Mercurial > hg4j

changeset 617:65c01508f002

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Rollback support for commands that modify repository. Strategy to keep complete copy of a file being changed
author Artem Tikhomirov <tikhomirov.artem@gmail.com>
date Wed, 15 May 2013 20:10:09 +0200
parents 5e0313485eef
children 7c0d2ce340b8
files src/org/tmatesoft/hg/core/HgAddRemoveCommand.java src/org/tmatesoft/hg/core/HgCommitCommand.java src/org/tmatesoft/hg/core/HgRevertCommand.java src/org/tmatesoft/hg/internal/COWTransaction.java src/org/tmatesoft/hg/internal/CommitFacility.java src/org/tmatesoft/hg/internal/CompleteRepoLock.java src/org/tmatesoft/hg/internal/DataAccessProvider.java src/org/tmatesoft/hg/internal/DirstateBuilder.java src/org/tmatesoft/hg/internal/FileUtils.java src/org/tmatesoft/hg/internal/RevlogStream.java src/org/tmatesoft/hg/internal/RevlogStreamWriter.java src/org/tmatesoft/hg/internal/Transaction.java src/org/tmatesoft/hg/repo/HgRepositoryFiles.java test/org/tmatesoft/hg/test/TestCommit.java
diffstat 14 files changed, 607 insertions(+), 42 deletions(-) [+]
line wrap: on
line diff
--- a/src/org/tmatesoft/hg/core/HgAddRemoveCommand.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/core/HgAddRemoveCommand.java	Wed May 15 20:10:09 2013 +0200
@@ -18,11 +18,14 @@
 
 import java.util.LinkedHashSet;
 
+import org.tmatesoft.hg.internal.COWTransaction;
 import org.tmatesoft.hg.internal.DirstateBuilder;
 import org.tmatesoft.hg.internal.DirstateReader;
 import org.tmatesoft.hg.internal.Internals;
+import org.tmatesoft.hg.internal.Transaction;
 import org.tmatesoft.hg.repo.HgManifest.Flags;
 import org.tmatesoft.hg.repo.HgRepository;
+import org.tmatesoft.hg.repo.HgRepositoryLock;
 import org.tmatesoft.hg.repo.HgRuntimeException;
 import org.tmatesoft.hg.util.CancelSupport;
 import org.tmatesoft.hg.util.CancelledException;
@@ -94,9 +97,12 @@
 	 * Perform scheduled addition/removal
 	 * 
 	 * @throws HgException subclass thereof to indicate specific issue with the command arguments or repository state
+	 * @throws HgRepositoryLockException if failed to lock the repo for modifications
 	 * @throws CancelledException if execution of the command was cancelled
 	 */
-	public void execute() throws HgException, CancelledException {
+	public void execute() throws HgException, HgRepositoryLockException, CancelledException {
+		final HgRepositoryLock wdLock = repo.getWorkingDirLock();
+		wdLock.acquire();
 		try {
 			final ProgressSupport progress = getProgressSupport(null);
 			final CancelSupport cancellation = getCancelSupport(null, true);
@@ -117,11 +123,24 @@
 				progress.worked(1);
 				cancellation.checkCancelled();
 			}
-			dirstateBuilder.serialize();
+			Transaction.Factory trFactory = new COWTransaction.Factory();
+			Transaction tr = trFactory.create(repo);
+			try {
+				dirstateBuilder.serialize(tr);
+				tr.commit();
+			} catch (RuntimeException ex) {
+				tr.rollback();
+				throw ex;
+			} catch (HgException ex) {
+				tr.rollback();
+				throw ex;
+			}
 			progress.worked(1);
 			progress.done();
 		} catch (HgRuntimeException ex) {
 			throw new HgLibraryFailureException(ex);
+		} finally {
+			wdLock.release();
 		}
 	}
 }
--- a/src/org/tmatesoft/hg/core/HgCommitCommand.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/core/HgCommitCommand.java	Wed May 15 20:10:09 2013 +0200
@@ -23,10 +23,13 @@
 import java.util.ArrayList;
 
 import org.tmatesoft.hg.internal.ByteArrayChannel;
+import org.tmatesoft.hg.internal.COWTransaction;
 import org.tmatesoft.hg.internal.CommitFacility;
+import org.tmatesoft.hg.internal.CompleteRepoLock;
 import org.tmatesoft.hg.internal.Experimental;
 import org.tmatesoft.hg.internal.FileContentSupplier;
 import org.tmatesoft.hg.internal.Internals;
+import org.tmatesoft.hg.internal.Transaction;
 import org.tmatesoft.hg.repo.HgChangelog;
 import org.tmatesoft.hg.repo.HgDataFile;
 import org.tmatesoft.hg.repo.HgInternals;
@@ -86,6 +89,7 @@
 
