core/rawdb: implement sequential reads in freezer_table (ethereum#23117)

* core/rawdb: implement sequential reads in freezer_table

* core/rawdb, ethdb: add sequential reader to db interface

* core/rawdb: lint nitpicks

* core/rawdb: fix some nitpicks

* core/rawdb: fix flaw with deferred reads not being performed

* core/rawdb: better documentation

core/rawdb/database.go

@@ -89,6 +89,11 @@ func (db *nofreezedb) Ancient(kind string, number uint64) ([]byte, error) {
 	return nil, errNotSupported
 }
 
+// ReadAncients returns an error as we don't have a backing chain freezer.
+func (db *nofreezedb) ReadAncients(kind string, start, max, maxByteSize uint64) ([][]byte, error) {
+	return nil, errNotSupported
+}
+
 // Ancients returns an error as we don't have a backing chain freezer.
 func (db *nofreezedb) Ancients() (uint64, error) {
 	return 0, errNotSupported

core/rawdb/freezer.go

@@ -180,6 +180,18 @@ func (f *freezer) Ancient(kind string, number uint64) ([]byte, error) {
 	return nil, errUnknownTable
 }
 
+// ReadAncients retrieves multiple items in sequence, starting from the index 'start'.
+// It will return
+//  - at most 'max' items,
+//  - at least 1 item (even if exceeding the maxByteSize), but will otherwise
+//   return as many items as fit into maxByteSize.
+func (f *freezer) ReadAncients(kind string, start, count, maxBytes uint64) ([][]byte, error) {
+	if table := f.tables[kind]; table != nil {
+		return table.RetrieveItems(start, count, maxBytes)
+	}
+	return nil, errUnknownTable
+}
+
 // Ancients returns the length of the frozen items.
 func (f *freezer) Ancients() (uint64, error) {
 	return atomic.LoadUint64(&f.frozen), nil

core/rawdb/freezer_table.go

@@ -70,6 +70,19 @@ func (i *indexEntry) marshallBinary() []byte {
 	return b
 }
 
+// bounds returns the start- and end- offsets, and the file number of where to
+// read there data item marked by the two index entries. The two entries are
+// assumed to be sequential.
+func (start *indexEntry) bounds(end *indexEntry) (startOffset, endOffset, fileId uint32) {
+	if start.filenum != end.filenum {
+		// If a piece of data 'crosses' a data-file,
+		// it's actually in one piece on the second data-file.
+		// We return a zero-indexEntry for the second file as start
+		return 0, end.offset, end.filenum
+	}
+	return start.offset, end.offset, end.filenum
+}
+
 // freezerTable represents a single chained data table within the freezer (e.g. blocks).
 // It consists of a data file (snappy encoded arbitrary data blobs) and an indexEntry
 // file (uncompressed 64 bit indices into the data file).
@@ -546,84 +559,183 @@ func (t *freezerTable) append(item uint64, encodedBlob []byte, wlock bool) (bool
 	return false, nil
 }
 
