Integrity Error on POST request raises Traceback

[massimiliano-della-rovere]

Scenario

The model User has the "unique=True" option set in the "email" and "username" fields.
A row with a duplicate value in any of the two fields.

What happens

The following Traceback:

Traceback (most recent call last):
  File "/home/mader/.virtualenvs/tcwa_backend/lib/python3.4/site-packages/tornado/web.py", line 1346, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "/home/mader/.virtualenvs/tcwa_backend/lib/python3.4/site-packages/tornado_restless/handler.py", line 486, in post
    self.finish()
  File "/home/mader/.virtualenvs/tcwa_backend/lib/python3.4/site-packages/tornado/web.py", line 863, in finish
    raise RuntimeError("finish() called twice.  May be caused "
RuntimeError: finish() called twice.  May be caused by using async operations without the @asynchronous decorator.

Proposed fix

It seems to work fine, but I am not sure there are other complications I have not yet met.

--- handler.py.orig     2015-03-15 19:43:59.000000000 +0100
+++ handler.py  2015-03-15 20:48:56.214286385 +0100
@@ -244,7 +244,8 @@
             result = self.patch_single(self.parse_pk(instance_id))

         self._call_postprocessor(result=result)
-        self.finish(result)
+        if not self._finished:
+            self.finish(result)

     def patch_many(self) -> dict:
         """
@@ -364,7 +365,8 @@
             result = self.delete_single(self.parse_pk(instance_id))

         self._call_postprocessor(result=result)
-        self.finish(result)
+        if not self._finished:
+            self.finish(result)

     def delete_many(self) -> dict:
         """
@@ -455,7 +457,8 @@
             result = self.put_single(self.parse_pk(instance_id))

         self._call_postprocessor(result=result)
-        self.finish(result)
+        if not self._finished:
+            self.finish(result)

     put_many = patch_many
     put_single = patch_single
@@ -480,7 +483,8 @@
         result = self.post_single()

         self._call_postprocessor(result=result)
-        self.finish(result)
+        if not self._finished:
+            self.finish(result)

     def post_single(self):
         """
@@ -691,7 +695,8 @@
             result = self.get_single(self.parse_pk(instance_id))

         self._call_postprocessor(result=result)
-        self.finish(result)
+        if not self._finished:
+            self.finish(result)

     def get_single(self, instance_id: list) -> dict:
         """