Lazily open ReadableChannel in BlockBasedFile.getOrLoadBlock

To avoid opening the readable channel in case of DfsBlockCache
hits. Also cleaning up typos around DfsBlockCache.

Change-Id: I615e349cb4838387c1e6743cdc384d1b81b54369
Signed-off-by: Minh Thai <mthai@google.com>

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

@@ -129,8 +129,8 @@ else if (size < cache.getBlockSize())
 	}
 
 	DfsBlock getOrLoadBlock(long pos, DfsReader ctx) throws IOException {
-		try (ReadableChannel rc = ctx.db.openFile(desc, ext)) {
-			return cache.getOrLoad(this, pos, ctx, () -> rc);
+		try (LazyChannel c = new LazyChannel(ctx, desc, ext)) {
+			return cache.getOrLoad(this, pos, ctx, c);
 		}
 	}
 
@@ -203,4 +203,41 @@ static int read(ReadableChannel rc, ByteBuffer buf) throws IOException {
 	static long elapsedMicros(long start) {
 		return (System.nanoTime() - start) / 1000L;
 	}
+
+	/**
+	 * A supplier of readable channel that opens the channel lazily.
+	 */
+	private static class LazyChannel
+			implements AutoCloseable, DfsBlockCache.ReadableChannelSupplier {
+		private final DfsReader ctx;
+		private final DfsPackDescription desc;
+		private final PackExt ext;
+
+		private ReadableChannel rc = null;
+
+		LazyChannel(DfsReader ctx, DfsPackDescription desc, PackExt ext) {
+			this.ctx = ctx;
+			this.desc = desc;
+			this.ext = ext;
+		}
+
+		@Override
+		public ReadableChannel get() throws IOException {
+			if (rc == null) {
+				synchronized (this) {
+					if (rc == null) {
+						rc = ctx.db.openFile(desc, ext);
+					}
+				}
+			}
+			return rc;
+		}
+
+		@Override
+		public void close() throws IOException {
+			if (rc != null) {
+				rc.close();
+			}
+		}
+	}
 }

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

@@ -754,7 +754,7 @@ interface RefLoader<T> {
 	 * Supplier for readable channel
 	 */
 	@FunctionalInterface
-	public interface ReadableChannelSupplier {
+	interface ReadableChannelSupplier {
 		/**
 		 * @return ReadableChannel
 		 * @throws IOException

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

@@ -212,7 +212,7 @@ public Consumer<Long> getRefLockWaitTimeConsumer() {
 	 *            consumer of wait time in milliseconds.
 	 * @return {@code this}
 	 */
-	public DfsBlockCacheConfig setReflockWaitTimeConsumer(Consumer<Long> c) {
+	public DfsBlockCacheConfig setRefLockWaitTimeConsumer(Consumer<Long> c) {
 		refLock = c;
 		return this;
 	}

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

@@ -128,9 +128,7 @@ public ByteBuffer read(long pos, int cnt) throws IOException {
 				open().setReadAheadBytes(readAhead);
 			}
 
-			DfsBlock block = cache.getOrLoad(file, pos, ctx, () -> {
-				return open();
-			});
+			DfsBlock block = cache.getOrLoad(file, pos, ctx, () -> open());
 			if (block.start == pos && block.size() >= cnt) {
 				return block.zeroCopyByteBuffer(cnt);
 			}