[darcs-users] push --and-apply

[John Meacham]

On Wed, Sep 17, 2003 at 01:29:58AM +0300, Aggelos Economopoulos wrote:
> On Tuesday 16 September 2003 16:26, David Roundy wrote:
> > On Tue, Sep 16, 2003 at 05:21:57AM +0300, Aggelos Economopoulos wrote:
> 
> [...]
> 
> > > Any clues?
> >
> > Yuck.  I also ran into this problem when testing the curl (as opposed to
> > libcurl) version.  I don't properly understand it, and so I came up with a
> > very crude hack for copyRemoteCmd, which was to retry every time I failed
> > due to a SIGVTALARM interrupt.  This worked because the error was only
> > sporadic.  It may also be sporadic in the scp case, I just don't know.
> >
> > As I understand it (and I don't very well), the virtual timer is an
> > interrupt timer that gets called when a certain amount of CPU time is used
> > by a process.  When it is called, the process dies, and that is what is
> > happening.  What I don't know is why the timer is getting set in the first
> > place.  My only guess is that for some reason execvp is setting the timer.
> > Either that or it is getting set when I fork.
> 
> After grepping a while in a ghc5 source tree, it seems neither is true. The 
> timer seems to be set for the program irrespective of the fork/exec syscalls. 
> The runtime appears to use it (the timer) for profiling itself (which is one 
> obvious usage of setitimer()/getitimer()) - see ghc-5/ghc/rts/Itimer.c . 
> Obviously the signal handlers are thrown away during the exec, so the new 
> process gets an unexpected SIGVTALRM and dies.
> 
> Unless I'm missing something, we are talking about a ghc bug. ghc has a 
> pPrPr_disableITimers function which it calls e.g. in executeFile, but some 
> paths leading to execve(2) are probably left unprotected. Copying 
> pPrPr_disableITimers from ghc (which does no error checking and unnecessarily 
> clears .it_interval) and calling it from read_ssh should work (if I'm right).
> 
> Does this sound logical?

Yeah, this is a known 'feature' of ghc. you need to turn off the VTALARM
before execing another program. see the 'system()' implementation in the
standard haskell libraries for an example of how to do so..
        John

-- 
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John Meacham - California Institute of Technology, Alum. - john at foo.net
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