 	/**
 	 * @throws HgException subclass thereof to indicate specific issue with the command arguments or repository state
+	 * @throws HgRepositoryLockException if failed to lock the repo for modifications
 	 * @throws IOException propagated IO errors from status walker over working directory
 	 * @throws CancelledException if execution of the command was cancelled
 	 */
@@ -93,6 +97,8 @@
 		if (message == null) {
 			throw new HgBadArgumentException("Shall supply commit message", null);
 		}
+		final CompleteRepoLock repoLock = new CompleteRepoLock(repo);
+		repoLock.acquire();
 		try {
 			int[] parentRevs = new int[2];
 			detectParentFromDirstate(parentRevs);
@@ -127,7 +133,18 @@
 			}
 			cf.branch(detectBranch());
 			cf.user(detectUser());
-			newRevision = cf.commit(message);
+			Transaction.Factory trFactory = new COWTransaction.Factory();
+			Transaction tr = trFactory.create(repo);
+			try {
+				newRevision = cf.commit(message, tr);
+				tr.commit();
+			} catch (RuntimeException ex) {
+				tr.rollback();
+				throw ex;
+			} catch (HgException ex) {
+				tr.rollback();
+				throw ex;
+			}
 			// TODO toClear list is awful
 			for (FileContentSupplier fcs : toClear) {
 				fcs.done();
@@ -135,6 +152,8 @@
 			return new Outcome(Kind.Success, "Commit ok");
 		} catch (HgRuntimeException ex) {
 			throw new HgLibraryFailureException(ex);
+		} finally {
+			repoLock.release();
 		}
 	}
 
--- a/src/org/tmatesoft/hg/core/HgRevertCommand.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/core/HgRevertCommand.java	Wed May 15 20:10:09 2013 +0200
@@ -21,13 +21,16 @@
 import java.util.LinkedHashSet;
 import java.util.Set;
 
+import org.tmatesoft.hg.internal.COWTransaction;
 import org.tmatesoft.hg.internal.CsetParamKeeper;
 import org.tmatesoft.hg.internal.DirstateBuilder;
 import org.tmatesoft.hg.internal.DirstateReader;
 import org.tmatesoft.hg.internal.Internals;
+import org.tmatesoft.hg.internal.Transaction;
 import org.tmatesoft.hg.repo.HgManifest;
 import org.tmatesoft.hg.repo.HgManifest.Flags;
 import org.tmatesoft.hg.repo.HgRepository;
+import org.tmatesoft.hg.repo.HgRepositoryLock;
 import org.tmatesoft.hg.repo.HgRuntimeException;
 import org.tmatesoft.hg.util.CancelSupport;
 import org.tmatesoft.hg.util.CancelledException;
@@ -99,6 +102,8 @@
 	 * @throws CancelledException if execution of the command was cancelled
 	 */
 	public void execute() throws HgException, CancelledException {
+		final HgRepositoryLock wdLock = repo.getWorkingDirLock();
+		wdLock.acquire();
 		try {
 			final ProgressSupport progress = getProgressSupport(null);
 			final CancelSupport cancellation = getCancelSupport(null, true);
@@ -155,11 +160,25 @@
 				progress.worked(1);
 				cancellation.checkCancelled();
 			}
-			dirstateBuilder.serialize();
+			Transaction.Factory trFactory = new COWTransaction.Factory();
+			Transaction tr = trFactory.create(repo);
+			try {
+				// TODO same code in HgAddRemoveCommand and similar in HgCommitCommand
+				dirstateBuilder.serialize(tr);
+				tr.commit();
+			} catch (RuntimeException ex) {
+				tr.rollback();
+				throw ex;
+			} catch (HgException ex) {
+				tr.rollback();
+				throw ex;
+			}
 			progress.worked(1);
 			progress.done();
 		} catch (HgRuntimeException ex) {
 			throw new HgLibraryFailureException(ex);
+		} finally {
+			wdLock.release();
 		}
 	}
 }
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/org/tmatesoft/hg/internal/COWTransaction.java	Wed May 15 20:10:09 2013 +0200
@@ -0,0 +1,176 @@
+/*
+ * Copyright (c) 2013 TMate Software Ltd
+ *  
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * For information on how to redistribute this software under
+ * the terms of a license other than GNU General Public License
+ * contact TMate Software at support@hg4j.com
+ */
+package org.tmatesoft.hg.internal;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+
+import org.tmatesoft.hg.core.HgIOException;
+import org.tmatesoft.hg.core.SessionContext;
+
+/**
+ * This transaction strategy makes a copy of original file and breaks origin hard links, if any.
+ * Changes are directed to actual repository files.
+ * 
+ * On commit, remove all backup copies
+ * On rollback, move all backup files in place of original
+ * 
+ * @author Artem Tikhomirov
+ * @author TMate Software Ltd.
+ */
