BEP: Improve the BuildEventTransport interface.

Better specify the BuildEventTransport interface. Besides clarifying threading
and blocking issues, this change also clarifies error handling/reporting.

After several discussions we concluded that the BuildEventTransport interface
should not provide error reporting / handling facilities, as there is not much
bazel could do with this information. Instead, a transport may decide for
itself if an error is fatal and abort the build or if an error should be logged
to the user's terminal or if it should be ignored.

Furthermore, changing the close() method lays the groundwork for an upcoming
change that will report the transport shutdown status to the user command
line.
PiperOrigin-RevId: 152408938

1 files changed, 161 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/buildeventstream/transports/FileTransport.java b/src/main/java/com/google/devtools/build/lib/buildeventstream/transports/FileTransport.java
new file mode 100644
index 0000000000..e691df1819
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/buildeventstream/transports/FileTransport.java
@@ -0,0 +1,161 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// Licensed 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 com.google.devtools.build.lib.buildeventstream.transports;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.util.concurrent.SettableFuture;
+import com.google.devtools.build.lib.buildeventstream.BuildEvent;
+import com.google.devtools.build.lib.buildeventstream.BuildEventConverters;
+import com.google.devtools.build.lib.buildeventstream.BuildEventTransport;
+import com.google.devtools.build.lib.buildeventstream.PathConverter;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.AsynchronousFileChannel;
+import java.nio.channels.CompletionHandler;
+import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
+import java.util.concurrent.Future;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * Non-blocking file transport.
+ *
+ * <p>Implementors of this class need to implement {@link #sendBuildEvent(BuildEvent)} which
+ * serializes the build event and writes it to file using {@link #writeData(byte[])}.
+ */
+abstract class FileTransport implements BuildEventTransport {
+
+  /**
+   * We use an {@link AsynchronousFileChannel} to perform non-blocking writes to a file. It get's
+   * tricky when it comes to {@link #close()}, as we may only complete the returned future when
+   * all writes have completed (succeeded or failed). Thus, we use a field
+   * {@link #outstandingWrites} to keep track of the number of writes that have not completed yet.
+   * It's simply incremented before a new write and decremented after a write has completed. When
+   * it's {@code 0} it's safe to complete the close future.
+   */
+
+  private static final Logger log = Logger.getLogger(FileTransport.class.getName());
+
+  @VisibleForTesting
+  final AsynchronousFileChannel ch;
+  private final WriteCompletionHandler completionHandler = new WriteCompletionHandler();
+  protected final BuildEventConverters converters;
+  // The offset in the file to begin the next write at.
+  private long writeOffset;
+  // Number of writes that haven't completed yet.
+  private long outstandingWrites;
+  // The future returned by close()
+  private SettableFuture<Void> closeFuture;
+
+  FileTransport(String path, final PathConverter pathConverter) {
+    try {
+      ch = AsynchronousFileChannel.open(Paths.get(path), StandardOpenOption.CREATE,
+          StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE);
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+    this.converters = new BuildEventConverters() {
+      @Override
+      public PathConverter pathConverter() {
+        return pathConverter;
+      }
+    };
+  }
+
+  synchronized void writeData(byte[] data) {
+    checkNotNull(data);
+    if (!ch.isOpen()) {
+      close();
+      return;
+    }
+    if (closing()) {
+      return;
+    }
+
+    outstandingWrites++;
+
+    ch.write(ByteBuffer.wrap(data), writeOffset, null, completionHandler);
+
+    writeOffset += data.length;
+  }
+
+  @Override
+  public synchronized Future<Void> close() {
+    if (closing()) {
+      return closeFuture;
+    }
+    closeFuture = SettableFuture.create();
+
+    if (writesComplete()) {
+      doClose();
+    }
+
+    return closeFuture;
+  }
+
+  private void doClose() {
+    try {
+      ch.force(true);
+      ch.close();
+    } catch (IOException e) {
+      log.log(Level.SEVERE, e.getMessage(), e);
+    } finally {
+      closeFuture.set(null);
+    }
+  }
+
+  private boolean closing() {
+    return closeFuture != null;
+  }
+
+  private boolean writesComplete() {
+    return outstandingWrites == 0;
+  }
+
+  /**
+   * Handler that's notified when a write completes.
+   */
+  private final class WriteCompletionHandler implements CompletionHandler<Integer, Void> {
+
+    @Override
+    public void completed(Integer result, Void attachment) {
+      countWriteAndTryClose();
+    }
+
+    @Override
+    public void failed(Throwable exc, Void attachment) {
+      log.log(Level.SEVERE, exc.getMessage(), exc);
+      countWriteAndTryClose();
+      // There is no point in trying to continue. Close the transport.
+      close();
+    }
+
+    private void countWriteAndTryClose() {
+      synchronized (FileTransport.this) {
+        checkState(outstandingWrites > 0);
+
+        outstandingWrites--;
+
+        if (closing() && writesComplete()) {
+          doClose();
+        }
+      }
+    }
+  }
+}