require should be made either threadsafe or guaranteed to run the discovered
file exactly once
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Key: JRUBY-3078
URL: http://jira.codehaus.org/browse/JRUBY-3078
Project: JRuby
Issue Type: Bug
Components: Core Classes/Modules
Reporter: Charles Oliver Nutter
Fix For: JRuby 1.1.6
In examining JRUBY-3057, we discovered that requires are not really as
thread-safe as they could be. They may cause two threads to execute the same
file at the same time, which is both a risk for concurrency and a violation of
require's implicit "execute exactly once" guarantee. A simple global
synchronization of require could lead two threads requiring the same files in
different orders to deadlock, so a full-on synchronization is not likely to be
possible. We must explore that possibility.
If it turns out that a general synchronization of require is not possible, we
must fall back on our alternative option: at least guarantee that require does
not ever execute a file that has been or is currently being required by another
thread. This is a much smaller goal, since it largely only requires
synchronizing on the list of loaded features and guaranteeing that features in
that list have at least "started" to load, if not finished (the latter
guarantee being impossible without a global synchronization).
JRUBY-3057 was fixed by avoiding concurrent requires due to time constraints
and the potential for an overreaching require synchronization to cause
catastrophic failures under heavily concurrent load. We should look into the
described require thread-safety options for a future release.
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