+public final class COWTransaction extends Transaction {
+	
+	private final FileUtils fileHelper;
+	private final List<RollbackEntry> entries = new LinkedList<RollbackEntry>();
+	
+	public COWTransaction(SessionContext.Source ctxSource) {
+		fileHelper = new FileUtils(ctxSource.getSessionContext().getLog());
+	}
+
+	@Override
+	public File prepare(File f) throws HgIOException {
+		if (!f.exists()) {
+			record(f, null);
+			return f;
+		}
+		if (known(f)) {
+			return f;
+		}
+		final File parentDir = f.getParentFile();
+		assert parentDir.canWrite();
+		File copy = new File(parentDir, f.getName() + ".hg4j.copy");
+		fileHelper.copy(f, copy);
+		final long lm = f.lastModified();
+		copy.setLastModified(lm);
+		File backup = new File(parentDir, f.getName() + ".hg4j.orig");
+		if (backup.exists()) {
+			backup.delete();
+		}
+		if (!f.renameTo(backup)) {
+			throw new HgIOException(String.format("Failed to backup %s to %s", f.getName(), backup.getName()), backup);
+		}
+		if (!copy.renameTo(f)) {
+			throw new HgIOException(String.format("Failed to bring on-write copy in place (%s to %s)", copy.getName(), f.getName()), copy);
+		}
+		f.setLastModified(lm);
+		record(f, backup);
+		return f;
+	}
+
+	@Override
+	public File prepare(File origin, File backup) throws HgIOException {
+		if (known(origin)) {
+			return origin;
+		}
+		fileHelper.copy(origin, backup);
+		final RollbackEntry e = record(origin, backup);
+		e.keepBackup = true;
+		return origin;
+	}
+
+	@Override
+	public void done(File f) throws HgIOException {
+		find(f).success = true;
+	}
+
+	@Override
+	public void failure(File f, IOException ex) {
+		find(f).failure = ex;
+	}
+
+	// XXX custom exception for commit and rollback to hold information about files rolled back
+	
+	@Override
+	public void commit() throws HgIOException {
+		for (Iterator<RollbackEntry> it = entries.iterator(); it.hasNext();) {
+			RollbackEntry e = it.next();
+			assert e.success;
+			if (e.failure != null) {
+				throw new HgIOException("Can't close transaction with a failure.", e.failure, e.origin);
+			}
+			if (!e.keepBackup && e.backup != null) {
+				e.backup.delete();
+			}
+			it.remove();
+		}
+	}
+
+	@Override
+	public void rollback() throws HgIOException {
+		LinkedList<RollbackEntry> success = new LinkedList<RollbackEntry>();
+		for (Iterator<RollbackEntry> it = entries.iterator(); it.hasNext();) {
+			RollbackEntry e = it.next();
+			e.origin.delete();
+			if (e.backup != null) {
+				if (!e.backup.renameTo(e.origin)) {
+					String msg = String.format("Transaction rollback failed, could not rename backup %s back to %s", e.backup.getName(), e.origin.getName());
+					throw new HgIOException(msg, e.origin);
+				}
+			}
+			success.add(e);
+			it.remove();
+		}
+	}
+
+	private RollbackEntry record(File origin, File backup) {
+		final RollbackEntry e = new RollbackEntry(origin, backup);
+		entries.add(e);
+		return e;
+	}
+
+	private boolean known(File f) {
+		for (RollbackEntry e : entries) {
+			if (e.origin.equals(f)) {
+				return true;
+			}
+		}
+		return false;
+	}
+	private RollbackEntry find(File f) {
+		for (RollbackEntry e : entries) {
+			if (e.origin.equals(f)) {
+				return e;
+			}
+		}
+		assert false;
+		return new RollbackEntry(f,f);
+	}
+
+	private static class RollbackEntry {
+		public final File origin;
+		public final File backup; // may be null to indicate file didn't exist
+		public boolean success = false;
+		public IOException failure = null;
+		public boolean keepBackup = false;
+		
+		public RollbackEntry(File o, File b) {
+			origin = o;
+			backup = b;
+		}
+	}
+	
+	public static class Factory implements Transaction.Factory {
+
+		public Transaction create(SessionContext.Source ctxSource) {
+			return new COWTransaction(ctxSource);
+		}
+		
+	}
+}
--- a/src/org/tmatesoft/hg/internal/CommitFacility.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/internal/CommitFacility.java	Wed May 15 20:10:09 2013 +0200
@@ -19,6 +19,7 @@
 import static org.tmatesoft.hg.repo.HgRepository.DEFAULT_BRANCH_NAME;
 import static org.tmatesoft.hg.repo.HgRepository.NO_REVISION;
 import static org.tmatesoft.hg.repo.HgRepositoryFiles.Branch;
+import static org.tmatesoft.hg.repo.HgRepositoryFiles.UndoBranch;
 import static org.tmatesoft.hg.util.LogFacility.Severity.Error;
 
