On 05/03/2014 12:29 AM, Stefan Hornburg (Racke) wrote:
On 05/02/2014 10:07 PM, Maxwell Carey wrote:
Is it possible to lock session files created by Dancer2::Session::YAML in order
to avoid race conditions when two processes try to write to the same session at
the same time? For example, if I have the following routes:
get '/foo' => sub {
my $foo = session 'foo';
session 'foo' => ++$foo;
};
get '/bar' => sub {
my $bar = session 'bar';
session 'bar' => ++$bar;
};
and both are called at the same time, one route "wins", resulting in either the
value of 'foo' or 'bar' being updated, but not both.
If it's not possible (or easy) to lock the file, I would be open to an
alternative storage backend, although I was hoping to keep things as simple as
possible, i.e.
I don't want to have to spin up a new MySQL database or equivalent. The
existing Dancer2::Session:: backends all seem somewhat...involved. Any ideas?
It makes a lot of sense to lock sessions, in any session backend (not only file
based). Locking file based sessions
isn't a big deal.
As it turns out, Dancer2::Core::Role::SessionFactory::File already uses
flock, with a shared lock for reads and an exclusive lock for writes.
The problem with my code is that the entire update operation needs to
occur inside a single lock, i.e. I need to lock the file before reading
the value and unlock it after writing the value. It seems like there's
no way to do this directly without mucking around in the internals. A
better approach might be to use a semaphore.
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