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

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 65156@debbugs.gnu.org
> Date: Tue, 08 Aug 2023 21:33:55 +0300
> From: 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?

I installed the patch below on the emacs-29 branch; please see if it
solves your problems with reading from pipes.

Paul, can there be a regular file that is not seekable?  If regular
files are always seekable, the patch can be simplified.

diff --git a/src/fileio.c b/src/fileio.c
index 995e414..55132f1 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4581,7 +4581,7 @@ because (1) it preserves some marker positions (in unchanged portions
       goto handled;
     }
 
-  if (seekable || !NILP (end))
+  if ((seekable && regular) || !NILP (end))
     total = end_offset - beg_offset;
   else
     /* For a special file, all we can do is guess.  */
@@ -4678,7 +4678,7 @@ because (1) it preserves some marker positions (in unchanged portions
 	   For a special file, where TOTAL is just a buffer size,
 	   so don't bother counting in HOW_MUCH.
 	   (INSERTED is where we count the number of characters inserted.)  */
-	if (seekable || !NILP (end))
+	if ((seekable && regular) || !NILP (end))
 	  how_much += this;
 	inserted += this;
       }