Fix UnionInputStream.read to be more Java-like

Relax the read() method to not block until exactly "len" bytes have
been read. Instead, return when one or more bytes have been read, up
to "len", so UnionInputStream more closely resembles InputStream's
read() method.

Change-Id: I3f632be8eb85a4a0baf27c9f067c8d817162de2b

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java

@@ -50,6 +50,7 @@
 
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.util.Arrays;
 
 import org.junit.Test;
@@ -238,4 +239,26 @@ public void close() throws IOException {
 			assertEquals("I AM A TEST", e.getMessage());
 		}
 	}
+
+	@Test
+	public void testNonBlockingPartialRead() throws Exception {
+		InputStream errorReadStream = new InputStream() {
+			@Override
+			public int read() throws IOException {
+				throw new IOException("Expected");
+			}
+		};
+		final UnionInputStream u = new UnionInputStream(
+				new ByteArrayInputStream(new byte[]{1,2,3}),
+				errorReadStream);
+		byte buf[] = new byte[10];
+		assertEquals(3, u.read(buf, 0, 10));
+		assertTrue(Arrays.equals(new byte[] {1,2,3}, slice(buf, 3)));
+		try {
+			u.read(buf, 0, 1);
+			fail("Expected exception from errorReadStream");
+		} catch (IOException e) {
+			assertEquals("Expected", e.getMessage());
+		}
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/util/io/UnionInputStream.java

@@ -138,23 +138,18 @@ else if (in == EOF)
 
 	@Override
 	public int read(byte[] b, int off, int len) throws IOException {
-		int cnt = 0;
-		while (0 < len) {
+		if (len == 0)
+			return 0;
+		for (;;) {
 			final InputStream in = head();
 			final int n = in.read(b, off, len);
-			if (0 < n) {
-				cnt += n;
-				off += n;
-				len -= n;
-			} else if (in == EOF)
-				return 0 < cnt ? cnt : -1;
-			else {
+			if (0 < n)
+				return n;
+			else if (in == EOF)
+				return -1;
+			else
 				pop();
-				if (0 < cnt)
-					break;
-			}
 		}
-		return cnt;
 	}
 
 	@Override