There was a Trac ticket about this:
http://dev.rubyonrails.org/ticket/9155
The problem could probably be fixed with one or more carefully placed
Thread#exclusive blocks. I don't think there is a good reason why
several threads have to be able to load new constants in parallel.
On K, 2008-04-23 at 10:04 +0100, Frederick Cheung wrote:
> Are there any claims as to whether Dependencies is thread safe? I was
> working on an odd startup problem one of our scripts has and it turns
> out that there seems to be a race condition:
>
> 2 threads spawn
>
> Thread 1 tries to reference Foo, hits const_missing
> Thread 2 tries to reference Foo, hits const_missing
> Thread 1 enters require_or_load (in dependencies.rb) and adds foo.rb
> to the loaded set
> Thread 2 performs the test on line 247 (if file_path && !
> loaded.include?(File.expand_path(file_path))) which fails because
> loaded does now contain foo.rb
> Thread 1 completes loading of foo.rb, Object.const_defined?(:Foo) now
> returns true
> Thread 2 hits line 253:
> elsif (parent = from_mod.parent) && parent != from_mod &&
> ! from_mod.parents.any? { |p| p.const_defined?(const_name) }
>
> which is also false since either from_mod is Object (in which case
> parent == from_mod) or from_mod.parents contains Object in which case
> the second part of the test
> Thread 2 then raises name_error.
>
> I've added some appropriately placed sleep statements into
> dependencies.rb to make this happen systematically rather than it just
> being blind luck. In my particular case we can probably work around
> this by explicitly requiring stuff, but this may be of interest to
> people thinking about thread safety in rails.
>
>
>
> Fred
>
>
> >
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