Refactor reading and writing heads in Repository

Add private methods which are used for reading and writing MERGE_HEAD
and CHERRY_PICK_HEAD files, as suggested in the comments on change
I947967fdc2f1d55016c95106b104c2afcc9797a1.

Change-Id: If4617a05ee57054b8b1fcba36a06a641340ecc0e
Signed-off-by: Robin Stocker <robin@nibor.org>

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

@@ -1158,15 +1158,8 @@ public abstract class Repository {
 		if (isBare() || getDirectory() == null)
 			throw new NoWorkTreeException();
 
-		File mergeHeadFile = new File(getDirectory(), Constants.MERGE_HEAD);
-		byte[] raw;
-		try {
-			raw = IO.readFully(mergeHeadFile);
-		} catch (FileNotFoundException notFound) {
-			return null;
-		}
-
-		if (raw.length == 0)
+		byte[] raw = readGitDirectoryFile(Constants.MERGE_HEAD);
+		if (raw == null)
 			return null;
 
 		LinkedList<ObjectId> heads = new LinkedList<ObjectId>();
@@ -1190,21 +1183,7 @@ public abstract class Repository {
 	 * @throws IOException
 	 */
 	public void writeMergeHeads(List<ObjectId> heads) throws IOException {
-		File mergeHeadFile = new File(gitDir, Constants.MERGE_HEAD);
-		if (heads != null) {
-			BufferedOutputStream bos = new BufferedOutputStream(
-					new FileOutputStream(mergeHeadFile));
-			try {
-				for (ObjectId id : heads) {
-					id.copyTo(bos);
-					bos.write('\n');
-				}
-			} finally {
-				bos.close();
-			}
-		} else {
-			FileUtils.delete(mergeHeadFile);
-		}
+		writeHeadsFile(heads, Constants.MERGE_HEAD);
 	}
 
 	/**
@@ -1223,16 +1202,8 @@ public abstract class Repository {
 		if (isBare() || getDirectory() == null)
 			throw new NoWorkTreeException();
 
-		File mergeHeadFile = new File(getDirectory(),
-				Constants.CHERRY_PICK_HEAD);
-		byte[] raw;
-		try {
-			raw = IO.readFully(mergeHeadFile);
-		} catch (FileNotFoundException notFound) {
-			return null;
-		}
-
-		if (raw.length == 0)
+		byte[] raw = readGitDirectoryFile(Constants.CHERRY_PICK_HEAD);
+		if (raw == null)
 			return null;
 
 		return ObjectId.fromString(raw, 0);
@@ -1248,18 +1219,54 @@ public abstract class Repository {
 	 * @throws IOException
 	 */
 	public void writeCherryPickHead(ObjectId head) throws IOException {
-		File cherryPickHeadFile = new File(gitDir, Constants.CHERRY_PICK_HEAD);
-		if (head != null) {
+		List<ObjectId> heads = (head != null) ? Collections.singletonList(head)
+				: null;
+		writeHeadsFile(heads, Constants.CHERRY_PICK_HEAD);
+	}
+
+	/**
+	 * Read a file from the git directory.
+	 *
+	 * @param filename
+	 * @return the raw contents or null if the file doesn't exist or is empty
+	 * @throws IOException
+	 */
+	private byte[] readGitDirectoryFile(String filename) throws IOException {
+		File file = new File(getDirectory(), filename);
+		try {
+			byte[] raw = IO.readFully(file);
+			return raw.length > 0 ? raw : null;
+		} catch (FileNotFoundException notFound) {
+			return null;
+		}
+	}
+
+	/**
+	 * Write the given heads to a file in the git directory.
+	 *
+	 * @param heads
+	 *            a list of object ids to write or null if the file should be
+	 *            deleted.
+	 * @param filename
+	 * @throws FileNotFoundException
+	 * @throws IOException
+	 */
+	private void writeHeadsFile(List<ObjectId> heads, String filename)
+			throws FileNotFoundException, IOException {
+		File headsFile = new File(getDirectory(), filename);
+		if (heads != null) {
 			BufferedOutputStream bos = new BufferedOutputStream(
-					new FileOutputStream(cherryPickHeadFile));
+					new FileOutputStream(headsFile));
 			try {
-				head.copyTo(bos);
-				bos.write('\n');
+				for (ObjectId id : heads) {
+					id.copyTo(bos);
+					bos.write('\n');
+				}
 			} finally {
 				bos.close();
 			}
 		} else {
-			FileUtils.delete(cherryPickHeadFile, FileUtils.SKIP_MISSING);
+			FileUtils.delete(headsFile, FileUtils.SKIP_MISSING);
 		}
 	}
 }