fix: InputStream.read returns 0 for a zero length read

core/src/main/java/io/roastedroot/zerofs/ZeroFsInputStream.java

@@ -66,6 +66,9 @@ public int read(byte[] b, int off, int len) throws IOException {
 
     private synchronized int readInternal(byte[] b, int off, int len) throws IOException {
         checkNotClosed();
+        if (len == 0) {
+            return 0;
+        }
         if (finished) {
             return -1;
         }

core/src/test/java/io/roastedroot/zerofs/ZeroFsInputStreamTest.java

@@ -106,6 +106,25 @@ public void testRead_partialArray_invalidInput() throws IOException {
         }
     }
 
+    @Test
+    public void testRead_zeroLength_returnsZero() throws IOException {
+        ZeroFsInputStream in = newInputStream(1, 2, 3);
+        byte[] bytes = new byte[10];
+        assertEquals(0, in.read(bytes, 0, 0));
+        assertArrayEquals(new byte[10], bytes); // nothing should be read
+        assertEquals(1, in.read()); // next read should still be the first byte
+    }
+
+    @Test
+    public void testRead_zeroLengthAtEof_returnsZero() throws IOException {
+        ZeroFsInputStream in = newInputStream(1, 2, 3);
+        assertEquals(3, in.read(new byte[3]));
+        assertEmpty(in); // we're at the end of the stream
+
+        // now try reading 0 bytes at the end of the stream
+        assertEquals(0, in.read(new byte[10], 0, 0));
+    }
+
     @Test
     public void testAvailable() throws IOException {
         ZeroFsInputStream in = newInputStream(1, 2, 3, 4, 5, 6, 7, 8);

core/src/test/java/io/roastedroot/zerofs/ZeroFsUnixLikeFileSystemTest.java

@@ -1077,7 +1077,7 @@ public void testDelete_file_whenOpenReferencesRemain() throws IOException {
         byte[] allBytes = new byte[105];
         int off = 0;
         int read;
-        while ((read = in.read(allBytes, off, allBytes.length - off)) != -1) {
+        while ((read = in.read(allBytes, off, allBytes.length - off)) > 0) {
             off += read;
         }
         assertArrayEquals(concat(bytes, moreBytes), allBytes);