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Id: 114349
Status: open
Priority: 0/
Queue: DBM-Deep

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Owner: Nobody in particular
Requestors: nick [...] van-laarhoven.org
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Subject: Race condition in DBM::Deep::File::_lock()
Date: Thu, 12 May 2016 15:58:03 +0200
To: bug-DBM-Deep [...] rt.cpan.org
From: Nick Hibma <nick [...] van-laarhoven.org>
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Gents, There is a race condition in the following code: sub _lock { ... if ($self->{locking}) { if (!$self->{locked}) { flock($self->{fh}, $type); # refresh end counter in case file has changed size my @stats = stat($self->{fh}); $self->{end} = $stats[7]; # double-check file inode, in case another process # has optimize()d our file while we were waiting. if (defined($self->{inode}) && $stats[1] != $self->{inode}) { $self->close; $self->open; #XXX This needs work $obj->{engine}->setup( $obj ); flock($self->{fh}, $type); # re-lock # This may not be necessary after re-opening $self->{end} = (stat($self->{fh}))[7]; # re-end } } … } If the file is optimised twice in rapid succession, the second flock will lock the wrong file. I’d suggest repeating until the lock, end stats and node check succeeds (note: code is untested!): sub _lock { my $self = shift; my ($obj, $type) = @_; $type = LOCK_EX unless defined $type; #XXX This is a temporary fix for Win32 and autovivification. It # needs to improve somehow. -RobK, 2008-03-09 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) { $type = LOCK_EX; } if (!defined($self->{fh})) { return; } #XXX This either needs to allow for upgrading a shared lock to an # exclusive lock or something else with autovivification. # -RobK, 2008-03-09 if ($self->{locking}) { while (!$self->{locked}) { flock($self->{fh}, $type); # refresh end counter in case file has changed size my @stats = stat($self->{fh}); $self->{end} = $stats[7]; # double-check file inode, in case another process # has optimize()d our file while we were waiting. if (defined($self->{inode}) && $stats[1] != $self->{inode}) { $self->close; $self->open; #XXX This needs work $obj->{engine}->setup( $obj ); } else { $self->{locked}++; } } return 1; } return; } Kind regards, Nick
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On Thu May 12 09:58:16 2016, nick@van-laarhoven.org wrote: Show quoted text
> Gents, > > There is a race condition in the following code:
[snip] Huh. You're right. Do you have any idea of how to go about writing a test for this?


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