PIL.UnidentifiedImageError (Empty image file)

[0xdade]

Version:
943b660

Describe the bug
There appears to be a scenario where the agent will take screenshots and pass along an empty file to the server. The server does a base64 decode and saves the results to a file and then tries to create a thumbnail based on the now-saved file. PIL reads an empty file and says "woah buddy this doesn't look like an image to me" and natlas throws an exception and doesn't save the document as a result.

To Reproduce
Steps to reproduce the behavior:

1. It's unclear to me exactly what is leading aquatone to produce an empty screenshot file at this time
2. Mock a submission with an empty screenshot and watch the fireworks

Expected behavior
If a file is empty, the agent shouldn't count it as a valid screenshot. If a file is empty, the server shouldn't count it as a valid screenshot, and it shouldn't attempt to do image manipulation on it.

Screenshots / Logs

[2020-07-25 00:22:59,784] ERROR in app: Exception on /api/submit [POST]
Traceback (most recent call last):
  File "/.venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/.venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/.venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/.venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/.venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/.venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/opt/natlas/natlas-server/app/auth/wrappers.py", line 49, in decorated_function
    return f(*args, **kwargs)
  File "/opt/natlas/natlas-server/app/api/routes.py", line 187, in submit
    newhost["screenshots"], newhost["num_screenshots"] = process_screenshots(
  File "/opt/natlas/natlas-server/app/api/processing/screenshot.py", line 52, in process_screenshots
    item["thumb_hash"] = create_thumbnail(fname, file_ext)
  File "/opt/natlas/natlas-server/app/api/processing/screenshot.py", line 10, in create_thumbnail
    thumb = Image.open(fname)
  File "/.venv/lib/python3.8/site-packages/PIL/Image.py", line 2895, in open
    raise UnidentifiedImageError(
PIL.UnidentifiedImageError: cannot identify image file '/data/media/original/e3/b0/e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855.png'

[tw-sentry-io]

Sentry issue: NATLAS-SERVER-13