Add tips for 413 connection reset issue

[greyli]

When uploading files exceed the limit set by MAX_CONTENT_LENGTH, the flask development server will reset the connection instead of return a 413 response. This behavior confused a lot of people:

• https://stackoverflow.com/questions/7371660/flask-resets-the-connection-instead-of-returning-a-413-when-the-file-is-to-big
• https://stackoverflow.com/questions/19911106/flask-file-upload-limit
• https://stackoverflow.com/questions/30414696/upload-file-larger-than-max-content-length-in-flask-results-connection-reset

[davidism]

Sounds like this should be fixed in Werkzeug. Also, avoid using the term "Flask development server", as Flask is not the thing providing the server.

[greyli]

It would be great if it can be fixed. It seems this issue was quite old, in this snippet that posted by Armin Ronacher in 2011, he said:

Some WSGI servers solve that problem for you, others do not. For instance the builtin Flask webserver is pretty dumb and will not attempt to fix this problem.

[davidism]

See #2188, which needs to be reported against Werkzeug.

[davidism]

Related to pallets/werkzeug#2620, which changes the development server so the 413 error will come through in most cases. In general though it's up to the client how they handle their input stream being closed before all input is sent.