transport: Add ReceiveCommandErrorHandler

This gives a chance to handle an exception for a user. For example, when
an IOException is thrown while executing
`walk.parseAny(cmd.getNewId())`, it's always handled as
REJECTED_MISSING_OBJECT. However, IOException can mean a Git storage IO
error. By introducing an error handler class, a user can add a custom
error handler for these cases.

Change-Id: I3e03a536e1d8e137cb0f6e596d71642e72adde9e
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceiveCommandErrorHandler.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2019, Google LLC  and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.transport;
+
+import java.io.IOException;
+import java.util.List;
+
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.transport.ReceiveCommand.Result;
+
+/**
+ * Exception handler for processing {@link ReceiveCommand}.
+ *
+ * @since 5.7
+ */
+public interface ReceiveCommandErrorHandler {
+	/**
+	 * Handle an exception thrown while validating the new commit ID.
+	 *
+	 * @param cmd
+	 *            offending command
+	 * @param e
+	 *            exception thrown
+	 */
+	default void handleNewIdValidationException(ReceiveCommand cmd,
+			IOException e) {
+		cmd.setResult(Result.REJECTED_MISSING_OBJECT, cmd.getNewId().name());
+	}
+
+	/**
+	 * Handle an exception thrown while validating the old commit ID.
+	 *
+	 * @param cmd
+	 *            offending command
+	 * @param e
+	 *            exception thrown
+	 */
+	default void handleOldIdValidationException(ReceiveCommand cmd,
+			IOException e) {
+		cmd.setResult(Result.REJECTED_MISSING_OBJECT, cmd.getOldId().name());
+	}
+
+	/**
+	 * Handle an exception thrown while checking if the update is fast-forward.
+	 *
+	 * @param cmd
+	 *            offending command
+	 * @param e
+	 *            exception thrown
+	 */
+	default void handleFastForwardCheckException(ReceiveCommand cmd,
+			IOException e) {
+		if (e instanceof MissingObjectException) {
+			cmd.setResult(Result.REJECTED_MISSING_OBJECT, e.getMessage());
+		} else {
+			cmd.setResult(Result.REJECTED_OTHER_REASON);
+		}
+	}
+
+	/**
+	 * Handle an exception thrown while checking if the update is fast-forward.
+	 *
+	 * @param cmds
+	 *            commands being processed
+	 * @param e
+	 *            exception thrown
+	 */
+	default void handleBatchRefUpdateException(List<ReceiveCommand> cmds,
+			IOException e) {
+		for (ReceiveCommand cmd : cmds) {
+			if (cmd.getResult() == Result.NOT_ATTEMPTED) {
+				cmd.reject(e);
+			}
+		}
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java

@@ -295,6 +295,10 @@ public Set<String> getCapabilities() {
 	/** Hook to validate the update commands before execution. */
 	private PreReceiveHook preReceive;
 
+	private ReceiveCommandErrorHandler receiveCommandErrorHandler = new ReceiveCommandErrorHandler() {
+		// Use the default implementation.
+	};
+
 	/** Hook to report on the commands after execution. */
 	private PostReceiveHook postReceive;
 
@@ -1020,6 +1024,17 @@ public List<ReceiveCommand> getAllCommands() {
 		return Collections.unmodifiableList(commands);
 	}
 
+	/**
+	 * Set an error handler for {@link ReceiveCommand}.
+	 *
+	 * @param receiveCommandErrorHandler
+	 * @since 5.7
+	 */
+	public void setReceiveCommandErrorHandler(
+			ReceiveCommandErrorHandler receiveCommandErrorHandler) {
+		this.receiveCommandErrorHandler = receiveCommandErrorHandler;
+	}
+
 	/**
 	 * Send an error message to the client.
 	 * <p>
@@ -1726,16 +1741,16 @@ private void validateCommands() {
 				try {
 					oldObj = walk.parseAny(cmd.getOldId());
 				} catch (IOException e) {
-					cmd.setResult(Result.REJECTED_MISSING_OBJECT,
+					receiveCommandErrorHandler
-							cmd.getOldId().name());
+							.handleOldIdValidationException(cmd, e);
 					continue;
 				}
 
 				try {
 					newObj = walk.parseAny(cmd.getNewId());
 				} catch (IOException e) {
-					cmd.setResult(Result.REJECTED_MISSING_OBJECT,
+					receiveCommandErrorHandler
-							cmd.getNewId().name());
+							.handleNewIdValidationException(cmd, e);
 					continue;
 				}
 
@@ -1743,16 +1758,14 @@ private void validateCommands() {
 						&& newObj instanceof RevCommit) {
 					try {
 						if (walk.isMergedInto((RevCommit) oldObj,
-								(RevCommit) newObj))
+								(RevCommit) newObj)) {
 							cmd.setTypeFastForwardUpdate();
-						else
+						} else {
-							cmd.setType(
+							cmd.setType(ReceiveCommand.Type.UPDATE_NONFASTFORWARD);
-									ReceiveCommand.Type.UPDATE_NONFASTFORWARD);
+						}
-					} catch (MissingObjectException e) {
-						cmd.setResult(Result.REJECTED_MISSING_OBJECT,
-								e.getMessage());
 					} catch (IOException e) {
-						cmd.setResult(Result.REJECTED_OTHER_REASON);
+						receiveCommandErrorHandler
+								.handleFastForwardCheckException(cmd, e);
 					}
 				} else {
 					cmd.setType(ReceiveCommand.Type.UPDATE_NONFASTFORWARD);
@@ -1831,11 +1844,9 @@ private void executeCommands() {
 		try {
 			batch.setPushCertificate(getPushCertificate());
 			batch.execute(walk, updating);
-		} catch (IOException err) {
+		} catch (IOException e) {
-			for (ReceiveCommand cmd : toApply) {
+			receiveCommandErrorHandler.handleBatchRefUpdateException(toApply,
-				if (cmd.getResult() == Result.NOT_ATTEMPTED)
+					e);
-					cmd.reject(err);
-			}
 		}
 	}