The size encoded in a entry header contains the header it self. (#266)

Co-authored-by: Fabian Gärtner <[email protected]>
Co-authored-by: Oleg Kovalov <[email protected]>
Co-authored-by: Tomasz Janiszewski <[email protected]>

Author: 3 people

queue/bytes_queue.go

@@ -38,19 +38,23 @@ type queueError struct {
 	message string
 }
 
-// getUvarintSize returns the number of bytes to encode x in uvarint format
-func getUvarintSize(x uint32) int {
-	if x < 128 {
-		return 1
-	} else if x < 16384 {
-		return 2
-	} else if x < 2097152 {
-		return 3
-	} else if x < 268435456 {
-		return 4
-	} else {
-		return 5
+// getNeededSize returns the number of bytes an entry of length need in the queue
+func getNeededSize(length int) int {
+	var header int
+	switch {
+	case length < 127: // 1<<7-1
+		header = 1
+	case length < 16382: // 1<<14-2
+		header = 2
+	case length < 2097149: // 1<<21 -3
+		header = 3
+	case length < 268435452: // 1<<28 -4
+		header = 4
+	default:
+		header = 5
 	}
+
+	return length + header
 }
 
 // NewBytesQueue initialize new bytes queue.
@@ -82,22 +86,21 @@ func (q *BytesQueue) Reset() {
 // Push copies entry at the end of queue and moves tail pointer. Allocates more space if needed.
 // Returns index for pushed data or error if maximum size queue limit is reached.
 func (q *BytesQueue) Push(data []byte) (int, error) {
-	dataLen := len(data)
-	headerEntrySize := getUvarintSize(uint32(dataLen))
+	neededSize := getNeededSize(len(data))
 
-	if !q.canInsertAfterTail(dataLen + headerEntrySize) {
-		if q.canInsertBeforeHead(dataLen + headerEntrySize) {
+	if !q.canInsertAfterTail(neededSize) {
+		if q.canInsertBeforeHead(neededSize) {
 			q.tail = leftMarginIndex
-		} else if q.capacity+headerEntrySize+dataLen >= q.maxCapacity && q.maxCapacity > 0 {
+		} else if q.capacity+neededSize >= q.maxCapacity && q.maxCapacity > 0 {
 			return -1, &queueError{"Full queue. Maximum size limit reached."}
 		} else {
-			q.allocateAdditionalMemory(dataLen + headerEntrySize)
+			q.allocateAdditionalMemory(neededSize)
 		}
 	}
 
 	index := q.tail
 
-	q.push(data, dataLen)
+	q.push(data, neededSize)
 
 	return index, nil
 }
@@ -120,9 +123,8 @@ func (q *BytesQueue) allocateAdditionalMemory(minimum int) {
 
 		if q.tail <= q.head {
 			if q.tail != q.head {
-				headerEntrySize := getUvarintSize(uint32(q.head - q.tail))
-				emptyBlobLen := q.head - q.tail - headerEntrySize
-				q.push(make([]byte, emptyBlobLen), emptyBlobLen)
+				// created slice is slightly larger then need but this is fine after only the needed bytes are copied
+				q.push(make([]byte, q.head-q.tail), q.head-q.tail)
 			}
 
 			q.head = leftMarginIndex
@@ -141,7 +143,7 @@ func (q *BytesQueue) push(data []byte, len int) {
 	headerEntrySize := binary.PutUvarint(q.headerBuffer, uint64(len))
 	q.copy(q.headerBuffer, headerEntrySize)
 
-	q.copy(data, len)
+	q.copy(data, len-headerEntrySize)
 
 	if q.tail > q.head {
 		q.rightMargin = q.tail
@@ -159,13 +161,12 @@ func (q *BytesQueue) copy(data []byte, len int) {
 
 // Pop reads the oldest entry from queue and moves head pointer to the next one
 func (q *BytesQueue) Pop() ([]byte, error) {
-	data, headerEntrySize, err := q.peek(q.head)
+	data, blockSize, err := q.peek(q.head)
 	if err != nil {
 		return nil, err
 	}
-	size := len(data)
 
-	q.head += headerEntrySize + size
+	q.head += blockSize
 	q.count--
 
 	if q.head == q.rightMargin {
@@ -238,7 +239,7 @@ func (q *BytesQueue) peek(index int) ([]byte, int, error) {
 	}
 
 	blockSize, n := binary.Uvarint(q.array[index:])
-	return q.array[index+n : index+n+int(blockSize)], n, nil
+	return q.array[index+n : index+int(blockSize)], int(blockSize), nil
 }
 
 // canInsertAfterTail returns true if it's possible to insert an entry of size of need after the tail of the queue

queue/bytes_queue_test.go

@@ -224,6 +224,31 @@ func TestAllocateAdditionalSpaceForInsufficientFreeFragmentedSpaceWhereTailIsBef
 	assertEqual(t, blob('d', 40), pop(queue))
 }
 
+func TestAllocateAdditionalSpaceForInsufficientFreeFragmentedSpaceWhereTailIsBeforeHead128(t *testing.T) {
+	t.Parallel()
+
+	// given
+	queue := NewBytesQueue(200, 0, false)
+
+	// when
+	queue.Push(blob('a', 30))  //  header + entry + left margin = 32 bytes
+	queue.Push(blob('b', 1))   // 32 + 128 + 1 = 161 bytes
+	queue.Push(blob('b', 125)) // 32 + 128 + 1 = 161 bytes
+	queue.Push(blob('c', 20))  //  160 + 20 + 1 = 182
+	queue.Pop()                // space freed at the beginning
+	queue.Pop()                // free 2 bytes
+	queue.Pop()                // free 126
+	queue.Push(blob('d', 30))  // 31 bytes used at the beginning, tail pointer is before head pointer, now free space is 128 bytes
+	queue.Push(blob('e', 160)) // invoke allocateAdditionalMemory but fill 127 bytes free space (It should be 128 bytes, but 127 are filled, leaving one byte unfilled)
+
+	// then
+	assertEqual(t, 400, queue.Capacity())
+	assertEqual(t, blob('d', 30), pop(queue))
+	assertEqual(t, blob(0, 126), pop(queue))  //126 bytes data with 2bytes header only possible as empty entry
+	assertEqual(t, blob('c', 20), pop(queue)) //The data is not expected
+	assertEqual(t, blob('e', 160), pop(queue))
+}
+
 func TestUnchangedEntriesIndexesAfterAdditionalMemoryAllocationWhereTailIsBeforeHead(t *testing.T) {
 	t.Parallel()