CachePartion.create_file contextmanager doesn't close storage file properly

The context manager "create_file" in "CachePartiton" class doesn't close/flush de file properly. This cause data lost when use file-like object storage (like S3Boto3Storage).

In order to reproduce:

On a fresh/new docker install, you have to configure an S3 like Storage for cache: (or for common storage, produces the same issue in other location)

MAYAN_DOCUMENTS_CACHE_STORAGE_BACKEND=storages.backends.s3boto3.S3Boto3Storage MAYAN_DOCUMENTS_CACHE_STORAGE_BACKEND_ARGUMENTS={'access_key':'','secret_key':'','bucket_name':'','default_acl':'private', 'location':'cache'}

In mayan frontend: upload a file and preview them. Then, on celery workers you get some errors and no preview is shown.

[2020-08-28 13:30:46,629: ERROR/ForkPoolWorker-1] Task mayan.apps.documents.tasks.task_generate_document_page_image[1410cdc4-2282-4fbe-91cb-2932480d394c] raised unexpected: AttributeError("'NoneType' object has no attribute 'size'",)
Traceback (most recent call last):
  File "~/.virtualenvs/mayan/lib/python3.6/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "~/.virtualenvs/mayan/lib/python3.6/site-packages/celery/app/trace.py", line 648, in __protected_call__
    return self.run(*args, **kwargs)
  File "~/projectes/mayan-edms/mayan/apps/documents/tasks.py", line 78, in task_generate_document_page_image
    return document_page.generate_image(user=user, **kwargs)
  File "~/projectes/mayan-edms/mayan/apps/documents/models/document_page_models.py", line 99, in generate_image
    image = self.get_image(transformations=transformation_list)
  File "~/projectes/mayan-edms/mayan/apps/documents/models/document_page_models.py", line 212, in get_image
    converter.transform(transformation=transformation)
  File "~/projectes/mayan-edms/mayan/apps/converter/classes.py", line 232, in transform
    self.image = transformation.execute_on(image=self.image)
  File "~/projectes/mayan-edms/mayan/apps/converter/transformations.py", line 422, in execute_on
    super(TransformationResize, self).execute_on(*args, **kwargs)
  File "~/projectes/mayan-edms/mayan/apps/converter/transformations.py", line 93, in execute_on
    self.aspect = 1.0 * image.size[0] / image.size[1]
AttributeError: 'NoneType' object has no attribute 'size'

In the cache storage, you can find a 0 bytes file.

This is due to that the context manager doesn't close(and flush) the file:

class CachePartition(models.Model):

    @contextmanager
    def create_file(self, filename):
        lock_id = 'cache_partition-create_file-{}-{}'.format(self.pk, filename)
        try:
            logger.debug('trying to acquire lock: %s', lock_id)
            lock = locking_backend.acquire_lock(lock_id)
            logger.debug('acquired lock: %s', lock_id)
            try:
                self.cache.prune()

                # Since open "wb+" doesn't create files force the creation of an
                # empty file.
                self.cache.storage.delete(
                    name=self.get_full_filename(filename=filename)
                )
                self.cache.storage.save(
                    name=self.get_full_filename(filename=filename),
                    content=ContentFile(content='')
                )

                try:
                    with transaction.atomic():
                        partition_file = self.files.create(filename=filename)
                        yield partition_file.open(mode='wb')
                        partition_file.update_size()
                except Exception as exception:
                    logger.error(
                        'Unexpected exception while trying to save new '
                        'cache file; %s', exception
                    )
                    self.cache.storage.delete(
                        name=self.get_full_filename(filename=filename)
                    )
                    raise
            finally:
                lock.release()
        except LockError:
            logger.debug('unable to obtain lock: %s' % lock_id)
            raise

When use a custom context manager, this is responsible of releasing the captured resources, for example: https://docs.python.org/3/library/contextlib.html

from contextlib import contextmanager

@contextmanager
def managed_resource(*args, **kwds):
    # Code to acquire resource, e.g.:
    resource = acquire_resource(*args, **kwds)
    try:
        yield resource
    finally:
        # Code to release resource, e.g.:
        release_resource(resource)