rkhachatryan commented on code in PR #28661: URL: https://github.com/apache/flink/pull/28661#discussion_r3724881811
########## flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/channel/FetchedChannelStateReaderImpl.java: ########## @@ -0,0 +1,533 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * The ASF licenses this file to You under the Apache License, Version 2.0 + * (the "License"); you may not use this file except in compliance with + * the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ + +package org.apache.flink.runtime.checkpoint.channel; + +import org.apache.flink.annotation.Internal; +import org.apache.flink.runtime.checkpoint.channel.FetchedChannelStateReader.SpillSegment; + +import javax.annotation.Nullable; + +import java.io.ByteArrayInputStream; +import java.io.DataInputStream; +import java.io.EOFException; +import java.io.IOException; +import java.io.InputStream; +import java.nio.file.Files; +import java.nio.file.Path; +import java.util.List; +import java.util.Optional; + +import static org.apache.flink.runtime.checkpoint.channel.AbstractSpillingHandler.SEGMENT_HEADER_BYTES; +import static org.apache.flink.util.Preconditions.checkState; + +/** + * The single {@link FetchedChannelStateReader} implementation over a {@link FetchedChannelState}'s + * spill files. + * + * <p>Reading is strictly sequential and never seeks mid-iteration. There is exactly one place that + * skips bytes: the very first {@link #nextSegment()} call, where a snapshot reader started mid-body + * discards the already-delivered prefix to land on the not-yet-delivered remainder. Every later + * call does no skipping at all — the previous body was read to its end, so the stream already sits + * on the next segment's header. This "skip only on first positioning" rule is what keeps the + * steady-state path free of any seek/skip. + * + * <p>The reader holds <b>two</b> {@link Position}s and nothing else duplicates them: + * + * <ul> + * <li>{@code current} — the live read position; its {@code readOffset} is exactly where the open + * file stream sits, advancing as the header and the consumer's body reads consume bytes (the + * latter outside the drainer lock). + * <li>{@code committed} — the delivered boundary; {@link SpillSegment#commit()} advances it from + * {@code current} (under the drainer lock). {@link #snapshot()} derives a new reader from it. + * </ul> + * + * <p>The "previous body fully read before advancing" rule is checked at the {@link #nextSegment()} + * entry (the first call is exempt — there is no previous segment). Body ownership is handed to the + * consumer, so the reader does not track body progress except through {@code current}. + */ +@Internal +final class FetchedChannelStateReaderImpl implements FetchedChannelStateReader { + + private final FetchedChannelStateSnapshot snapshot; + private final FetchedChannelState channelState; + private final List<Path> files; + + /** Live read position; {@code readOffset} is where the open stream physically sits. */ + private final Position current; + + /** Delivered boundary; {@link SpillSegment#commit()} advances it from {@link #current}. */ + private final Position committed; + + /** Open stream over {@code current.fileIndex}, or {@code null} before the first read. */ + @Nullable private InputStream fileStream; + + /** Size of the file currently open. */ + private long currentFileSize; + + /** Body view of the segment returned by the last {@link #nextSegment()}, or {@code null}. */ + @Nullable private BoundedSegmentStream currentBody; + + private boolean positioned; + private boolean closed; + + FetchedChannelStateReaderImpl(FetchedChannelStateSnapshot snapshot) { + this.snapshot = snapshot; + this.channelState = snapshot.channelState(); + this.files = channelState.files(); + // Must copy the position so that this reader's commits do not mutate the snapshot's state. + this.committed = snapshot.position().copy(); + this.current = committed.copy(); + } + + @Override + public Optional<SpillSegment> nextSegment() { + checkState(!closed, "FetchedChannelStateReader is closed"); + checkState( + currentBody == null || currentBody.remaining() == 0, + "Previous segment body not fully consumed before advancing: %s bytes left", + currentBody == null ? 0 : currentBody.remaining()); + try { + if (!positioned) { + positioned = true; + return firstSegment(); + } + return followingSegment(); + } catch (IOException e) { + throw new RuntimeException("Failed to read segment", e); + } + } + + /** + * First positioning — the only path that may skip bytes. Opens the file at the committed header + * offset and reads the header. A snapshot may resume in the middle of a segment: the committed + * {@code readOffset} says how many body bytes were already delivered, and that prefix is + * skipped so the returned body starts at the not-yet-delivered remainder. If the segment was + * already fully delivered (prefix == whole body), it is exhausted here and we move on to the + * next one. + */ + private Optional<SpillSegment> firstSegment() throws IOException { + // The committed read offset may sit mid-body (after a partial commit), but the header lives + // at segmentStartOffset. Capture how much was already delivered, then rewind the live read + // offset to the header