Fix read_file_backwards.

We weren't actually reading backwards because we always did "seek from start". Now we do some "seek from current" with a negative offset, and a "seek from end".

Author: thejpster

tests/read_file.rs

@@ -150,27 +150,31 @@ fn read_file_backwards() {
 
     const CHUNK_SIZE: u32 = 100;
     let length = volume_mgr.file_length(test_file).expect("file length");
-    let mut offset = length - CHUNK_SIZE;
     let mut read = 0;
 
+    // go to end
+    volume_mgr.file_seek_from_end(test_file, 0).expect("seek");
+
     // We're going to read the file backwards in chunks of 100 bytes. This
     // checks we didn't make any assumptions about only going forwards.
     while read < length {
+        // go to start of next chunk
         volume_mgr
-            .file_seek_from_start(test_file, offset)
+            .file_seek_from_current(test_file, -(CHUNK_SIZE as i32))
             .expect("seek");
+        // read chunk
         let mut buffer = [0u8; CHUNK_SIZE as usize];
         let len = volume_mgr.read(test_file, &mut buffer).expect("read");
         assert_eq!(len, CHUNK_SIZE as usize);
         contents.push_front(buffer.to_vec());
         read += CHUNK_SIZE;
-        if offset >= CHUNK_SIZE {
-            offset -= CHUNK_SIZE;
-        }
+        // go to start of chunk we just read
+        volume_mgr
+            .file_seek_from_current(test_file, -(CHUNK_SIZE as i32))
+            .expect("seek");
     }
 
     assert_eq!(read, length);
-    assert_eq!(offset, 0);
 
     let flat: Vec<u8> = contents.iter().flatten().copied().collect();