Re: insert-file-contents and fifos

Re: insert-file-contents and fifos

[Richard Stallman]

I don't really know how to use fifos, so I can't make a test case.
Can you send a specific test case with which to observe this problem?

Re: insert-file-contents and fifos

[Vivek]

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On Sat, 4 May 2002, Richard Stallman wrote:

> I don't really know how to use fifos, so I can't make a test case.
> Can you send a specific test case with which to observe this problem?

The attached perl script, when run, will create a fifo called .sig-dyn,
daemonise, and sit attached to it, emitting a signature each time
.sig-dyn is read from. If ~/sigd/sigfile (I do means "sigd", it's not a
typo) is not present, then it uses fortune(6) as a source.

GNU emacs 20 was capable of reading from this, 21 is not. The change
appears to be the introduction of the new read_non_regular function
in fileio.c : a strace shows emacs fetching the data from the file
( you can see the data being read ), and then aborting. Presumably
the state of a fifo is sufficiently different from a non-regular file
that read_non_regular reacts as if an error has occurred.

There is a (possibly architecturally bad) patch on savannah.gnu.org
that restores the old GNU emacs 20 behaviour specifically for fifos,
while leaving all other file types to the new code. The patch is
against the version of GNU emacs 21.1 (21.1-7) shipped with Debian
GNU/Linux (woody) a while ago now - woody ships with 21.2-1 now.

If there are any problems with getting the sigerator working, let me know.
( It should be pretty straightforward, I usually run it from my home
  directory, but it should be Ok anywhere it has write permission ).


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#!/usr/bin/perl

use Fcntl;
use strict;
use vars qw(@sig $sig $lidx);

my $FIFO = '.sig-dyn';
#$SIG{PIPE} = sub { warn("Broken pipe: $!\n") };
$SIG{PIPE} = 'IGNORE';
$SIG{HUP} = \&readsigs;
$SIG{TERM} = sub { exit(0) };

sub readsigs ()
{
    chdir;
    if(sysopen(SIGS, join('/',$ENV{HOME}, qw(sigd sigfile)), O_RDONLY, undef))
    {
	$/ = "--\n";
	chomp(@sig = <SIGS>);
	close(SIGS);
	$sig =
	  sub
	  {
	      my $idx;
	      for($idx = $lidx; $idx == $lidx; $idx = int(rand(@sig))) {}
	      join('', "-- \n", $sig[$lidx = $idx]);
	  };
    }
    else { $sig = sub { "-- \n".`fortune -s` } }

    unless(-p $FIFO)
    {
	rename($FIFO, "$FIFO~");	
	system(mkfifo => $FIFO)
	  && system(mknod => $FIFO, 'p')
	    && die("mk{nod|fifo} failed: $!\n");
    }
}

readsigs();

fork() && exit(0);

SIGLOOP:
{
    my $wi = '';
    my $wo = '';

    while (-p $FIFO)
    {
	my $s = $sig->();
	my $l = length($s);
	sysopen(FIFO, $FIFO, O_WRONLY|O_SYNC, undef)
          || die("sysopen(FH, $FIFO, O_WRONLY|O_SYNC, undef) failed: $!\n");
        syswrite(FIFO, $s, $l, 0);
        close(FIFO);
        select(undef(), undef(), undef(), 0.5);
    }
}

warn("$FIFO is not a pipe: exiting\n");
exit(0);

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Re: insert-file-contents and fifos

[Richard Stallman]

    GNU emacs 20 was capable of reading from this, 21 is not. The change
    appears to be the introduction of the new read_non_regular function
    in fileio.c : a strace shows emacs fetching the data from the file
    ( you can see the data being read ), and then aborting. Presumably
    the state of a fifo is sufficiently different from a non-regular file
    that read_non_regular reacts as if an error has occurred.

Could you step through it and see why this happens?
The code looks straightforward, so I don't see why it would fail.

Re: insert-file-contents and fifos

[Vivek]

On Mon, 6 May 2002, Richard Stallman wrote:

>     GNU emacs 20 was capable of reading from this, 21 is not. The change
>     appears to be the introduction of the new read_non_regular function
>     in fileio.c : a strace shows emacs fetching the data from the file
>     ( you can see the data being read ), and then aborting. Presumably
>     the state of a fifo is sufficiently different from a non-regular file
>     that read_non_regular reacts as if an error has occurred.
>
> Could you step through it and see why this happens?
> The code looks straightforward, so I don't see why it would fail.

Maybe I'm being dense, or just don't understand what the code is meant to
do, but:

`read_non_regular' _always_ calls

  Fsignal (Qquit, Qnil);

just after `emacs_read' (which succeeds, afaict)

and the call to `read_non_regular' in `insert-file-contents' is wrapped
in an `internal_condition_case_1' call: so is it _ever_ possible for
a read of a non regular file to succeed, since:

val = internal_condition_case_1 ( read_non_regular, Qnil, Qerror,
				  read_non_regular_quit );

if (NILP (val))
  {
    read_quit = 1;
    break;
  }

So, according to my (albeit limited) understanding, we're calling
internal_condition_case_1, which calls read_non_regular, which
will _always_ signal '(quit . nil), which will therefore always
result in a 'nil return from internal_condition_case_1 : can
insert-file-contents read any non regular files at all? I tried
with both ~/.sig-dyn and /dev/fd0 and both resulted in the same
behaviour: a successful read(2) of the available data (or the first
65536 bytes in thcase of /dev/fd0) followed by a bailout.

I'm confused... for the record, here's the read_non_regular that I have:

static Lisp_Object
read_non_regular ()
{
  int nbytes;

  immediate_quit = 1;
  QUIT;

  nbytes = emacs_read (non_regular_fd,
		       BEG_ADDR + PT_BYTE - 1 + non_regular_inserted,
		       non_regular_nbytes);

  Fsignal (Qquit, Qnil);
  immediate_quit = 0;

  return make_number (nbytes);
}

Re: insert-file-contents and fifos

[Richard Stallman]

    `read_non_regular' _always_ calls

      Fsignal (Qquit, Qnil);

    just after `emacs_read' (which succeeds, afaict)

How strange--the 21.2 code really has that absurd call to Fsignal.
The latest development version does not have it; I think it was
inserted by mistake.

Eli, could you delete that Fsignal call?


read_non_regular ()
{
  int nbytes;
  
  immediate_quit = 1;
  QUIT;
  nbytes = emacs_read (non_regular_fd,
		       BEG_ADDR + PT_BYTE - 1 + non_regular_inserted,
		       non_regular_nbytes);
  Fsignal (Qquit, Qnil);
  immediate_quit = 0;
  return make_number (nbytes);
}

Re: insert-file-contents and fifos

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Date: Mon, 6 May 2002 22:58:41 -0600 (MDT)
> 
> Eli, could you delete that Fsignal call?

Done.