werkzeug times out when exception is raised

[RazerM]

I found this within a large Flask application but I managed to reproduce it.

SQLAlchemy's exceptions contain an .orig attribute with the original DBAPI exception. I only raised it to see it in the werkzeug debugger, and found that the debugger page never loads.

from werkzeug.wrappers import Request, Response
from sqlalchemy import create_engine, text
from sqlalchemy.exc import DatabaseError

engine = create_engine('sqlite://')


@Request.application
def application(request):
    try:
        engine.execute(text('SELECT undefined_function()'))
    except DatabaseError as exc:
        # if we just re-raise, it works fine
        # raise

        # exc.orig is definitely an exception
        assert isinstance(exc.orig, Exception)

        # werkzeug doesn't return a response when this is raised
        raise exc.orig
    return Response('This is unreachable')


if __name__ == '__main__':
    from werkzeug.serving import run_simple
    run_simple('localhost', 4000, application, use_debugger=True)

The same happens for a PostgreSQL (psycopg2) connection.

[RazerM]

Using git bisect I found 3245e58

[davidism]

Thanks for tracking this down. This is the first time I've seen an exception context cycle. I added cycle detection to the formatter.