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Re: dd SIGUSR1 race

From: Federico Simoncelli
Subject: Re: dd SIGUSR1 race
Date: Fri, 26 Sep 2014 11:39:58 -0400 (EDT)

----- Original Message -----
> From: "Pádraig Brady" <address@hidden>
> To: "Bernhard Voelker" <address@hidden>
> Cc: "Federico Simoncelli" <address@hidden>, "Coreutils" <address@hidden>
> Sent: Friday, September 26, 2014 5:05:30 PM
> Subject: Re: dd SIGUSR1 race
> We'd definitely leave the existing SIGUSR1 handling in place
> but we could augment it to allow avoiding races.
> Option1:
>   Unblock SIGUSR1 explicitly, allowing the parent to mask that and avoid
>   races,
> Option2:
>   Set the handler even if SIGUSR1 was set to ignored.
> Note the default behavior for SIGINFO on BSD is to discard it,
> so that doesn't have the "kill dd by mistake" issue, but does it seem
> have the possibility to lose SIGINFO, so that for robustness
> they'd have to be resent after a timeout.
> Now that doesn't seem too onerous since you'd have to be dealing
> with timeouts anyway, so one could take the same approach with existing GNU
> dd
> implementations. I.E. just set SIGUSR1 to ignore in the parent
> before the fork and then reset.
> Another reason to favor Option2 is that can be easily controlled
> from the shell using: trap '' USR1
> Note we only print stats on receiving the SIGUSR1 so I don't see
> much issue in requisitioning it (even though not strictly POSIX compliant).
> Also we can even suppress that stats output if problematic with status=none.

Does it mean that the controlling process cannot reliably use SIGUSR1?

Currently vdsm[1] has a special meaning for SIGUSR1 and it makes wide
use of "dd" for several reasons.

Will we have a short moment before/after forking for dd where SIGUSR1
is masked and the signal is not handled, or is there some special
trick we could use?
(I thought of double-forking... but again, we shouldn't be forced to
use cumbersome strategies to have progress report).

Using signals is a big advantage for sysadmins that once they've
launched the process they can monitor it at any time in case it seems
to take too long.

Anyway for controlling processes dealing with signals is harder (they
are asynchronous, you don't know if they're lost or not resulting in
missing stats or too many stats, the initial sleep that you added in
the script uses .5 which is an arbitrary value, etc.)

Many other commands are providing an explicit flag: wget, curl,
qemu-img, etc.



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