Directly use the FileChannel from ChannelLock for WRITE LockedFileChannel

This fixes behaviour on Windows, where the we cannot open another
FileChannel on the write-locked path. We have to use the channel that we
locked.

Author: tpietzsch

src/main/java/org/janelia/saalfeldlab/n5/ChannelLock.java

@@ -25,10 +25,19 @@ private ChannelLock(final FileChannel channel, final FileLock lock) {
 	}
 
 	public void close() throws IOException {
-		lock.release();
+
+		// NB: We do not call lock.release() here, because it may throw an
+		// exception if the channel is already closed. Instead, we just close
+		// the channel. This will automatically release the lock. (And it is ok
+		// to close an already closed channel.)
+
 		channel.close();
 	}
 
+	FileChannel getChannel() {
+		return channel;
+	}
+
 	/**
 	 * Create a {@link FileChannel} on the given {@code path} and lock it with a
 	 * system-level {@link FileLock}. If there is an existing overlapping file

src/main/java/org/janelia/saalfeldlab/n5/KeyLockState.java

@@ -177,23 +177,8 @@ LockedFileChannel acquireWrite() throws IOException {
 			}
 
 			// We have a WRITE ChannelLock.
-			// Try to open a FileChannel.
-			final FileChannel channel;
-			try {
-				channel = FileChannel.open(path, StandardOpenOption.READ, StandardOpenOption.WRITE, StandardOpenOption.CREATE);
-			} catch (IOException e) {
-				// Something went wrong. Back off.
-				try {
-					channelLock.close();
-				} finally {
-					channelLockMutex.release();
-				}
-				throw e;
-			}
-
-			// We have a FileChannel.
 			// Create a LockedFileChannel that will releaseWrite() when it is closed.
-			return new LockedFileChannel(channel, this::releaseWrite);
+			return new LockedFileChannel(channelLock.getChannel(), this::releaseWrite);
 		} catch (InterruptedException e) {
 			throw new IOException(e);
 		}
@@ -214,22 +199,8 @@ LockedFileChannel tryAcquireWrite() {
 		}
 
 		// We have a WRITE ChannelLock.
-		// Try to open a FileChannel.
-		final FileChannel channel;
-		try {
-			channel = FileChannel.open(path, StandardOpenOption.READ, StandardOpenOption.WRITE, StandardOpenOption.CREATE);
-		} catch (IOException e) {
-			// Something went wrong. Back off.
-			try {
-				releaseChannelLock();
-			} catch (IOException ignored) {
-			}
-			return null;
-		}
-
-		// We have a FileChannel.
 		// Create a LockedFileChannel that will releaseWrite() when it is closed.
-		return new LockedFileChannel(channel, this::releaseWrite);
+		return new LockedFileChannel(channelLock.getChannel(), this::releaseWrite);
 	}
 
 	void releaseWrite() throws IOException {