How to handle exceptions raised from a Generator wired to a Response?

[wrkhenddher]

Hello folks,

Is there any hook or facility to finish streaming content once an exception is raised within a generator?

Our current solution is to wrap the generator with a try/except block and yield "Output truncated - Exception..." when an exception is caught.

# Refer to https://github.com/pallets/flask/issues/2992

@app.route('/streamingerror')
def streaming_error_route():
    def inner_gen():
        try: 
            yield 'hello'
            raise Exception('error from streaming_error_route')
            yield 'bye'
        except Exception as e:
            logging.exception("Exception in inner_gen")
            yield f"Output truncated - {e}"
            raise e
    return Response(stream_with_context(inner_gen()))

Here is the traceback:

❯ FLASK_APP=flask_issue.py FLASK_DEBUG=1 python -m flask run
 * Serving Flask app 'flask_issue.py'
 * Debug mode: on
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://127.0.0.1:5000
Press CTRL+C to quit
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 127-710-189
127.0.0.1 - - [13/Nov/2023 11:57:44] "GET /streamingerror HTTP/1.1" 200 -
ERROR:root:Exception in inner_gen
Traceback (most recent call last):
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue.py", line 27, in inner_gen
    raise Exception('error from streaming_error_route')
Exception: error from streaming_error_route
Debugging middleware caught exception in streamed response at a point where response headers were already sent.
Traceback (most recent call last):
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue/lib/python3.8/site-packages/werkzeug/wsgi.py", line 256, in __next__
    return self._next()
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue/lib/python3.8/site-packages/werkzeug/wrappers/response.py", line 32, in _iter_encoded
    for item in iterable:
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue/lib/python3.8/site-packages/flask/helpers.py", line 115, in generator
    yield from gen
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue.py", line 32, in inner_gen
    raise e
  File "/Users/henddher/Documents/github/gdcapi_gene_expr/flask_issue.py", line 27, in inner_gen
    raise Exception('error from streaming_error_route')
Exception: error from streaming_error_route

The main problems are:

• The HTTP/1.0 200 OK header has been sent but content is truncated due to the exception.
• The Content-Length header is absent.
• Without the "Output truncated - Exception...", there is no way for the enduser to know there was an error.

Perhaps the more fundamental question is:

How should one deal with abrupt terminations within a generator wired to a response?

Bit of extra context:

❯ python --version
Python 3.8.16

❯ pip freeze --all
blinker==1.7.0
click==8.1.7
Flask==3.0.0
importlib-metadata==6.8.0
itsdangerous==2.1.2
Jinja2==3.1.2
MarkupSafe==2.1.3
pip==22.0.4
setuptools==56.0.0
Werkzeug==3.0.1
zipp==3.17.0

PS/
See same question in flask's Q&A:
it was closed as NA