2 files changed, 87 insertions, 3 deletions
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/IOTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/IOTest.java
new file mode 100644
index 0000000000..10a858fbfb
--- /dev/null
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/IOTest.java
@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2021, Thomas Wolf <thomas.wolf@paranor.ch> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.util;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+
+import org.junit.Test;
+
+public class IOTest {
+
+	private static final byte[] DATA = "abcdefghijklmnopqrstuvwxyz"
+			.getBytes(StandardCharsets.US_ASCII);
+
+	private byte[] initBuffer(int size) {
+		byte[] buffer = new byte[size];
+		for (int i = 0; i < size; i++) {
+			buffer[i] = (byte) ('0' + (i % 10));
+		}
+		return buffer;
+	}
+
+	private int read(byte[] buffer, int from) throws IOException {
+		try (InputStream in = new ByteArrayInputStream(DATA)) {
+			return IO.readFully(in, buffer, from);
+		}
+	}
+
+	@Test
+	public void readFullyBufferShorter() throws Exception {
+		byte[] buffer = initBuffer(9);
+		int length = read(buffer, 0);
+		assertEquals(buffer.length, length);
+		assertArrayEquals(buffer, Arrays.copyOfRange(DATA, 0, length));
+	}
+
+	@Test
+	public void readFullyBufferLonger() throws Exception {
+		byte[] buffer = initBuffer(50);
+		byte[] initial = Arrays.copyOf(buffer, buffer.length);
+		int length = read(buffer, 0);
+		assertEquals(DATA.length, length);
+		assertArrayEquals(Arrays.copyOfRange(buffer, 0, length), DATA);
+		assertArrayEquals(Arrays.copyOfRange(buffer, length, buffer.length),
+				Arrays.copyOfRange(initial, length, initial.length));
+	}
+
+	@Test
+	public void readFullyBufferShorterOffset() throws Exception {
+		byte[] buffer = initBuffer(9);
+		byte[] initial = Arrays.copyOf(buffer, buffer.length);
+		int length = read(buffer, 6);
+		assertEquals(3, length);
+		assertArrayEquals(Arrays.copyOfRange(buffer, 0, 6),
+				Arrays.copyOfRange(initial, 0, 6));
+		assertArrayEquals(Arrays.copyOfRange(buffer, 6, buffer.length),
+				Arrays.copyOfRange(DATA, 0, 3));
+	}
+
+	@Test
+	public void readFullyBufferLongerOffset() throws Exception {
+		byte[] buffer = initBuffer(50);
+		byte[] initial = Arrays.copyOf(buffer, buffer.length);
+		int length = read(buffer, 40);
+		assertEquals(10, length);
+		assertArrayEquals(Arrays.copyOfRange(buffer, 0, 40),
+				Arrays.copyOfRange(initial, 0, 40));
+		assertArrayEquals(Arrays.copyOfRange(buffer, 40, buffer.length),
+				Arrays.copyOfRange(DATA, 0, 10));
+	}
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/IO.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/IO.java
index 680d90392d..6d5694e435 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/IO.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/IO.java
@@ -246,7 +246,7 @@ public class IO {
 	 *            buffer that must be fully populated, [off, off+len).
 	 * @param off
 	 *            position within the buffer to start writing to.
-	 * @return number of bytes in buffer or stream, whichever is shortest
+	 * @return number of bytes read
 	 * @throws java.io.IOException
 	 *             there was an error reading from the stream.
 	 */
@@ -254,8 +254,8 @@ public class IO {
 			throws IOException {
 		int r;
 		int len = 0;
-		while ((r = fd.read(dst, off, dst.length - off)) >= 0
-				&& len < dst.length) {
+		while (off < dst.length
+				&& (r = fd.read(dst, off, dst.length - off)) >= 0) {
 			off += r;
 			len += r;
 		}