-// getBounds returns the indexes for the item
-// returns start, end, filenumber and error
-func (t *freezerTable) getBounds(item uint64) (uint32, uint32, uint32, error) {
-	buffer := make([]byte, indexEntrySize)
-	var startIdx, endIdx indexEntry
-	// Read second index
-	if _, err := t.index.ReadAt(buffer, int64((item+1)*indexEntrySize)); err != nil {
-		return 0, 0, 0, err
-	}
-	endIdx.unmarshalBinary(buffer)
-	// Read first index (unless it's the very first item)
-	if item != 0 {
-		if _, err := t.index.ReadAt(buffer, int64(item*indexEntrySize)); err != nil {
-			return 0, 0, 0, err
-		}
-		startIdx.unmarshalBinary(buffer)
-	} else {
+// getIndices returns the index entries for the given from-item, covering 'count' items.
+// N.B: The actual number of returned indices for N items will always be N+1 (unless an
+// error is returned).
+// OBS: This method assumes that the caller has already verified (and/or trimmed) the range
+// so that the items are within bounds. If this method is used to read out of bounds,
+// it will return error.
+func (t *freezerTable) getIndices(from, count uint64) ([]*indexEntry, error) {
+	// Apply the table-offset
+	from = from - uint64(t.itemOffset)
+	// For reading N items, we need N+1 indices.
+	buffer := make([]byte, (count+1)*indexEntrySize)
+	if _, err := t.index.ReadAt(buffer, int64(from*indexEntrySize)); err != nil {
+		return nil, err
+	}
+	var (
+		indices []*indexEntry
+		offset  int
+	)
+	for i := from; i <= from+count; i++ {
+		index := new(indexEntry)
+		index.unmarshalBinary(buffer[offset:])
+		offset += indexEntrySize
+		indices = append(indices, index)
+	}
+	if from == 0 {
 		// Special case if we're reading the first item in the freezer. We assume that
 		// the first item always start from zero(regarding the deletion, we
 		// only support deletion by files, so that the assumption is held).
 		// This means we can use the first item metadata to carry information about
 		// the 'global' offset, for the deletion-case
-		return 0, endIdx.offset, endIdx.filenum, nil
+		indices[0].offset = 0
+		indices[0].filenum = indices[1].filenum
 	}
-	if startIdx.filenum != endIdx.filenum {
-		// If a piece of data 'crosses' a data-file,
-		// it's actually in one piece on the second data-file.
-		// We return a zero-indexEntry for the second file as start
-		return 0, endIdx.offset, endIdx.filenum, nil
-	}
-	return startIdx.offset, endIdx.offset, endIdx.filenum, nil
+	return indices, nil
 }
 
 // Retrieve looks up the data offset of an item with the given number and retrieves
 // the raw binary blob from the data file.
 func (t *freezerTable) Retrieve(item uint64) ([]byte, error) {
-	blob, err := t.retrieve(item)
+	items, err := t.RetrieveItems(item, 1, 0)
 	if err != nil {
 		return nil, err
 	}
-	if t.noCompression {
-		return blob, nil
+	return items[0], nil
+}
+
+// RetrieveItems returns multiple items in sequence, starting from the index 'start'.
+// It will return at most 'max' items, but will abort earlier to respect the
+// 'maxBytes' argument. However, if the 'maxBytes' is smaller than the size of one
+// item, it _will_ return one element and possibly overflow the maxBytes.
+func (t *freezerTable) RetrieveItems(start, count, maxBytes uint64) ([][]byte, error) {
+	// First we read the 'raw' data, which might be compressed.
+	diskData, sizes, err := t.retrieveItems(start, count, maxBytes)
+	if err != nil {
+		return nil, err
 	}
-	return snappy.Decode(nil, blob)
+	var (
+		output     = make([][]byte, 0, count)
+		offset     int // offset for reading
+		outputSize int // size of uncompressed data
+	)
+	// Now slice up the data and decompress.
+	for i, diskSize := range sizes {
+		item := diskData[offset : offset+diskSize]
+		offset += diskSize
+		decompressedSize := diskSize
+		if !t.noCompression {
+			decompressedSize, _ = snappy.DecodedLen(item)
+		}
+		if i > 0 && uint64(outputSize+decompressedSize) > maxBytes {
+			break
+		}
+		if !t.noCompression {
+			data, err := snappy.Decode(nil, item)
+			if err != nil {
+				return nil, err
+			}
+			output = append(output, data)
+		} else {
+			output = append(output, item)
+		}
+		outputSize += decompressedSize
+	}
+	return output, nil
 }
 
