clintropolis commented on code in PR #19397: URL: https://github.com/apache/druid/pull/19397#discussion_r3206343297
########## processing/src/main/java/org/apache/druid/segment/AsyncCursorHolder.java: ########## @@ -0,0 +1,298 @@ +/* + * 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.druid.segment; + +import com.google.errorprone.annotations.concurrent.GuardedBy; +import org.apache.druid.error.DruidException; +import org.apache.druid.frame.processor.ReturnOrAwait; +import org.apache.druid.java.util.common.Either; + +import javax.annotation.Nullable; +import java.io.Closeable; +import java.util.ArrayList; +import java.util.List; + +/** + * Closeable wrapper around an asynchronously-loaded {@link CursorHolder}, returned by + * {@link CursorFactory#makeCursorHolderAsync}. Designed to make lifecycle management safe even when the holder is + * still loading: callers receive a single Closeable handle and can register it once with their cleanup machinery, + * regardless of where the underlying load is in its lifecycle. + * <p> + * The hazard this exists to avoid: returning a {@code ListenableFuture<CursorHolder>} (or similar future-of-Closeable) + * makes correct cleanup error-prone, where canceling the future or letting a caller fail before consuming the future + * can orphan the produced holder, leaking the underlying resources. By exposing a Closeable that internally tracks the + * load and disposes whatever has materialized, callers don't have to write that bookkeeping themselves. + * <p> + * <h3>Producer protocol</h3> + * Producers feed results in via {@link #set(CursorHolder)} or {@link #setException(Throwable)}, both of which return + * a boolean. If they return {@code false}, this wrapper has already been closed and the producer is responsible for + * closing whatever holder it just produced. + * Producers may pass a {@link Runnable} canceler at construction time which runs on {@link #close()} when the wrapper + * is closed before the {@link #set} has been called, giving the producer an opportunity to abort its work. The canceler + * is best-effort: a producer may have already produced the holder by the time it observes cancellation, in which case + * its {@link #set} call will return false and it must close the holder it tried to set. + * <p> + * <h3>Consumer protocol</h3> + * Consumers wait for {@link #isReady()} via {@link #addReadyCallback}, and {@link #release()} to transfer ownership of + * the {@link CursorHolder} (or throw the producer exception). Calling {@link #release()} before {@link #isReady()} + * returns {@code true}, multiple times, or after this holder has been closed will throw a {@link DruidException}. + * <p> + * For example (using {@link ReturnOrAwait} to show intended yield-then-resume usage pattern): + * <pre>{@code + * if (asyncHolder == null) { + * asyncHolder = cursorFactory.makeCursorHolderAsync(spec); + * closer.register(asyncHolder); // safe at any lifecycle point, close() handles in-flight loads + * } + * if (!asyncHolder.isReady()) { + * SettableFuture<?> awaitFuture = SettableFuture.create(); + * asyncHolder.addReadyCallback(() -> awaitFuture.set(null)); + * return ReturnOrAwait.awaitAllFutures(List.of(awaitFuture)); + * } + * final CursorHolder holder = asyncHolder.release(); // ownership transfers to the caller + * // ... use holder; close it when done (or hand it to a component that owns its lifecycle) ... + * }</pre> + */ +public class AsyncCursorHolder implements Closeable +{ + /** + * Completed {@link AsyncCursorHolder} backed by an already available {@link CursorHolder} + */ + public static AsyncCursorHolder completed(CursorHolder holder) + { + final AsyncCursorHolder result = new AsyncCursorHolder(null); + result.set(holder); + return result; + } + + @Nullable + private final Runnable canceler; + + @GuardedBy("this") + @Nullable + private CursorHolder result = null; + @GuardedBy("this") + @Nullable + private Throwable error = null; + @GuardedBy("this") + private boolean closed = false; + @GuardedBy("this") + private boolean disposed = false; + @GuardedBy("this") + private final List<Runnable> readyCallbacks = new ArrayList<>(); + + /** + * @param canceler optional callback invoked from {@link #close()} when the wrapper is closed before the load has + * completed ({@link #set} or {@link #setException}). Producers that support cancellation should + * provide one; producers that don't can pass {@code null}, in which case {@link #close()} just stops + * observing the result. + */ + public AsyncCursorHolder(@Nullable Runnable canceler) + { + this.canceler = canceler; + } + + /** + * Allows producer to mark the load successful with the given holder. Returns {@code true} if accepted, {@code false} + * if this wrapper has already been closed, in which case the producer is responsible for closing {@link CursorHolder} + * itself. Throws {@link DruidException} if the load was already completed (from prior calls to this method or + * {@link #setException}). + * <p> + * Callbacks registered via {@link #addReadyCallback} fire outside the lock to avoid re-entrancy deadlocks. Review Comment: oh, i guess bad wording; like its more like avoiding problems like if the callback needs to hold some other lock we don't have any weird ordering issues like if that lock is some lock shared between callback thread and producing thread, will try to clarify -- 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] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