 import java.io.File;
@@ -94,7 +95,9 @@
 		user = userName;
 	}
 	
-	public Nodeid commit(String message) throws HgIOException, HgRepositoryLockException {
+	// this method doesn't roll transaction back in case of failure, caller's responsibility
+	// this method expects repository to be locked, if needed
+	public Nodeid commit(String message, Transaction transaction) throws HgIOException, HgRepositoryLockException {
 		final HgChangelog clog = repo.getRepo().getChangelog();
 		final int clogRevisionIndex = clog.getRevisionCount();
 		ManifestRevision c1Manifest = new ManifestRevision(null, null);
@@ -161,7 +164,7 @@
 				// that would attempt to access newly added file after commit would fail
 				// (despite the fact the file is in there)
 			}
-			RevlogStreamWriter fileWriter = new RevlogStreamWriter(repo, contentStream);
+			RevlogStreamWriter fileWriter = new RevlogStreamWriter(repo, contentStream, transaction);
 			Nodeid fileRev = fileWriter.addRevision(bac.toArray(), clogRevisionIndex, fp.first(), fp.second());
 			newManifestRevision.put(df.getPath(), fileRev);
 			touchInDirstate.add(df.getPath());
@@ -172,7 +175,7 @@
 		for (Map.Entry<Path, Nodeid> me : newManifestRevision.entrySet()) {
 			manifestBuilder.add(me.getKey().toString(), me.getValue());
 		}
-		RevlogStreamWriter manifestWriter = new RevlogStreamWriter(repo, repo.getImplAccess().getManifestStream());
+		RevlogStreamWriter manifestWriter = new RevlogStreamWriter(repo, repo.getImplAccess().getManifestStream(), transaction);
 		Nodeid manifestRev = manifestWriter.addRevision(manifestBuilder.build(), clogRevisionIndex, manifestParents.first(), manifestParents.second());
 		//
 		// Changelog
@@ -181,7 +184,7 @@
 		changelogBuilder.branch(branch == null ? DEFAULT_BRANCH_NAME : branch);
 		changelogBuilder.user(String.valueOf(user));
 		byte[] clogContent = changelogBuilder.build(manifestRev, message);
-		RevlogStreamWriter changelogWriter = new RevlogStreamWriter(repo, repo.getImplAccess().getChangelogStream());
+		RevlogStreamWriter changelogWriter = new RevlogStreamWriter(repo, repo.getImplAccess().getChangelogStream(), transaction);
 		Nodeid changesetRev = changelogWriter.addRevision(clogContent, clogRevisionIndex, p1Commit, p2Commit);
 		// TODO move fncache update to an external facility, along with dirstate and bookmark update
 		if (!newlyAddedFiles.isEmpty() && repo.fncacheInUse()) {
@@ -201,14 +204,19 @@
 		}
 		String oldBranchValue = DirstateReader.readBranch(repo);
 		String newBranchValue = branch == null ? DEFAULT_BRANCH_NAME : branch;
+		// TODO undo.dirstate and undo.branch as described in http://mercurial.selenic.com/wiki/FileFormats#undo..2A
 		if (!oldBranchValue.equals(newBranchValue)) {
-			File branchFile = repo.getRepositoryFile(Branch);
+			File branchFile = transaction.prepare(repo.getRepositoryFile(Branch), repo.getRepositoryFile(UndoBranch));
 			FileOutputStream fos = null;
 			try {
 				fos = new FileOutputStream(branchFile);
 				fos.write(newBranchValue.getBytes(EncodingHelper.getUTF8()));
 				fos.flush();
+				fos.close();
+				fos = null;
+				transaction.done(branchFile);
 			} catch (IOException ex) {
+				transaction.failure(branchFile, ex);
 				repo.getLog().dump(getClass(), Error, ex, "Failed to write branch information, error ignored");
 			} finally {
 				try {
@@ -220,7 +228,7 @@
 				}
 			}
 		}
-		// bring dirstate up to commit state
+		// bring dirstate up to commit state, TODO share this code with HgAddRemoveCommand
 		final DirstateBuilder dirstateBuilder = new DirstateBuilder(repo);
 		dirstateBuilder.fillFrom(new DirstateReader(repo, new Path.SimpleSource()));
 		for (Path p : removals) {
@@ -230,12 +238,11 @@
 			dirstateBuilder.recordUncertain(p);
 		}
 		dirstateBuilder.parents(changesetRev, Nodeid.NULL);
-		dirstateBuilder.serialize();
+		dirstateBuilder.serialize(transaction);
 		// update bookmarks
 		if (p1Commit != NO_REVISION || p2Commit != NO_REVISION) {
 			repo.getRepo().getBookmarks().updateActive(p1Cset, p2Cset, changesetRev);
 		}
-		// TODO undo.dirstate and undo.branch as described in http://mercurial.selenic.com/wiki/FileFormats#undo..2A
 		// TODO Revisit: might be reasonable to send out a "Repo changed" notification, to clear
 		// e.g. cached branch, tags and so on, not to rely on file change detection methods?
 		// The same notification might come useful once Pull is implemented
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/org/tmatesoft/hg/internal/CompleteRepoLock.java	Wed May 15 20:10:09 2013 +0200
@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2013 TMate Software Ltd
+ *  
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * For information on how to redistribute this software under
+ * the terms of a license other than GNU General Public License
+ * contact TMate Software at support@hg4j.com
+ */
+package org.tmatesoft.hg.internal;
+
+import static org.tmatesoft.hg.util.LogFacility.Severity.Error;
+
+import org.tmatesoft.hg.core.HgRepositoryLockException;
+import org.tmatesoft.hg.repo.HgRepository;
+import org.tmatesoft.hg.repo.HgRepositoryLock;
+import org.tmatesoft.hg.util.LogFacility;
+
+/**
+ * Helper to lock both storage and working directory
+ * 
+ * @author Artem Tikhomirov
+ * @author TMate Software Ltd.
+ */
+public final class CompleteRepoLock {
+
+	private final HgRepository repo;
+	private HgRepositoryLock wdLock, storeLock;
+
+	public CompleteRepoLock(HgRepository hgRepo) {
+		repo = hgRepo;
+	}
+
+	public void acquire() throws HgRepositoryLockException {
+		wdLock = repo.getWorkingDirLock();
+		storeLock = repo.getStoreLock();
+		wdLock.acquire();
+		try {
+			storeLock.acquire();
+		} catch (HgRepositoryLockException ex) {
+			try {
+				wdLock.release();
+			} catch (HgRepositoryLockException e2) {
+				final LogFacility log = repo.getSessionContext().getLog();
+				log.dump(getClass(), Error, e2, "Nested exception ignored once failed to acquire store lock");
+			}
+			throw ex;
+		}
+
+	}
+	
+	public void release() throws HgRepositoryLockException {
+		try {
+			storeLock.release();
+		} catch (HgRepositoryLockException ex) {
+			try {
+				wdLock.release();
+			} catch (HgRepositoryLockException e2) {
+				final LogFacility log = repo.getSessionContext().getLog();
+				log.dump(getClass(), Error, e2, "Nested exception ignored when releasing working directory lock");
+			}
+			throw ex;
+		}
+		wdLock.release();
+	}
+}
--- a/src/org/tmatesoft/hg/internal/DataAccessProvider.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/internal/DataAccessProvider.java	Wed May 15 20:10:09 2013 +0200
@@ -29,6 +29,7 @@
 import java.nio.MappedByteBuffer;
 import java.nio.channels.FileChannel;
 
