bug#65156: 29.1; Reading from pipe with --insert or insert-file-contents no longer supported

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 8 Aug 2023 20:20:23 +0200
> From: Lucas Werkmeister <mail@lucaswerkmeister.de>
> 
> Launch graphical Emacs with standard input attached to a pipe, then
> attempt to insert /dev/stdin, for example:
> 
>      echo test | emacs -Q --insert /dev/stdin
>      echo test | emacs -Q --eval '(insert-file-contents "/dev/stdin")'
> 
> This now results in an error (and nothing inserted into the buffer):
> "Maximum buffer size exceeded".

The doc string says:

  When inserting data from a special file (e.g., /dev/urandom), you
  can’t specify VISIT or BEG, and END should be specified to avoid
  inserting unlimited data into the buffer.

> Previously, this used to work; git bisect identifies cb4579ed6b ("Allow
> inserting parts of /dev/urandom with insert-file-contents", bug#18370)
> as the first bad commit.
> 
> Other (non-stdin) pipes are also affected. Testing with a named pipe 
> shows that the error only occurs after the pipe is first written to:
> 
>      # first terminal:
>      mkfifo /tmp/fifo
>      emacs -Q
>      M-x insert-file /tmp/fifo
>      # second terminal:
>      { echo x; sleep 10; echo y; } > /tmp/fifo
> 
> Before you run the command in the second terminal, you can observe that 
> Emacs is just waiting for input from the pipe; as soon as you run the 
> other command, Emacs shows the error before the sleep finishes.

Lars, Paul, any suggestions?