Re: How to handle terminal detection in a daemon calling git?

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On Jun 1, 2012, at 4:53 AM, Jeff King wrote:

On Thu, May 31, 2012 at 08:39:42AM -0500, Travis P wrote:

Here's what I learned this morning: it appears to work when I don't
close STDIN.

   #close $_ for *STDIN, *STDOUT, *STDERR; # What I was doing. Fails.
   close $_ for *STDOUT, *STDERR;  # Tried this, it works.
   *STDOUT = $log_fh;
   *STDERR = $log_fh;

Yeah, don't do that.  This can cause subtle bugs in subprocesses. For
example:

 1. You don't have a descriptor 0, because it is closed.

 2. Some part of the program opens a new descriptor (e.g., to read a
file, making a pipe, etc). This becomes descriptor 0, because it is
    the lowest unused descriptor.

 3. The program wants to redirect its stdin (e.g., because it is
    forking and exec'ing a child). So it calls dup2(fd, 0), closing
    what was at stdin previously, which might have been valuable.

The right thing to do is to redirect stdin from /dev/null, not close it
entirely.

Ah, yes, that's a normal thing I would think about in C.  In Perl,
I imagined that those details were handled by Perl.

With that in mind, I'm still seeing strange behavior when I do this,
where it looks to me like I'm closing and then immediately assigning
STDIN:

 my $null_in_fh;
 open($null_in_fh, '<', '/dev/null') or die;
close *STDIN; # this appears to mess things up, even with the following assignment
 *STDIN = $null_in_fh;

But if I don't do the close STDIN (with or without the glob), then things work.
Maybe that assignment doesn't work for some Perl-ish reason.

Because, if I just do this
  open(STDIN, '<', '/dev/null') or die;
even after the close and/or assignment, then all appears okay.

I'll stick with this last technique and just chalk it up to something
to something about Perl I don't understand.  (I had the idea that
maybe I wanted to redirect /dev/null to STDIN at some point anyway.)

Thanks for your comments.

-Travis



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