Null out the data field when skipping past the end of a GridFS file and modify the GridFSTest to ensure skipping from the beginning of a GridFSFile works

Author: Daniel Gottlieb

src/main/com/mongodb/gridfs/GridFSDBFile.java

@@ -173,6 +173,7 @@ public long skip(long numBytesToSkip) throws IOException {
                 offsetInFile = _currentChunkIdx * _chunkSize + _offset;
             if (numBytesToSkip + offsetInFile >= _length) {
                 _currentChunkIdx = _numChunks;
+                _data = null;
                 return _length - offsetInFile;
             }
 
@@ -186,8 +187,6 @@ public long skip(long numBytesToSkip) throws IOException {
         }
 
         final int _numChunks;
-        //Math trick to ensure the _lastChunkSize is between 1 and _chunkSize
-        //final long _lastChunkSize = ((_length - 1) % _chunkSize) + 1;
 
         int _currentChunkIdx = -1;
         int _offset = 0;

src/test/com/mongodb/gridfs/GridFSTest.java

@@ -222,12 +222,9 @@ public void testInputStreamSkipping() throws Exception {
 
         inputStream = (GridFSDBFile.MyInputStream)savedFile.getInputStream();
 
-        int position = 0;
-        assertEquals((byte)(position++ % 251), (byte)inputStream.read());
-
         long skipped = inputStream.skip(1);
         assertEquals(1, skipped);
-        position += 1;
+        int position = 1;
         assertEquals((byte)(position++ % 251), (byte)inputStream.read());
 
         skipped = inputStream.skip(chunkSize);