detect incomplete body before reading next request

[pajod]

**DO NOT MERGE: ** The attached patch only checks that we have read the entire expected body after processing is done. It merely assist in testing and demonstrating that there is a problem, it does not prevent replying to truncated input yet.

I wonder if there is a way to approach #3234 with just one length check. We have one NoMoreData now, but it appears we want two variants - a) done reading, and b) unable to read more.

[benoitc]

I don't understand the point there. We don't crash in the midldle of the appliication processing. How is the application supposed to handle such error there?

[pajod]

We don't crash in the middle of the application processing

We do raise InvalidHeaderName when we see something funny in chunked trailer fields. To me that feels like sufficient precedent for demanding that the application (and, where failing that, Gunicorn) can reasonably deal with the result of invalid or truncated input up to and including processing even the very last octet of arbitrary-length input.

EDIT: I have not yet made much progress on determining what "reasonably deal with" means in this context, hence the repro-only state of this PR.

[benoitc]

We don't crash in the middle of the application processing

We do raise InvalidHeaderName when we see something funny in chunked trailer fields. To me that feels like sufficient precedent for demanding that the application (and, where failing that, Gunicorn) can reasonably deal with the result of invalid or truncated input up to and including processing even the very last octet of arbitrary-length input.

fair enough :) please go ahead then :) We shoudl probably document this somewhere also

[benoitc]

maybe adding a reason there?