != nil && len(end) == 0) {
+		return nil, errKeyEmpty
+	}
+	return newMemDBIteratorMtxChoice(db, start, end, false, false), nil
+}
+
+// ReverseIteratorNoMtx makes an iterator with no mutex.
+func (db *MemDB) ReverseIteratorNoMtx(start, end []byte) (Iterator, error) {
+	if (start != nil && len(start) == 0) || (end != nil && len(end) == 0) {
+		return nil, errKeyEmpty
+	}
+	return newMemDBIteratorMtxChoice(db, start, end, true, false), nil
+}

memdb_iterator.go

@@ -21,24 +21,34 @@ type memDBIterator struct {
 	item   *item
 	start  []byte
 	end    []byte
+	useMtx bool
 }
 
 var _ Iterator = (*memDBIterator)(nil)
 
 // newMemDBIterator creates a new memDBIterator.
 func newMemDBIterator(db *MemDB, start []byte, end []byte, reverse bool) *memDBIterator {
+	return newMemDBIteratorMtxChoice(db, start, end, reverse, true)
+}
+
+func newMemDBIteratorMtxChoice(db *MemDB, start []byte, end []byte, reverse bool, useMtx bool) *memDBIterator {
 	ctx, cancel := context.WithCancel(context.Background())
 	ch := make(chan *item, chBufferSize)
 	iter := &memDBIterator{
 		ch:     ch,
 		cancel: cancel,
 		start:  start,
 		end:    end,
+		useMtx: useMtx,
 	}
 
-	db.mtx.RLock()
+	if useMtx {
+		db.mtx.RLock()
+	}
 	go func() {
-		defer db.mtx.RUnlock()
+		if useMtx {
+			defer db.mtx.RUnlock()
+		}
 		// Because we use [start, end) for reverse ranges, while btree uses (start, end], we need
 		// the following variables to handle some reverse iteration conditions ourselves.
 		var (