bug#34763: 27.0.50; url-retrieve-synchronously misbehaves inside eldoc-documentation-function

[Dmitry Gutov <dgutov@yandex.ru>]

On 15.03.2019 17:49, Eli Zaretskii wrote:

> The code in the functions that react to quit-flag being non-nil should
> be audited.  They don't look clean to me, FWIW, and we don't generally
> do such things from Lisp, we normally quit on the C level.  Maybe this
> whole sub-feature, whereby we show a message for interrupted
> transfers, should be rethought.

Apparently the idea is to handle quits, including contexts where 
inhibit-quit is t (947612be2c71d2478179694e8dfa538b9e8e07c1).

So url-retrieve-synchronously uses with-local-quit around 
accept-process-output, and I guess the idea was to handle quits inside 
debugging output calls.

So url-http-debug also does the job of clearing the sentinel and the 
filter of the connection's process.

But maybe that's not enough: it doesn't kill the process and/or remove 
it from url-http-open-connections. (I'm guessing 
url-http-mark-connection-as-free would normally be called later if the 
sentinel wasn't removed).

I'm not quite clear on how error handling should work here anyway. 
Asynchronous code is tricky that way.