Fix corrupted S3 downloads when file object in append mode

s3transfer/compat.py

@@ -54,6 +54,20 @@ def accepts_kwargs(func):
     MAXINT = sys.maxint
 
 
+def writes_are_seekable(fileobj):
+    """Backwards compat function to determine if writes to a fileobj are seekable
+
+    :param fileobj: The file-like object to determine if writes are seekable
+
+    :returns: True, if in append mode. False, otherwise.
+    """
+    APPEND_ONLY_MODE = 'a'
+    is_seekable = seekable(fileobj)
+    if hasattr(fileobj, 'mode') and APPEND_ONLY_MODE in fileobj.mode:
+        is_seekable = False
+    return is_seekable
+
+
 def seekable(fileobj):
     """Backwards compat function to determine if a fileobj is seekable
 

s3transfer/download.py

@@ -24,7 +24,7 @@
     ReadTimeoutError
 
 from s3transfer.compat import SOCKET_ERROR
-from s3transfer.compat import seekable
+from s3transfer.compat import seekable, writes_are_seekable
 from s3transfer.exceptions import RetriesExceededError
 from s3transfer.futures import IN_MEMORY_DOWNLOAD_TAG
 from s3transfer.utils import random_file_extension
@@ -202,7 +202,7 @@ def _get_temp_fileobj(self):
 class DownloadSeekableOutputManager(DownloadOutputManager):
     @classmethod
     def is_compatible(cls, download_target, osutil):
-        return seekable(download_target)
+        return writes_are_seekable(download_target)
 
     def get_fileobj_for_io_writes(self, transfer_future):
         # Return the fileobj provided to the future.

tests/unit/test_compat.py

@@ -17,7 +17,7 @@
 from botocore.compat import six
 
 from tests import unittest
-from s3transfer.compat import seekable, readable
+from s3transfer.compat import seekable, readable, writes_are_seekable
 
 
 class ErrorRaisingSeekWrapper(object):
@@ -49,6 +49,12 @@ def test_seekable_fileobj(self):
         with open(self.filename, 'w') as f:
             self.assertTrue(seekable(f))
 
+    def test_non_seekable_append_fileobj(self):
+        with open(self.filename, 'a') as f:
+            self.assertFalse(writes_are_seekable(f))
+        with open(self.filename, 'a+') as f:
+            self.assertFalse(writes_are_seekable(f))
+
     def test_non_file_like_obj(self):
         # Fails becase there is no seekable(), seek(), nor tell()
         self.assertFalse(seekable(object()))