Making the boosted double SV tagger thread-safe (76X)

[ferencek]

fastjet::contrib::Njettiness is not thread-safe and therefore should not be used as a data member of an ESProducer. This PR addresses the problem by defining a local instance of fastjet::contrib::Njettiness instead of using a global instance defined as a data member of the CandidateBoostedDoubleSecondaryVertexComputer class.

This PR is urgent since it fixes CMSSW crashes occurring in the multi-threaded jobs.

[cmsbuild]

A new Pull Request was created by @ferencek (Dinko Ferenček) for CMSSW_7_6_X.

It involves the following packages:

RecoBTag/SecondaryVertex

@cmsbuild, @cvuosalo, @davidlange6, @slava77 can you please review it and eventually sign? Thanks.
@ferencek, @acaudron, @pvmulder, @imarches this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[slava77]

@cmsbuild please test
@ferencek how much slower is this solution?

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10165/console

[ferencek]

@slava77, haven't checked the speed but I'm afraid there is no alternative at the moment.

@Dr15Jones @rappoccio

[cmsbuild]

+1
Tested at: b7970d9
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12679/10165/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12679/10165/summary.html

[cvuosalo]

@ferencek: Today it was reported that this bug fix is essential to enable multi-threaded 76X running, which is always crashing right now. Please add this information to the description of this PR. It is important for reviewers to know this PR is very urgent.

[ferencek]

@cvuosalo, both @davidlange6 and @smuzaffar were included in the original email from @Dr15Jones so I think all relevant people are already in the loop and know that this PR and #12680 are critical to include.

[slava77]

@ferencek
it should be clear from the PR that it fixes crashes in multi-threaded jobs (including CMSSW_7_6_2, if I understand correctly).
We need good documentation of the impact of the update for release self-documentation.
This can't be replaced with private email threads.

[ferencek]

@slava77, point taken. The PRs were simply a reaction to the email Chris sent. I personally did not run any jobs and probably did not fully appreciate the scale of the problem. I have updated the PR description.

[cvuosalo]

+1

For #12679 b7970d9

Urgent bug fix for to prevent multi-threaded jobs from crashing. There should be no change in monitored quantities. #12680 is the 80X version of this PR.

The code change is satisfactory, and Jenkins tests against baseline CMSSW_7_6_X_2015-12-04-1100 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_6_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar