The last chunk can be accepted asynchronously

The server is allowed to respond with a 202 for the last chunk, to indicate it's processing that
last chunk asynchronously.  This fleshes out the process for checking for completed process of the
file in that case, and improves the file status key definitions.

Author: warsaw

peps/pep-0694.rst

@@ -298,6 +298,7 @@ the following keys:
     wishes to communicate to the end user.  These notices are specific to the overall session, not
     to any particular file in the session.
 
+
 .. _session-links:
 
 Session Links
@@ -330,8 +331,15 @@ The ``files`` key contains a mapping from the names of the files uploaded in thi
 sub-mapping with the following keys:
 
 ``status``
-    A string with the same values and semantics as the :ref:`session status key <session-status>`,
-    except that it indicates the status of the specific referenced file.
+    A string with valid values ``partial``, ``pending``, ``complete``, and ``error``.  If a file
+    upload has not seen an ``Upload-Complete: ?1`` header, then ``partial`` will be returned.  If
+    ``Upload-Complete: ?1`` resulted in a ``202 Accepted``, then ``pending`` will be returned until
+    asynchronous processing of the last chunk and the full file has been completed.  If a ``201
+    Created`` was returned, or the last chunk processing is finished, ``complete`` will be returned.
+    If there was an error during upload, then clients should not assume the file is in any usable
+    state, ``error`` will be returned and it's best to :ref:`cancel or delete <cancel-an-upload>`
+    the file and start over.  This action would remove the file name from the ``files`` key of the
+    :ref:`session status response body <session-response>`.
 
 ``link``
     The *absolute* URL that the client should use to reference this specific file.  This URL is used
@@ -523,10 +531,13 @@ For the second chunk representing bytes 1000 through 1999, include the following
 These requests would continue sequentially until the last chunk is ready to be uploaded.
 
 For each successful chunk, the server **MUST** respond with a ``202 Accepted`` header, except for
-the final chunk, which **MUST** be a ``201 Created``, and as with non-chunked uploads, the body of
-these responses has no content.
+the final chunk, which **MUST** be either:
 
-.. _complete-the-upload:
+* ``201 Created`` if the server accepts and processes the last chunk synchronously, completing the
+  file upload.
+* ``202 Accepted`` if the server accepts the last chunk, but must process it asynchronously.  In
+  this case, the client should query the :ref:`session status <session-response>` periodically until
+  the uploaded :ref:`file status <session-file>` transitions to ``complete``.
 
 The final chunk of data **MUST** include the ``Upload-Complete: ?1`` header, since at that point the
 entire file has been uploaded.