+import org.tmatesoft.hg.core.HgIOException;
 import org.tmatesoft.hg.core.SessionContext;
 import org.tmatesoft.hg.util.LogFacility;
 
@@ -80,7 +81,7 @@
 			return new DataAccess();
 		}
 		try {
-			FileChannel fc = new FileInputStream(f).getChannel();
+			FileChannel fc = new FileInputStream(f).getChannel(); // FIXME SHALL CLOSE FIS, not only channel
 			long flen = fc.size();
 			if (!shortRead && flen > mapioMagicBoundary) {
 				// TESTS: bufLen of 1024 was used to test MemMapFileAccess
@@ -100,12 +101,26 @@
 		return new DataAccess(); // non-null, empty.
 	}
 	
-	public DataSerializer createWriter(File f, boolean createNewIfDoesntExist) {
+	public DataSerializer createWriter(final Transaction tr, File f, boolean createNewIfDoesntExist) throws HgIOException {
 		if (!f.exists() && !createNewIfDoesntExist) {
 			return new DataSerializer();
 		}
 		try {
-			return new StreamDataSerializer(context.getLog(), new FileOutputStream(f, true));
+			final File transactionFile = tr.prepare(f);
+			return new StreamDataSerializer(context.getLog(), new FileOutputStream(transactionFile, true)) {
+				@Override
+				public void done() {
+					super.done();
+					// FIXME invert RevlogStreamWriter to send DataSource here instead of grabbing DataSerializer
+					// besides, DataSerializer#done is invoked regardless of whether write was successful or not,
+					// while Transaction#done() assumes there's no error
+					try {
+						tr.done(transactionFile);
+					} catch (HgIOException ex) {
+						context.getLog().dump(DataAccessProvider.class, Error, ex, null);
+					}
+				}
+			};
 		} catch (final FileNotFoundException ex) {
 			context.getLog().dump(getClass(), Error, ex, null);
 			return new DataSerializer() {
--- a/src/org/tmatesoft/hg/internal/DirstateBuilder.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/internal/DirstateBuilder.java	Wed May 15 20:10:09 2013 +0200
@@ -16,6 +16,9 @@
  */
 package org.tmatesoft.hg.internal;
 
+import static org.tmatesoft.hg.repo.HgRepositoryFiles.Dirstate;
+import static org.tmatesoft.hg.repo.HgRepositoryFiles.UndoDirstate;
+
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
@@ -32,7 +35,6 @@
 import org.tmatesoft.hg.repo.HgDirstate.Record;
 import org.tmatesoft.hg.repo.HgInvalidStateException;
 import org.tmatesoft.hg.repo.HgManifest.Flags;
-import org.tmatesoft.hg.repo.HgRepositoryFiles;
 import org.tmatesoft.hg.util.Path;
 
 /**
@@ -149,13 +151,15 @@
 		}
 	}
 	
-	public void serialize() throws HgIOException {
-		File dirstateFile = hgRepo.getRepositoryFile(HgRepositoryFiles.Dirstate);
+	public void serialize(Transaction tr) throws HgIOException {
+		File dirstateFile = tr.prepare(hgRepo.getRepositoryFile(Dirstate), hgRepo.getRepositoryFile(UndoDirstate));
 		try {
 			FileChannel dirstate = new FileOutputStream(dirstateFile).getChannel();
 			serialize(dirstate);
 			dirstate.close();
+			tr.done(dirstateFile);
 		} catch (IOException ex) {
+			tr.failure(dirstateFile, ex);
 			throw new HgIOException("Can't write down new directory state", ex, dirstateFile);
 		}
 	}
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/org/tmatesoft/hg/internal/FileUtils.java	Wed May 15 20:10:09 2013 +0200
@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2013 TMate Software Ltd
+ *  
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * For information on how to redistribute this software under
+ * the terms of a license other than GNU General Public License
+ * contact TMate Software at support@hg4j.com
+ */
+package org.tmatesoft.hg.internal;
+
+import static org.tmatesoft.hg.util.LogFacility.Severity.Debug;
+
+import java.io.Closeable;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+
+import org.tmatesoft.hg.core.HgIOException;
+import org.tmatesoft.hg.util.LogFacility;
+import org.tmatesoft.hg.util.LogFacility.Severity;
+
+/**
+ * 
+ * @author Artem Tikhomirov
+ * @author TMate Software Ltd.
+ */
+final class FileUtils {
+	
+	private final LogFacility log;
+	
+	public static void copyFile(File from, File to) throws HgIOException {
+		new FileUtils(new StreamLogFacility(Debug, true, System.err)).copy(from, to);
+	}
+
+	public FileUtils(LogFacility logFacility) {
+		log = logFacility;
+	}
+
+	public void copy(File from, File to) throws HgIOException {
+		FileInputStream fis = null;
+		FileOutputStream fos = null;
+		try {
+			fis = new FileInputStream(from);
+			fos = new FileOutputStream(to);
+			FileChannel input = fis.getChannel();
+			FileChannel output = fos.getChannel();
+			long count = input.size();
+			long pos = 0;
+			int zeroCopied = 0; // flag to prevent hang-up
+			do {
+				long c = input.transferTo(pos, count, output);
+				pos += c;
+				count -= c;
+				if (c == 0) {
+					if (++zeroCopied == 3) {
+						String m = String.format("Can't copy %s to %s, transferTo copies 0 bytes. Position: %d, bytes left:%d", from.getName(), to.getName(), pos, count);
+						throw new IOException(m);
+					}
+				} else {
+					// reset
+					zeroCopied = 0;
+				}
+			} while (count > 0);
+			fos.close();
+			fos = null;
+			fis.close();
+			fis = null;
+		} catch (IOException ex) {
+			// not in finally because I don't want to loose exception from fos.close()
+			closeQuietly(fis);
+			closeQuietly(fos);
+			String m = String.format("Failed to copy %s to %s", from.getName(), to.getName());
+			throw new HgIOException(m, ex, from);
+		}
+	}
+	
+	public void closeQuietly(Closeable stream) {
+		if (stream != null) {
+			try {
+				stream.close();
+			} catch (IOException ex) {
+				// ignore
+				log.dump(getClass(), Severity.Warn, ex, "Exception while closing stream quietly");
+			}
+		}
+	}
+
+}
--- a/src/org/tmatesoft/hg/internal/RevlogStream.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/internal/RevlogStream.java	Wed May 15 20:10:09 2013 +0200
@@ -29,6 +29,7 @@
 import java.util.List;
 import java.util.zip.Inflater;
 
