Set ROOT histogram fitting to be thread-safe

[Dr15Jones]

In order to fit different root histograms on different threads one
must call the undocumented function TMinuitMinimizer::UseStaticMinuit
and pass it false.
This will fix crashes seen in the CMSSW_7_6 IBs.

[cmsbuild]

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

Set ROOT histogram fitting to be thread-safe

It involves the following packages:

RecoHI/HiTracking

@cmsbuild, @cvuosalo, @slava77 can you please review it and eventually sign? Thanks.
@jazzitup, @dgulhan, @echapon, @yenjie, @kurtejung, @istaslis, @mandrenguyen, @richard-cms, @yetkinyilmaz 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

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9115/console

[cmsbuild]

+1
Tested at: 15a24e3
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12033/9115/summary.html

The following merge commits were also included on top of IB + this PR after doing git cms-merge-topic:
64fce4e
You can see more details here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12033/9115/git-log-recent-commits
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12033/9115/git-merge-result

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12033/9115/summary.html

[cvuosalo]

+1

For #12033 15a24e3

Fix to make ROOT histogram fitting thread-safe to prevent crashes. There should be no change in monitored quantities. #12031 and #12032 are the 76X and 80X versions of this PR, and ithey've already been approved by Reco.

The code change is satisfactory, and Jenkins tests against baseline CMSSW_7_5_X_2015-10-21-1100 show no significant differences, as expected.

[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) and once validation in the development release cycle CMSSW_7_6_X is complete. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1