jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsObjDatabase.java

/*
 * 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
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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.AnyObjectId;
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),

		/**
		 * RefTreeGraph pack was created by Git garbage collection.
		 *
		 * @see DfsGarbageCollector
		 */
		GC_TXN(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;
	}

	/**
	 * Does the requested object exist in this database?
	 * <p>
	 * This differs from ObjectDatabase's implementation in that we can selectively
	 * ignore unreachable (garbage) objects.
	 *
	 * @param objectId
	 *            identity of the object to test for existence of.
	 * @param avoidUnreachableObjects
	 *            if true, ignore objects that are unreachable.
	 * @return true if the specified object is stored in this database.
	 * @throws IOException
	 *             the object store cannot be accessed.
	 */
	public boolean has(AnyObjectId objectId, boolean avoidUnreachableObjects)
			throws IOException {
		try (ObjectReader or = newReader()) {
			or.setAvoidUnreachableObjects(avoidUnreachableObjects);
			return or.has(objectId);
		}
	}

	/**
	 * 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;
		}
	}
}