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

From: Travis <git@castle.fastmail.fm>
Date: Jun 1, 2012, 13:52 UTC
Message-ID: <827737B4-F5EE-4FD3-9B1A-710047836080@castle.fastmail.fm>
In-Reply-To: <20120601095333.GF32340@sigill.intra.peff.net>

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

Show 27 quoted lines

> 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