so we open the file there and read the header, not mid-body. + int deliveredPrefix = (int) current.deliveredBodyBytes(); + current.rewindToSegmentStart(); + + if (!openCurrentFile()) { + return Optional.empty(); + } + SegmentHeader header = readHeaderAtCurrent(); + checkState( + deliveredPrefix <= header.bufferLength, + "Delivered offset %s exceeds segment length %s", + deliveredPrefix, + header.bufferLength); + + if (deliveredPrefix == header.bufferLength) { + // This segment was already fully delivered before the snapshot; nothing remains in it. + // Skip its whole body to reach the next segment's header, then take the steady path. + skipBody(header.bufferLength); + return followingSegment(); + } + + // Discard the already-delivered prefix (the one and only skip in this class), then hand out + // the remainder. alreadyDelivered is carried so commit() records the boundary from the + // head. + skipBody(deliveredPrefix); + currentBody = + new BoundedSegmentStream(header.bufferLength - deliveredPrefix, deliveredPrefix); + return Optional.of(new Segment(header.channelInfo, currentBody)); + } + + /** + * Steady-state path — no skipping. The previous body was read to its end, so the stream sits + * exactly on this segment's header (or at the current file's end, in which case we roll to the + * next file). Reads the header and returns the whole-body view. + */ + private Optional<SpillSegment> followingSegment() throws IOException { + if (!openCurrentFile()) { + return Optional.empty(); + } + SegmentHeader header = readHeaderAtCurrent(); + currentBody = new BoundedSegmentStream(header.bufferLength); + return Optional.of(new Segment(header.channelInfo, currentBody)); + } + + @Override + public FetchedChannelStateSnapshot snapshot() { + checkState(!closed, "FetchedChannelStateReader is closed"); + return new FetchedChannelStateSnapshot(channelState, committed.copy()); + } + + @Override + public void close() throws IOException { + if (closed) { + return; + } + closed = true; + try { + closeFileStream(); + } finally { + snapshot.release(); + } + } + + // ------------------------------------------------------------------------------------------- + // Sequential IO over the spill files; all of it advances current.readOffset / current.fileIndex + // ------------------------------------------------------------------------------------------- + + /** + * Ensures a file is open with the stream positioned at {@code current}'s read offset, ready to + * read this segment's header. Rolls to the next file when the current one is exhausted. Returns + * false when no segment remains. + * + * <p>{@code current.segmentStartOffset} is set to where the header begins, so a later {@link + * SpillSegment#commit()} records the right segment for the snapshot to resume from. + */ + private boolean openCurrentFile() throws IOException { + if (current.fileIndex >= files.size()) { + return false; + } + openFileAndSeek(); + if (current.readOffset < currentFileSize) { + current.startSegmentHere(); + return true; + } + // Current file fully read: move to the next file's first segment. + closeFileStream(); + current.rollToNextFile(); + if (current.fileIndex >= files.size()) { + return false; + } + openFileAndSeek(); + if (current.readOffset < currentFileSize) { + current.startSegmentHere(); + return true; + } + return false; + } + + /** Reads the 12-byte header at the current read offset; advances past it. */ + private SegmentHeader readHeaderAtCurrent() throws IOException { + byte[] headerBytes = new byte[SEGMENT_HEADER_BYTES]; + readFully(headerBytes); + DataInputStream h = new DataInputStream(new ByteArrayInputStream(headerBytes)); + int gateIdx = h.readInt(); + int channelIdx = h.readInt(); + int bufferLength = h.readInt(); + checkState(bufferLength >= 0, "negative segment length: %s", bufferLength); + return new SegmentHeader(new InputChannelInfo(gateIdx, channelIdx), bufferLength); + } + + /** + * Ensures the file at {@code current.fileIndex} is open with the stream positioned at {@code + * current.readOffset}. If a stream is already open it is left as-is: sequential reading + * guarantees it is already there. + */ + private void openFileAndSeek() throws IOException { + if (fileStream != null) { + return; + } + Path path = files.get(current.fileIndex); + currentFileSize = Files.size(path); + InputStream in = Files.newInputStream(path); + try { + skipOnStream(in, current.readOffset, path); + } catch (IOException e) { + in.close(); + throw e; + } + fileStream = in; + } + + /** Skips {@code count} body bytes on the open stream, advancing the read offset. */ + private void skipBody(long count) throws IOException { + if (count > 0) { + skipOnStream(fileStream, count, files.get(current.fileIndex)); + current.advanceReadOffset(count); + } + } + + /** Skips exactly {@code count} bytes on {@code in}, failing loud if the file ends early. */ + private void skipOnStream(InputStream in, long count, Path path) throws IOException { Review Comment: Yes, this comment is about `skipOnStream`, sorry for misplacing. I think `skipOnStream` can be replaced by `IOUtils#skipFully`. `readFully` as well - it was the [next](https://github.com/apache/flink/pull/28661#discussion_r3693955986) comment -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
