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    * Copyright (C) 2011, Google Inc.
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
    *
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  package org.eclipse.jgit.internal.storage.dfs;
  
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.eclipse.jgit.internal.storage.pack.PackExt;
  import org.eclipse.jgit.lib.ObjectDatabase;
  import org.eclipse.jgit.lib.ObjectInserter;
  import org.eclipse.jgit.lib.ObjectReader;
  
  /** Manages objects stored in {@link DfsPackFile} on a storage system. */
  public abstract class DfsObjDatabase extends ObjectDatabase {
  	private static final PackList NO_PACKS = new PackList(new DfsPackFile[0]);
  
  	/** Sources for a pack file. */
  	public static enum PackSource {
  		/** The pack is created by ObjectInserter due to local activity. */
  		INSERT(0),
  
  		/**
  		 * The pack is created by PackParser due to a network event.
  		 * <p>
  		 * A received pack can be from either a push into the repository, or a
  		 * fetch into the repository, the direction doesn't matter. A received
  		 * pack was built by the remote Git implementation and may not match the
  		 * storage layout preferred by this version. Received packs are likely
  		 * to be either compacted or garbage collected in the future.
  		 */
  		RECEIVE(0),
  
  		/**
  		 * Pack was created by Git garbage collection by this implementation.
  		 * <p>
  		 * This source is only used by the {@link DfsGarbageCollector} when it
  		 * builds a pack file by traversing the object graph and copying all
  		 * reachable objects into a new pack stream.
  		 *
  		 * @see DfsGarbageCollector
  		 */
  		GC(1),
  
  		/**
  		 * The pack was created by compacting multiple packs together.
  		 * <p>
  		 * Packs created by compacting multiple packs together aren't nearly as
  		 * efficient as a fully garbage collected repository, but may save disk
  		 * space by reducing redundant copies of base objects.
  		 *
  		 * @see DfsPackCompactor
 		 */
 		COMPACT(1),
 
 		/**
 		 * Pack was created by Git garbage collection.
 		 * <p>
 		 * This pack contains only unreachable garbage that was found during the
 		 * last GC pass. It is retained in a new pack until it is safe to prune
 		 * these objects from the repository.
 		 */
 		UNREACHABLE_GARBAGE(2);
 
 		final int category;
 
 		PackSource(int category) {
 			this.category = category;
 		}
 	}
 
 	private final AtomicReference<PackList> packList;
 
 	private final DfsRepository repository;
 
 	private DfsReaderOptions readerOptions;
 
 	/**
 	 * Initialize an object database for our repository.
 	 *
 	 * @param repository
 	 *            repository owning this object database.
 	 *
 	 * @param options
 	 *            how readers should access the object database.
 	 */
 	protected DfsObjDatabase(DfsRepository repository,
 			DfsReaderOptions options) {
 		this.repository = repository;
 		this.packList = new AtomicReference<PackList>(NO_PACKS);
 		this.readerOptions = options;
 	}
 
 	/** @return configured reader options, such as read-ahead. */
 	public DfsReaderOptions getReaderOptions() {
 		return readerOptions;
 	}
 
 	@Override
 	public ObjectReader newReader() {
 		return new DfsReader(this);
 	}
 
 	@Override
 	public ObjectInserter newInserter() {
 		return new DfsInserter(this);
 	}
 
 	/**
 	 * Scan and list all available pack files in the repository.
 	 *
 	 * @return list of available packs. The returned array is shared with the
 	 *         implementation and must not be modified by the caller.
 	 * @throws IOException
 	 *             the pack list cannot be initialized.
 	 */
 	public DfsPackFile[] getPacks() throws IOException {
 		return scanPacks(NO_PACKS).packs;
 	}
 
 	/** @return repository owning this object database. */
 	protected DfsRepository getRepository() {
 		return repository;
 	}
 
 	/**
 	 * List currently known pack files in the repository, without scanning.
 	 *
 	 * @return list of available packs. The returned array is shared with the
 	 *         implementation and must not be modified by the caller.
 	 */
 	public DfsPackFile[] getCurrentPacks() {
 		return packList.get().packs;
 	}
 
 	/**
 	 * Generate a new unique name for a pack file.
 	 *
 	 * @param source
 	 *            where the pack stream is created.
 	 * @return a unique name for the pack file. Must not collide with any other
 	 *         pack file name in the same DFS.
 	 * @throws IOException
 	 *             a new unique pack description cannot be generated.
 	 */
 	protected abstract DfsPackDescription newPack(PackSource source)
 			throws IOException;
 
