bug#25214: #25214 26.0.50; Interacting with user from threads other than the primary

[Eli Zaretskii <eliz@gnu.org>]

> From: Zhang Haijun <ccsmile2008@outlook.com>
> CC: "25214@debbugs.gnu.org" <25214@debbugs.gnu.org>
> Date: Thu, 7 Feb 2019 01:39:54 +0000
> 
> 1. Create a fd to receive the input request event. This fd can only be watched by main thread or a special keybord sheduler thread.
> 2. If a thread need input, it sends an input request event(containning its thread id, prompt …) to the fd and then goto background.

How do you "send input request to the fd"?  A program cannot both read
and write to the same fd within the same process, at least not
portably.

> 3. The main thread(or keyboard sheduler thread) receives the event and put it into in-box and show a notification to user.

If another thread writes to some fd, it means the main thread is not
running, so it cannot receive the event, even if we did find the way
of having an fd which can be read by one thread and written by all the
others.