--- /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);
+		}
+		
+	}
+}