 	/**
 	 * Commit a pack and index pair that was written to the DFS.
 	 * <p>
 	 * Committing the pack/index pair makes them visible to readers. The JGit
 	 * DFS code always writes the pack, then the index. This allows a simple
 	 * commit process to do nothing if readers always look for both files to
 	 * exist and the DFS performs atomic creation of the file (e.g. stream to a
 	 * temporary file and rename to target on close).
 	 * <p>
 	 * During pack compaction or GC the new pack file may be replacing other
 	 * older files. Implementations should remove those older files (if any) as
 	 * part of the commit of the new file.
 	 * <p>
 	 * This method is a trivial wrapper around
 	 * {@link #commitPackImpl(Collection, Collection)} that calls the
 	 * implementation and fires events.
 	 *
 	 * @param desc
 	 *            description of the new packs.
 	 * @param replaces
 	 *            if not null, list of packs to remove.
 	 * @throws IOException
 	 *             the packs cannot be committed. On failure a rollback must
 	 *             also be attempted by the caller.
 	 */
 	protected void commitPack(Collection<DfsPackDescription> desc,
 			Collection<DfsPackDescription> replaces) throws IOException {
 		commitPackImpl(desc, replaces);
 		getRepository().fireEvent(new DfsPacksChangedEvent());
 	}
 
 	/**
 	 * Implementation of pack commit.
 	 *
 	 * @see #commitPack(Collection, Collection)
 	 *
 	 * @param desc
 	 *            description of the new packs.
 	 * @param replaces
 	 *            if not null, list of packs to remove.
 	 * @throws IOException
 	 *             the packs cannot be committed.
 	 */
 	protected abstract void commitPackImpl(Collection<DfsPackDescription> desc,
 			Collection<DfsPackDescription> replaces) throws IOException;
 
 	/**
 	 * Try to rollback a pack creation.
 	 * <p>
 	 * JGit DFS always writes the pack first, then the index. If the pack does
 	 * not yet exist, then neither does the index. A safe DFS implementation
 	 * would try to remove both files to ensure they are really gone.
 	 * <p>
 	 * A rollback does not support failures, as it only occurs when there is
 	 * already a failure in progress. A DFS implementor may wish to log
 	 * warnings/error messages when a rollback fails, but should not send new
 	 * exceptions up the Java callstack.
 	 *
 	 * @param desc
 	 *            pack to delete.
 	 */
 	protected abstract void rollbackPack(Collection<DfsPackDescription> desc);
 
 	/**
 	 * List the available pack files.
 	 * <p>
 	 * The returned list must support random access and must be mutable by the
 	 * caller. It is sorted in place using the natural sorting of the returned
 	 * DfsPackDescription objects.
 	 *
 	 * @return available packs. May be empty if there are no packs.
 	 * @throws IOException
 	 *             the packs cannot be listed and the object database is not
 	 *             functional to the caller.
 	 */
 	protected abstract List<DfsPackDescription> listPacks() throws IOException;
 
 	/**
 	 * Open a pack, pack index, or other related file for reading.
 	 *
 	 * @param desc
 	 *            description of pack related to the data that will be read.
 	 *            This is an instance previously obtained from
 	 *            {@link #listPacks()}, but not necessarily from the same
 	 *            DfsObjDatabase instance.
 	 * @param ext
 	 *            file extension that will be read i.e "pack" or "idx".
 	 * @return channel to read the file.
 	 * @throws FileNotFoundException
 	 *             the file does not exist.
 	 * @throws IOException
 	 *             the file cannot be opened.
 	 */
 	protected abstract ReadableChannel openFile(
 			DfsPackDescription desc, PackExt ext)
 			throws FileNotFoundException, IOException;
 
