Re: [gentoo-portage-dev] Re: [PATCH] econf: update configure/config.{sub,guess} atomically to avoid races

[Mike Frysinger <vapier@gentoo.org>]

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On Friday 20 December 2013 22:07:17 Ryan Hill wrote:
> On Tue, 17 Dec 2013 18:28:12 -0500 Mike Frysinger wrote:
> > Use $BASHPID which will be unique even in subshells.
> 
> This requires bash 4.0.  I'm all for it, but haters gonna hate.

ugh.  fun fact: this is used in multiple places in portage already, as well as 
some eclasses.

it can be re-implemented with:
	read -r pid _ < /proc/self/stat; echo $pid

but that doesn't work in a subshell, so you couldn't do:
	bashpid() { local pid; read -r pid _ < /proc/self/stat; echo $pid; }
	echo ${BASHPID:-$(bashpid)}
because now you'd be a sub-subshell.  in this case you want the parent pid, 
but that isn't easy to get as the 2nd field of stat is the process name, and 
i'm not sure that won't have spaces (since it might be the name of the shell 
script).  plus it isn't terribly portable.

this seems reasonable portable though:
	bashpid() { sh -c 'echo $PPID'; }
	echo ${BASHPID:-$(bashpid)}
even if it does have the overhead of running another bash process.  but 
considering this only impacts bash-3.2, maybe we simply don't care.
-mike

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