read full file when doing diff

but avoid reading file at all or full file when file is too big for diffing

lib/ansible/plugins/action/__init__.py

@@ -594,17 +594,18 @@ class ActionBase(with_metaclass(ABCMeta, object)):
                     diff['before'] = dest_contents
 
             if source_file:
+                st = os.stat(source)
+                if st[stat.ST_SIZE] > C.MAX_FILE_SIZE_FOR_DIFF:
+                    diff['src_larger'] = C.MAX_FILE_SIZE_FOR_DIFF
+                else:
                     display.debug("Reading local copy of the file %s" % source)
                     try:
                         src = open(source)
-                    src_contents = src.read(8192)
-                    st = os.stat(source)
+                        src_contents = src.read()
                     except Exception as e:
                         raise AnsibleError("Unexpected error while reading source (%s) for diff: %s " % (source, str(e)))
                     if "\x00" in src_contents:
                         diff['src_binary'] = 1
-                elif st[stat.ST_SIZE] > C.MAX_FILE_SIZE_FOR_DIFF:
-                    diff['src_larger'] = C.MAX_FILE_SIZE_FOR_DIFF
                     else:
                         diff['after_header'] = source
                         diff['after'] = src_contents