 	/**
 	 * Open a pack, pack index, or other related file for writing.
 	 *
 	 * @param desc
 	 *            description of pack related to the data that will be written.
 	 *            This is an instance previously obtained from
 	 *            {@link #newPack(PackSource)}.
 	 * @param ext
 	 *            file extension that will be written i.e "pack" or "idx".
 	 * @return channel to write the file.
 	 * @throws IOException
 	 *             the file cannot be opened.
 	 */
 	protected abstract DfsOutputStream writeFile(
 			DfsPackDescription desc, PackExt ext) throws IOException;
 
 	void addPack(DfsPackFile newPack) throws IOException {
 		PackList o, n;
 		do {
 			o = packList.get();
 			if (o == NO_PACKS) {
 				// The repository may not have needed any existing objects to
 				// complete the current task of creating a pack (e.g. push of a
 				// pack with no external deltas). Because we don't scan for
 				// newly added packs on missed object lookups, scan now to
 				// make sure all older packs are available in the packList.
 				o = scanPacks(o);
 
 				// Its possible the scan identified the pack we were asked to
 				// add, as the pack was already committed via commitPack().
 				// If this is the case return without changing the list.
 				for (DfsPackFile p : o.packs) {
 					if (p == newPack)
 						return;
 				}
 			}
 
 			DfsPackFile[] packs = new DfsPackFile[1 + o.packs.length];
 			packs[0] = newPack;
 			System.arraycopy(o.packs, 0, packs, 1, o.packs.length);
 			n = new PackList(packs);
 		} while (!packList.compareAndSet(o, n));
 	}
 
 	private PackList scanPacks(final PackList original) throws IOException {
 		PackList o, n;
 		synchronized (packList) {
 			do {
 				o = packList.get();
 				if (o != original) {
 					// Another thread did the scan for us, while we
 					// were blocked on the monitor above.
 					//
 					return o;
 				}
 				n = scanPacksImpl(o);
 				if (n == o)
 					return n;
 			} while (!packList.compareAndSet(o, n));
 		}
 		getRepository().fireEvent(new DfsPacksChangedEvent());
 		return n;
 	}
 
 	private PackList scanPacksImpl(PackList old) throws IOException {
 		DfsBlockCache cache = DfsBlockCache.getInstance();
 		Map<DfsPackDescription, DfsPackFile> forReuse = reuseMap(old);
 		List<DfsPackDescription> scanned = listPacks();
 		Collections.sort(scanned);
 
 		List<DfsPackFile> list = new ArrayList<DfsPackFile>(scanned.size());
 		boolean foundNew = false;
 		for (DfsPackDescription dsc : scanned) {
 			DfsPackFile oldPack = forReuse.remove(dsc);
 			if (oldPack != null) {
 				list.add(oldPack);
 			} else {
 				list.add(cache.getOrCreate(dsc, null));
 				foundNew = true;
 			}
 		}
 
 		for (DfsPackFile p : forReuse.values())
 			p.close();
 		if (list.isEmpty())
 			return new PackList(NO_PACKS.packs);
 		if (!foundNew)
 			return old;
 		return new PackList(list.toArray(new DfsPackFile[list.size()]));
 	}
 
 	private static Map<DfsPackDescription, DfsPackFile> reuseMap(PackList old) {
 		Map<DfsPackDescription, DfsPackFile> forReuse
 			= new HashMap<DfsPackDescription, DfsPackFile>();
 		for (DfsPackFile p : old.packs) {
 			if (p.invalid()) {
 				// The pack instance is corrupted, and cannot be safely used
 				// again. Do not include it in our reuse map.
 				//
 				p.close();
 				continue;
 			}
 
 			DfsPackFile prior = forReuse.put(p.getPackDescription(), p);
 			if (prior != null) {
 				// This should never occur. It should be impossible for us
 				// to have two pack files with the same name, as all of them
 				// came out of the same directory. If it does, we promised to
 				// close any PackFiles we did not reuse, so close the second,
 				// readers are likely to be actively using the first.
 				//
 				forReuse.put(prior.getPackDescription(), prior);
 				p.close();
 			}
 		}
 		return forReuse;
 	}
 
 	/** Clears the cached list of packs, forcing them to be scanned again. */
 	protected void clearCache() {
 		packList.set(NO_PACKS);
 	}
 
 	@Override
 	public void close() {
 		// PackList packs = packList.get();
 		packList.set(NO_PACKS);
 
 		// TODO Close packs if they aren't cached.
 		// for (DfsPackFile p : packs.packs)
 		// p.close();
 	}
 
 	private static final class PackList {
 		/** All known packs, sorted. */
 		final DfsPackFile[] packs;
 
 		PackList(final DfsPackFile[] packs) {
 			this.packs = packs;
 		}
 	}
 }