pybind/rados: track completions before calling aio functions

Tracking completions is critical for memory safety - if the
aio function succeeds, the completion must be tracked. However,
if a KeyboardInterrupt or similar arrives between the call and
the tracking, the completion will not be tracked.

Fix this by tracking the completion before the aio call, and
explicitly cleaning up in the failure case.

This leaves the opposite problem, where an unexpected exception
(other than simple error return from the aio function) will cause
the completion to not be freed until the Ioctx is destroyed, but
that is a relatively minor issue.

src/pybind/rados/rados.pyx

@@ -1802,13 +1802,13 @@ cdef class Ioctx(object):
             uint64_t _offset = offset
 
         completion = self.__get_completion(oncomplete, onsafe)
-
+        self.__track_completion(completion)
         with nogil:
             ret = rados_aio_write(self.io, _object_name, completion.rados_comp,
                                 _to_write, size, _offset)
         if ret < 0:
+            completion._cleanup()
             raise make_ex(ret, "error writing object %s" % object_name)
-        self.__track_completion(completion)
         return completion
 
     def aio_write_full(self, object_name, to_write,
@@ -1844,13 +1844,14 @@ cdef class Ioctx(object):
             size_t size = len(to_write)
 
         completion = self.__get_completion(oncomplete, onsafe)
+        self.__track_completion(completion)
         with nogil:
             ret = rados_aio_write_full(self.io, _object_name,
                                     completion.rados_comp,
                                     _to_write, size)
         if ret < 0:
+            completion._cleanup()
             raise make_ex(ret, "error writing object %s" % object_name)
-        self.__track_completion(completion)
         return completion
 
     def aio_append(self, object_name, to_append, oncomplete=None, onsafe=None):
@@ -1884,13 +1885,14 @@ cdef class Ioctx(object):
             size_t size = len(to_append)
 
         completion = self.__get_completion(oncomplete, onsafe)
+        self.__track_completion(completion)
         with nogil:
             ret = rados_aio_append(self.io, _object_name,
                                 completion.rados_comp,
                                 _to_append, size)
         if ret < 0:
+            completion._cleanup()
             raise make_ex(ret, "error appending object %s" % object_name)
-        self.__track_completion(completion)
         return completion
 
     def aio_flush(self):
@@ -1946,12 +1948,13 @@ cdef class Ioctx(object):
         completion = self.__get_completion(oncomplete_, None)
         completion.buf = PyBytes_FromStringAndSize(NULL, length)
         ret_buf = PyBytes_AsString(completion.buf)
+        self.__track_completion(completion)
         with nogil:
             ret = rados_aio_read(self.io, _object_name, completion.rados_comp,
                                 ret_buf, _length, _offset)
         if ret < 0:
+            completion._cleanup()
             raise make_ex(ret, "error reading %s" % object_name)
-        self.__track_completion(completion)
         return completion
 
     def aio_remove(self, object_name, oncomplete=None, onsafe=None):
@@ -1977,12 +1980,13 @@ cdef class Ioctx(object):
             char* _object_name = object_name
 
         completion = self.__get_completion(oncomplete, onsafe)
+        self.__track_completion(completion)
         with nogil:
             ret = rados_aio_remove(self.io, _object_name,
                                 completion.rados_comp)
         if ret < 0:
+            completion._cleanup()
             raise make_ex(ret, "error removing %s" % object_name)
-        self.__track_completion(completion)
         return completion
 
     def require_ioctx_open(self):