+import org.tmatesoft.hg.core.HgIOException;
 import org.tmatesoft.hg.core.Nodeid;
 import org.tmatesoft.hg.repo.HgInternals;
 import org.tmatesoft.hg.repo.HgInvalidControlFileException;
@@ -101,14 +102,14 @@
 		return dataAccess.createReader(getDataFile(), false);
 	}
 	
-	/*package*/ DataSerializer getIndexStreamWriter() {
+	/*package*/ DataSerializer getIndexStreamWriter(Transaction tr) throws HgIOException {
 		DataAccessProvider dataAccess = repo.getDataAccess();
-		return dataAccess.createWriter(indexFile, true);
+		return dataAccess.createWriter(tr, indexFile, true);
 	}
 	
-	/*package*/ DataSerializer getDataStreamWriter() {
+	/*package*/ DataSerializer getDataStreamWriter(Transaction tr) throws HgIOException {
 		DataAccessProvider dataAccess = repo.getDataAccess();
-		return dataAccess.createWriter(getDataFile(), true);
+		return dataAccess.createWriter(tr, getDataFile(), true);
 	}
 	
 	/**
--- a/src/org/tmatesoft/hg/internal/RevlogStreamWriter.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/internal/RevlogStreamWriter.java	Wed May 15 20:10:09 2013 +0200
@@ -19,9 +19,11 @@
 import static org.tmatesoft.hg.internal.Internals.REVLOGV1_RECORD_SIZE;
 import static org.tmatesoft.hg.repo.HgRepository.NO_REVISION;
 
+import java.io.File;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 
+import org.tmatesoft.hg.core.HgIOException;
 import org.tmatesoft.hg.core.Nodeid;
 import org.tmatesoft.hg.core.SessionContext;
 import org.tmatesoft.hg.repo.HgInvalidControlFileException;
@@ -38,24 +40,27 @@
 
 	private final DigestHelper dh = new DigestHelper();
 	private final RevlogCompressor revlogDataZip;
+	private final Transaction transaction;
 	private int lastEntryBase, lastEntryIndex;
 	private byte[] lastEntryContent;
 	private Nodeid lastEntryRevision;
 	private IntMap<Nodeid> revisionCache = new IntMap<Nodeid>(32);
 	private RevlogStream revlogStream;
 	
-	public RevlogStreamWriter(SessionContext.Source ctxSource, RevlogStream stream) {
+	public RevlogStreamWriter(SessionContext.Source ctxSource, RevlogStream stream, Transaction tr) {
 		assert ctxSource != null;
 		assert stream != null;
+		assert tr != null;
 				
 		revlogDataZip = new RevlogCompressor(ctxSource.getSessionContext());
 		revlogStream = stream;
+		transaction = tr;
 	}
 	
