Well after dive into jruby-rack code and jar file finally I found the problem.

jruby-rack-0.9.4.jar doesn't contain rack/lock.rb!!! that's it.

Rack::Lock is just used with some database adapters that's the reason why it just crashes sometimes.

This is resolved with 0.9.5, but...

If jruby-rack didn't bundle its own version of Rack this wouldn't be an issue and people could choose whatever version of Rack was suitable for their environment.

Agree, actually as jruby-rack user I didn't mind to have that dependency, I mean, warble already vendorize rack gem and it could be a requirement for jruby-rack custom usages.

Is there a known workaround to this problem ?

you can use a 0.9.5 snapshot

http://snapshots.repository.codehaus.org/org/jruby/rack/jruby-rack/0.9.5-SNAPSHOT/jruby-rack-0.9.5-20090625.190033-5.jar

Gary Weaver suggested the following work around:

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Having trouble getting into Kenai at the moment, but the workaround to your issue below should be to use the latest jruby-rack. How to override the one packaged in warbler is:

1. Additional Java .jar files to include. Note that if .jar files are placed
2. in lib (and not otherwise excluded) then they need not be mentioned here.
3. JRuby and JRuby-Rack are pre-loaded in this list. Be sure to include your
4. own versions if you directly set the value
5. config.java_libs += FileList["lib/java/*.jar"]
   config.java_libs.delete_if {|f| f =~ /jruby-rack/ || f =~ /jruby-complete/ }
   config.java_libs += FileList["lib/jruby-complete*.jar"]
   config.java_libs += FileList["lib/jruby-rack*.jar"]

then put the jruby-rack and jruby-complete jars you want to use in the lib/ dir.

The jruby-rack snapshot containing this fix is:
http://snapshots.repository.codehaus.org/org/jruby/rack/jruby-rack/0.9.5-SNAPSHOT/jruby-rack-0.9.5-20090625.190033-5.jar

You'll need to find the jruby jar you want to use and put it in the lib dir or remove that part from above.

My related blog entries:
http://stufftohelpyouout.blogspot.com/2009/07/fix-ioerrorio-errorbroken-pipestrange.html
http://stufftohelpyouout.blogspot.com/2009/06/how-to-get-warbler-to-include-custom.html

If you could post this into the Kenai ticket if it works for you, that would be great.
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(just got my Kenai acct. stuff straightened out)

A few more notes about the workaround:

• Do warble war:clean first to make sure it removes previous versions of jruby-rack, or it won't fix it (I did that. ugh!).
• Nick was the one who described how to do this for me and told me that the new jar fixed it (and he noted that in the ticket also).

Gary

As of 7a14ac4, JRuby-Rack allows a newer rack to be loaded and doesn't force its vendored copy on you.