Take back ownership of files from aiohttp for retrying requests.

Fix #1809

discord/file.py

@@ -25,6 +25,7 @@ DEALINGS IN THE SOFTWARE.
 """
 
 import os.path
+import io
 
 class File:
     """A parameter object used for :meth:`abc.Messageable.send`
@@ -52,11 +53,28 @@ class File:
         Whether the attachment is a spoiler.
     """
 
-    __slots__ = ('fp', 'filename', '_true_fp')
+    __slots__ = ('fp', 'filename', '_original_pos', '_owner', '_closer')
 
     def __init__(self, fp, filename=None, *, spoiler=False):
         self.fp = fp
-        self._true_fp = None
+
+        if isinstance(fp, io.IOBase):
+            if not (fp.seekable() and fp.readable()):
+                raise ValueError('File buffer {!r} must be seekable and readable'.format(fp))
+            self.fp = fp
+            self._original_pos = fp.tell()
+            self._owner = False
+        else:
+            self.fp = open(fp, 'rb')
+            self._original_pos = 0
+            self._owner = True
+
+        # aiohttp only uses two methods from IOBase
+        # read and close, since I want to control when the files
+        # close, I need to stub it so it doesn't close unless
+        # I tell it to
+        self._closer = self.fp.close
+        self.fp.close = lambda: None
 
         if filename is None:
             if isinstance(fp, str):
@@ -66,15 +84,22 @@ class File:
         else:
             self.filename = filename
 
-        if spoiler and not self.filename.startswith('SPOILER_'):
+        if spoiler and self.filename is not None and not self.filename.startswith('SPOILER_'):
             self.filename = 'SPOILER_' + self.filename
 
-    def open_file(self):
-        fp = self.fp
-        if isinstance(fp, str):
-            self._true_fp = fp = open(fp, 'rb')
-        return fp
+    def reset(self, *, seek=True):
+        # The `seek` parameter is needed because
+        # the retry-loop is iterated over multiple times
+        # starting from 0, as an implementation quirk
+        # the resetting must be done at the beginning
+        # before a request is done, since the first index
+        # is 0, and thus false, then this prevents an
+        # unnecessary seek since it's the first request
+        # done.
+        if seek:
+            self.fp.seek(self._original_pos)
 
     def close(self):
-        if self._true_fp:
-            self._true_fp.close()
+        self.fp.close = self._closer
+        if self._owner:
+            self._closer()