Support reading first SHA-1 from large FETCH_HEAD files

When reading refs, avoid reading huge files that were put there
accidentally, but still read the top of e.g. FETCH_HEAD, which
may be longer than our limit. We're only interested in the first line
anyway.

Bug: 340880
Change-Id: I11029b9b443f22019bf80bd3dd942b48b531bc11
Signed-off-by: Carsten Pfieffer <carsten.pfeiffer@gebit.de>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/RefDirectory.java

@@ -850,9 +850,10 @@ private LooseRef scanRef(LooseRef ref, String name) throws IOException {
 		} else if (modified == 0)
 			return null;
 
+		final int limit = 4096;
 		final byte[] buf;
 		try {
-			buf = IO.readFully(path, 4096);
+			buf = IO.readSome(path, limit);
 		} catch (FileNotFoundException noFile) {
 			return null; // doesn't exist; not a reference.
 		}
@@ -862,6 +863,9 @@ private LooseRef scanRef(LooseRef ref, String name) throws IOException {
 			return null; // empty file; not a reference.
 
 		if (isSymRef(buf, n)) {
+			if (n == limit)
+				return null; // possibly truncated ref
+
 			// trim trailing whitespace
 			while (0 < n && Character.isWhitespace(buf[n - 1]))
 				n--;

org.eclipse.jgit/src/org/eclipse/jgit/util/IO.java

@@ -77,6 +77,47 @@ public static final byte[] readFully(final File path)
 		return IO.readFully(path, Integer.MAX_VALUE);
 	}
 
+	/**
+	 * Read at most limit bytes from the local file into memory as a byte array.
+	 *
+	 * @param path
+	 *            location of the file to read.
+	 * @param limit
+	 *            maximum number of bytes to read, if the file is larger than
+	 *            only the first limit number of bytes are returned
+	 * @return complete contents of the requested local file. If the contents
+	 *         exceeds the limit, then only the limit is returned.
+	 * @throws FileNotFoundException
+	 *             the file does not exist.
+	 * @throws IOException
+	 *             the file exists, but its contents cannot be read.
+	 */
+	public static final byte[] readSome(final File path, final int limit)
+			throws FileNotFoundException, IOException {
+		FileInputStream in = new FileInputStream(path);
+		try {
+			byte[] buf = new byte[limit];
+			int cnt = 0;
+			for (;;) {
+				int n = in.read(buf, cnt, buf.length - cnt);
+				if (n <= 0)
+					break;
+				cnt += n;
+			}
+			if (cnt == buf.length)
+				return buf;
+			byte[] res = new byte[cnt];
+			System.arraycopy(buf, 0, res, 0, cnt);
+			return res;
+		} finally {
+			try {
+				in.close();
+			} catch (IOException ignored) {
+				// do nothing
+			}
+		}
+	}
+
 	/**
 	 * Read an entire local file into memory as a byte array.
 	 *