avoid shredding empty files, also x/0

also cleaned up unused import and exception var

lib/ansible/parsing/vault/__init__.py

@@ -71,7 +71,7 @@ try:
 except ImportError:
     pass
 
-from ansible.compat.six import PY3, byte2int
+from ansible.compat.six import PY3
 from ansible.utils.unicode import to_unicode, to_bytes
 
 HAS_ANY_PBKDF2HMAC = HAS_PBKDF2 or HAS_PBKDF2HMAC
@@ -236,22 +236,24 @@ class VaultEditor:
         """
 
         file_len = os.path.getsize(tmp_path)
-        max_chunk_len = min(1024*1024*2, file_len)
 
-        passes = 3
-        with open(tmp_path,  "wb") as fh:
-            for _ in range(passes):
-                fh.seek(0,  0)
-                # get a random chunk of data, each pass with other length
-                chunk_len = random.randint(max_chunk_len//2, max_chunk_len)
-                data = os.urandom(chunk_len)
+        if file_len > 0: # avoid work when file was empty
+            max_chunk_len = min(1024*1024*2, file_len)
 
-                for _ in range(0, file_len // chunk_len):
-                    fh.write(data)
-                fh.write(data[:file_len % chunk_len])
+            passes = 3
+            with open(tmp_path,  "wb") as fh:
+                for _ in range(passes):
+                    fh.seek(0,  0)
+                    # get a random chunk of data, each pass with other length
+                    chunk_len = random.randint(max_chunk_len//2, max_chunk_len)
+                    data = os.urandom(chunk_len)
 
-                assert(fh.tell() == file_len) # FIXME remove this assert once we have unittests to check its accuracy
-                os.fsync(fh)
+                    for _ in range(0, file_len // chunk_len):
+                        fh.write(data)
+                    fh.write(data[:file_len % chunk_len])
+
+                    assert(fh.tell() == file_len) # FIXME remove this assert once we have unittests to check its accuracy
+                    os.fsync(fh)
 
 
     def _shred_file(self, tmp_path):
@@ -273,7 +275,7 @@ class VaultEditor:
 
         try:
             r = call(['shred', tmp_path])
-        except OSError as e:
+        except OSError:
             # shred is not available on this system, or some other error occured.
             r = 1