 	/**
 	 * @return nodeid of added revision
 	 */
-	public Nodeid addRevision(byte[] content, int linkRevision, int p1, int p2) {
+	public Nodeid addRevision(byte[] content, int linkRevision, int p1, int p2) throws HgIOException {
 		lastEntryRevision = Nodeid.NULL;
 		int revCount = revlogStream.revisionCount();
 		lastEntryIndex = revCount == 0 ? NO_REVISION : revCount - 1;
@@ -85,7 +90,7 @@
 		indexFile = dataFile = activeFile = null;
 		try {
 			//
-			activeFile = indexFile = revlogStream.getIndexStreamWriter();
+			activeFile = indexFile = revlogStream.getIndexStreamWriter(transaction);
 			final boolean isInlineData = revlogStream.isInlineData();
 			HeaderWriter revlogHeader = new HeaderWriter(isInlineData);
 			revlogHeader.length(content.length, compressedLen);
@@ -101,7 +106,7 @@
 			if (isInlineData) {
 				dataFile = indexFile;
 			} else {
-				dataFile = revlogStream.getDataStreamWriter();
+				dataFile = revlogStream.getDataStreamWriter(transaction);
 			}
 			activeFile = dataFile;
 			if (useCompressedData) {
@@ -124,12 +129,8 @@
 			revlogStream.revisionAdded(lastEntryIndex, lastEntryRevision, lastEntryBase, lastEntryOffset);
 		} catch (IOException ex) {
 			String m = String.format("Failed to write revision %d", lastEntryIndex+1, null);
-			HgInvalidControlFileException t = new HgInvalidControlFileException(m, ex, null);
-			if (activeFile == dataFile) {
-				throw revlogStream.initWithDataFile(t);
-			} else {
-				throw revlogStream.initWithIndexFile(t);
-			}
+			// FIXME proper file in the exception based on activeFile == dataFile || indexFile 
+			throw new HgIOException(m, ex, new File(revlogStream.getDataFileName()));
 		} finally {
 			indexFile.done();
 			if (dataFile != null && dataFile != indexFile) {
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/org/tmatesoft/hg/internal/Transaction.java	Wed May 15 20:10:09 2013 +0200
@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2013 TMate Software Ltd
+ *  
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * For information on how to redistribute this software under
+ * the terms of a license other than GNU General Public License
+ * contact TMate Software at support@hg4j.com
+ */
+package org.tmatesoft.hg.internal;
+
+import java.io.File;
+import java.io.IOException;
+
+import org.tmatesoft.hg.core.HgIOException;
+import org.tmatesoft.hg.core.SessionContext;
+import org.tmatesoft.hg.repo.HgInvalidStateException;
+
+/**
+ * Implementation strategies possible:<ul>
+ * <li> Get a copy, write changes to origin, keep copy as backup till #commit
+ *   <p>(-) doesn't break hard links 
+ * <li> Get a copy, write changes to a copy, on commit rename copy to origin. 
+ *   <p>(-) What if we read newly written data (won't find it);
+ *   <p>(-) complex #commit
+ *   <p>(+) simple rollback
+ * <li> Get a copy, rename origin to backup (breaks hard links), rename copy to origin, write changes 
+ *   <p>(+) Modified file is in place right away;
+ *   <p>(+) easy #commit
+ * <li> Do not copy, just record file size, truncate to that size on rollback
+ * <li> ...?
+ * </ul> 
+ * @author Artem Tikhomirov
+ * @author TMate Software Ltd.
+ */
+public abstract class Transaction {
+	/**
+	 * Record the file is going to be modified during this transaction, obtain actual
+	 * destination to write to.
+	 */
+	public abstract File prepare(File f) throws HgIOException;
+	/**
+	 * overwrites backup if exists, backup is kept after successful {@link #commit()}
+	 */
+	public abstract File prepare(File origin, File backup) throws HgIOException;
+	/**
+	 * Tell that file was successfully processed
+	 */
+	public abstract void done(File f) throws HgIOException;
+	/**
+	 * optional?
+	 */
+	public abstract void failure(File f, IOException ex);
+	/**
+	 * Complete the transaction
+	 */
+	public abstract void commit() throws HgIOException;
+	/**
+	 * Undo all the changes
+	 */
+	public abstract void rollback() throws HgIOException;
+
+	public interface Factory {
+		public Transaction create(SessionContext.Source ctxSource);
+	}
+
+	public static class NoRollback extends Transaction {
+
+		@Override
+		public File prepare(File f) throws HgIOException {
+			return f;
+		}
+
+		@Override
+		public File prepare(File origin, File backup) throws HgIOException {
+			return origin;
+		}
+
+		@Override
+		public void done(File f) throws HgIOException {
+			// no-op
+		}
+
+		@Override
+		public void failure(File f, IOException ex) {
+			// no-op
+		}
+
+		@Override
+		public void commit() throws HgIOException {
+			// no-op
+		}
+
+		@Override
+		public void rollback() throws HgIOException {
+			throw new HgInvalidStateException("This transaction doesn't support rollback");
+		}
+	}
+}
--- a/src/org/tmatesoft/hg/repo/HgRepositoryFiles.java	Tue May 14 17:31:35 2013 +0200
+++ b/src/org/tmatesoft/hg/repo/HgRepositoryFiles.java	Wed May 15 20:10:09 2013 +0200
@@ -30,7 +30,8 @@
 	HgSub(".hgsub"), HgSubstate(".hgsubstate"),
 	LastMessage(false, "last-message.txt"),
 	Bookmarks(false, "bookmarks"), BookmarksCurrent(false, "bookmarks.current"),
-	Branch(false, "branch");
+	Branch(false, "branch"), 
+	UndoBranch(false, "undo.branch"), UndoDirstate(false, "undo.dirstate");
 
 	private final String fname;
 	private final boolean livesInWC; 
--- a/test/org/tmatesoft/hg/test/TestCommit.java	Tue May 14 17:31:35 2013 +0200
+++ b/test/org/tmatesoft/hg/test/TestCommit.java	Wed May 15 20:10:09 2013 +0200
@@ -34,10 +34,13 @@
 import org.tmatesoft.hg.core.HgStatus.Kind;
 import org.tmatesoft.hg.core.HgStatusCommand;
 import org.tmatesoft.hg.core.Nodeid;
+import org.tmatesoft.hg.core.SessionContext;
 import org.tmatesoft.hg.internal.ByteArrayChannel;
+import org.tmatesoft.hg.internal.COWTransaction;
 import org.tmatesoft.hg.internal.CommitFacility;
 import org.tmatesoft.hg.internal.FileContentSupplier;
 import org.tmatesoft.hg.internal.Internals;
+import org.tmatesoft.hg.internal.Transaction;
 import org.tmatesoft.hg.repo.HgDataFile;
 import org.tmatesoft.hg.repo.HgLookup;
 import org.tmatesoft.hg.repo.HgRepository;
@@ -55,7 +58,14 @@
 
