Implement zero-copy for single window objects

Objects that fall completely within a single window can be worked
with in a zero-copy fashion, provided that the window is backed by
a normal byte[] and not by a ByteBuffer.

This works for a surprising number of objects.  The default window
size is 8 KiB, but most deltas are quite a bit smaller than that.
Objects smaller than 1/2 of the window size have a very good chance
of falling completely within a window's array, which means we can
work with them without copying their data around.

Larger objects, or objects which are unlucky enough to span over a
window boundary, get copied through the temporary buffer.  We pay
a tiny penalty to realize we can't use the zero-copy code path,
but its easier than trying to keep track of two adjacent windows.

With this change (as well as everything preceeding it), packing
is actually a bit faster.  Some crude benchmarks based on cloning
linux-2.6.git (~324 MiB, 1,624,785 objects) over localhost using
C git client and JGit daemon shows we get better throughput, and
slightly better times:

  Total Time    | Throughput
  (old)  (now)  | (old)          (now)
  --------------+---------------------------
  2m45s  2m37s  | 12.49 MiB/s    21.17 MiB/s
  2m42s  2m36s  | 16.29 MiB/s    22.63 MiB/s
  2m37s  2m31s  | 16.07 MiB/s    21.92 MiB/s

Change-Id: I48b2c8d37f08d7bf5e76c5a8020cde4a16ae3396
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/ByteArrayWindow.java

@@ -45,6 +45,9 @@
 
 package org.eclipse.jgit.lib;
 
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.zip.CRC32;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 
@@ -80,4 +83,19 @@ final class ByteArrayWindow extends ByteWindow {
 			o += inf.inflate(b, o, b.length - o);
 		return o;
 	}
+
+	void crc32(CRC32 out, long pos, int cnt) {
+		out.update(array, (int) (pos - start), cnt);
+	}
+
+	void write(OutputStream out, long pos, int cnt) throws IOException {
+		out.write(array, (int) (pos - start), cnt);
+	}
+
+	void check(Inflater inf, byte[] tmp, long pos, int cnt)
+			throws DataFormatException {
+		inf.setInput(array, (int) (pos - start), cnt);
+		while (inf.inflate(tmp, 0, tmp.length) > 0)
+			continue;
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/lib/PackFile.java

@@ -338,18 +338,22 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 		final long dataOffset = src.copyOffset + headerCnt;
 		final long dataLength;
 		final long expectedCRC;
+		final ByteArrayWindow quickCopy;
 
 		// Verify the object isn't corrupt before sending. If it is,
 		// we report it missing instead.
 		//
 		try {
 			dataLength = findEndOffset(src.copyOffset) - dataOffset;
+			quickCopy = curs.quickCopy(this, dataOffset, dataLength);
 
 			if (idx().hasCRC32Support()) {
 				// Index has the CRC32 code cached, validate the object.
 				//
 				expectedCRC = idx().findCRC32(src);
-
+				if (quickCopy != null) {
+					quickCopy.crc32(crc1, dataOffset, (int) dataLength);
+				} else {
 					long pos = dataOffset;
 					long cnt = dataLength;
 					while (cnt > 0) {
@@ -359,6 +363,7 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 						pos += n;
 						cnt -= n;
 					}
+				}
 				if (crc1.getValue() != expectedCRC) {
 					setCorrupt(src.copyOffset);
 					throw new CorruptObjectException(MessageFormat.format(
@@ -370,10 +375,13 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 				// now while inflating the raw data to get zlib to tell us
 				// whether or not the data is safe.
 				//
-				long pos = dataOffset;
-				long cnt = dataLength;
 				Inflater inf = curs.inflater();
 				byte[] tmp = new byte[1024];
+				if (quickCopy != null) {
+					quickCopy.check(inf, tmp, dataOffset, (int) dataLength);
+				} else {
+					long pos = dataOffset;
+					long cnt = dataLength;
 					while (cnt > 0) {
 						final int n = (int) Math.min(cnt, buf.length);
 						readFully(pos, buf, 0, n, curs);
@@ -384,7 +392,8 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 						pos += n;
 						cnt -= n;
 					}
-				if (!inf.finished()) {
+				}
+				if (!inf.finished() || inf.getBytesRead() != dataLength) {
 					setCorrupt(src.copyOffset);
 					throw new EOFException(MessageFormat.format(
 							JGitText.get().shortCompressedStreamAt,
@@ -413,7 +422,14 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 			throw gone;
 		}
 
-		if (dataLength <= buf.length) {
+		if (quickCopy != null) {
+			// The entire object fits into a single byte array window slice,
+			// and we have it pinned.  Write this out without copying.
+			//
+			out.writeHeader(src, inflatedLength);
+			quickCopy.write(out, dataOffset, (int) dataLength);
+
+		} else if (dataLength <= buf.length) {
 			// Tiny optimization: Lots of objects are very small deltas or
 			// deflated commits that are likely to fit in the copy buffer.
 			//

org.eclipse.jgit/src/org/eclipse/jgit/lib/WindowCursor.java

@@ -166,6 +166,15 @@ final class WindowCursor extends ObjectReader implements ObjectReuseAsIs {
 		}
 	}
 
+	ByteArrayWindow quickCopy(PackFile p, long pos, long cnt)
+			throws IOException {
+		pin(p, pos);
+		if (window instanceof ByteArrayWindow
+				&& window.contains(p, pos + (cnt - 1)))
+			return (ByteArrayWindow) window;
+		return null;
+	}
+
 	Inflater inflater() {
 		prepareInflater();
 		return inf;