-// retrieve looks up the data offset of an item with the given number and retrieves
-// the raw binary blob from the data file. OBS! This method does not decode
-// compressed data.
-func (t *freezerTable) retrieve(item uint64) ([]byte, error) {
+// retrieveItems reads up to 'count' items from the table. It reads at least
+// one item, but otherwise avoids reading more than maxBytes bytes.
+// It returns the (potentially compressed) data, and the sizes.
+func (t *freezerTable) retrieveItems(start, count, maxBytes uint64) ([]byte, []int, error) {
 	t.lock.RLock()
 	defer t.lock.RUnlock()
 	// Ensure the table and the item is accessible
 	if t.index == nil || t.head == nil {
-		return nil, errClosed
+		return nil, nil, errClosed
 	}
-	if atomic.LoadUint64(&t.items) <= item {
-		return nil, errOutOfBounds
+	itemCount := atomic.LoadUint64(&t.items) // max number
+	// Ensure the start is written, not deleted from the tail, and that the
+	// caller actually wants something
+	if itemCount <= start || uint64(t.itemOffset) > start || count == 0 {
+		return nil, nil, errOutOfBounds
 	}
-	// Ensure the item was not deleted from the tail either
-	if uint64(t.itemOffset) > item {
-		return nil, errOutOfBounds
+	if start+count > itemCount {
+		count = itemCount - start
 	}
-	startOffset, endOffset, filenum, err := t.getBounds(item - uint64(t.itemOffset))
-	if err != nil {
-		return nil, err
+	var (
+		output     = make([]byte, maxBytes) // Buffer to read data into
+		outputSize int                      // Used size of that buffer
+	)
+	// readData is a helper method to read a single data item from disk.
+	readData := func(fileId, start uint32, length int) error {
+		// In case a small limit is used, and the elements are large, may need to
+		// realloc the read-buffer when reading the first (and only) item.
+		if len(output) < length {
+			output = make([]byte, length)
+		}
+		dataFile, exist := t.files[fileId]
+		if !exist {
+			return fmt.Errorf("missing data file %d", fileId)
+		}
+		if _, err := dataFile.ReadAt(output[outputSize:outputSize+length], int64(start)); err != nil {
+			return err
+		}
+		outputSize += length
+		return nil
 	}
-	dataFile, exist := t.files[filenum]
-	if !exist {
-		return nil, fmt.Errorf("missing data file %d", filenum)
+	// Read all the indexes in one go
+	indices, err := t.getIndices(start, count)
+	if err != nil {
+		return nil, nil, err
 	}
-	// Retrieve the data itself, decompress and return
-	blob := make([]byte, endOffset-startOffset)
-	if _, err := dataFile.ReadAt(blob, int64(startOffset)); err != nil {
-		return nil, err
+	var (
+		sizes      []int               // The sizes for each element
+		totalSize  = 0                 // The total size of all data read so far
+		readStart  = indices[0].offset // Where, in the file, to start reading
+		unreadSize = 0                 // The size of the as-yet-unread data
+	)
+
+	for i, firstIndex := range indices[:len(indices)-1] {
+		secondIndex := indices[i+1]
+		// Determine the size of the item.
+		offset1, offset2, _ := firstIndex.bounds(secondIndex)
+		size := int(offset2 - offset1)
+		// Crossing a file boundary?
+		if secondIndex.filenum != firstIndex.filenum {
+			// If we have unread data in the first file, we need to do that read now.
+			if unreadSize > 0 {
+				if err := readData(firstIndex.filenum, readStart, unreadSize); err != nil {
+					return nil, nil, err
+				}
+				unreadSize = 0
+			}
+			readStart = 0
+		}
+		if i > 0 && uint64(totalSize+size) > maxBytes {
+			// About to break out due to byte limit being exceeded. We don't
+			// read this last item, but we need to do the deferred reads now.
+			if unreadSize > 0 {
+				if err := readData(secondIndex.filenum, readStart, unreadSize); err != nil {
+					return nil, nil, err
+				}
+			}
+			break
+		}
+		// Defer the read for later
+		unreadSize += size
+		totalSize += size
+		sizes = append(sizes, size)
+		if i == len(indices)-2 || uint64(totalSize) > maxBytes {
+			// Last item, need to do the read now
+			if err := readData(secondIndex.filenum, readStart, unreadSize); err != nil {
+				return nil, nil, err
+			}
+			break
+		}
 	}
-	t.readMeter.Mark(int64(len(blob) + 2*indexEntrySize))
-	return blob, nil
+	return output[:outputSize], sizes, nil
 }
 
 // has returns an indicator whether the specified number data