Fix a race condition in edm::Ref

[Dr15Jones]

The code which was used to dereference an edm::Ref had a race condition where one thread could have updated the cache right after another thread checked to see if the cache had been updated. This would lead to productGetter() returning a null value which was then used to attempt a function call. The code now only calls productGetter() once and handles the case where it was changed since the last check.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_5_X.

Fix a race condition in edm::Ref

It involves the following packages:

DataFormats/Common

@cmsbuild, @Dr15Jones can you please review it and eventually sign? Thanks.
@makortel, @wddgit, @wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[Dr15Jones]

please test

[Dr15Jones]

@davidlange6 This isn't a problem now but will be a problem in the future when we are using more than one thread per Event. Therefore this doesn't have to go into 7_5.

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: 87dd209
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-9648/5517/summary.html

[Dr15Jones]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1