From: Jeff King <peff@peff.net>
Date: Sat, 02 Jun 2012 16:51:32 GMT
Subject: Re: How to handle terminal detection in a daemon calling git?
Message-ID: <20120602165132.GE15017@sigill.intra.peff.net>
In-Reply-To: <827737B4-F5EE-4FD3-9B1A-710047836080@castle.fastmail.fm>

On Fri, Jun 01, 2012 at 08:52:04AM -0500, Travis wrote:

> 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;

Keep in mind that STDIN is a perl filehandle, not a file descriptor.
When you close it, you close the filehandle and its underlying
descriptor. Then you assign another filehandle to STDIN, which has its
own descriptor.

Try this:

  strace perl -e '
    open($null, "</dev/null");
    close *STDIN;
    *STDIN = $null;
    <STDIN>;
  '

you'll see that it is doing something like:

  open("/dev/null", O_RDONLY)             = 3
  close(0)                                = 0
  read(3, "", 8192)                       = 0

The final read comes from the new descriptor, and we never re-opened
descriptor 0. You are basically just pointing the name STDIN to a new
handle, not doing anything with the underlying descriptor.

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

Yeah, this is the right way to re-open a descriptor in perl, because
open() does the magic to dup2 the newly opened descriptor onto STDIN's
descriptor.

> Thanks for your comments.

No problem. Glad it is working now.

-Peff