 	@Rule
 	public ErrorCollectorExt errorCollector = new ErrorCollectorExt();
-
+	
+	private final Transaction.Factory trFactory = new COWTransaction.Factory();
+//	{
+//		public Transaction create(Source ctxSource) {
+//			return new Transaction.NoRollback();
+//		}
+//	};
+	
 	@Test
 	public void testCommitToNonEmpty() throws Exception {
 		File repoLoc = RepoUtils.initEmptyTempRepo("test-commit2non-empty");
@@ -68,7 +78,9 @@
 		// just changed endings are in the patch!
 		HgDataFile df = hgRepo.getFileNode("file1");
 		cf.add(df, new ByteArraySupplier("hello\nworld".getBytes()));
-		Nodeid secondRev = cf.commit("SECOND");
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid secondRev = cf.commit("SECOND", tr);
+		tr.commit();
 		//
 		List<HgChangeset> commits = new HgLogCommand(hgRepo).execute();
 		errorCollector.assertEquals(2, commits.size());
@@ -96,7 +108,9 @@
 		final byte[] initialContent = "hello\nworld".getBytes();
 		cf.add(df, new ByteArraySupplier(initialContent));
 		String comment = "commit 1";
-		Nodeid c1Rev = cf.commit(comment);
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid c1Rev = cf.commit(comment,  tr);
+		tr.commit();
 		List<HgChangeset> commits = new HgLogCommand(hgRepo).execute();
 		errorCollector.assertEquals(1, commits.size());
 		HgChangeset c1 = commits.get(0);
@@ -130,7 +144,9 @@
 		FileContentSupplier contentProvider = new FileContentSupplier(fileD);
 		cf.add(dfD, contentProvider);
 		cf.branch("branch1");
-		Nodeid commitRev1 = cf.commit("FIRST");
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid commitRev1 = cf.commit("FIRST",  tr);
+		tr.commit();
 		contentProvider.done();
 		//
 		List<HgChangeset> commits = new HgLogCommand(hgRepo).range(parentCsetRevIndex+1, TIP).execute();
@@ -158,7 +174,9 @@
 		FileContentSupplier contentProvider = new FileContentSupplier(new File(repoLoc, "xx"));
 		cf.add(hgRepo.getFileNode("xx"), contentProvider);
 		cf.forget(hgRepo.getFileNode("d"));
-		Nodeid commitRev = cf.commit("Commit with add/remove cmd");
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid commitRev = cf.commit("Commit with add/remove cmd",  tr);
+		tr.commit();
 		contentProvider.done();
 		//
 		List<HgChangeset> commits = new HgLogCommand(hgRepo).changeset(commitRev).execute();
@@ -191,20 +209,22 @@
 		FileContentSupplier contentProvider = new FileContentSupplier(fileD);
 		cf.add(dfD, contentProvider);
 		cf.branch("branch1");
-		Nodeid commitRev1 = cf.commit("FIRST");
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid commitRev1 = cf.commit("FIRST",  tr);
 		contentProvider.done();
 		//
 		RepoUtils.modifyFileAppend(fileD, " 2 \n");
 		cf.add(dfD, contentProvider = new FileContentSupplier(fileD));
 		cf.branch("branch2");
-		Nodeid commitRev2 = cf.commit("SECOND");
+		Nodeid commitRev2 = cf.commit("SECOND",  tr);
 		contentProvider.done();
 		//
 		RepoUtils.modifyFileAppend(fileD, " 2 \n");
 		cf.add(dfD, contentProvider = new FileContentSupplier(fileD));
 		cf.branch(DEFAULT_BRANCH_NAME);
-		Nodeid commitRev3 = cf.commit("THIRD");
+		Nodeid commitRev3 = cf.commit("THIRD",  tr);
 		contentProvider.done();
+		tr.commit();
 		//
 		List<HgChangeset> commits = new HgLogCommand(hgRepo).range(parentCsetRevIndex+1, TIP).execute();
 		assertEquals(3, commits.size());
@@ -313,7 +333,9 @@
 		CommitFacility cf = new CommitFacility(Internals.getInstance(hgRepo), parentCsetRevIndex);
 		cf.add(hgRepo.getFileNode("a"), new FileContentSupplier(new File(repoLoc, "a")));
 		cf.branch(branch);
-		Nodeid commit = cf.commit("FIRST");
+		Transaction tr = newTransaction(hgRepo);
+		Nodeid commit = cf.commit("FIRST",  tr);
+		tr.commit();
 		errorCollector.assertEquals("commit with branch shall update WC", branch, hgRepo.getWorkingCopyBranchName());
 		
 		ExecHelper eh = new ExecHelper(new OutputParser.Stub(), repoLoc);
@@ -331,6 +353,10 @@
 		verifyRun.run("hg", "verify");
 		errorCollector.assertEquals("hg verify", 0, verifyRun.getExitValue());
 	}
+	
+	private Transaction newTransaction(SessionContext.Source ctxSource) {
+		return trFactory.create(ctxSource);
+	}
 
 	public static void main(String[] args) throws Exception {
 		new TestCommit().testCommitToEmpty();