Secondary input source chooses first file to open randomly

[wmtan]

The secondary input source, used for mixing pileup, needs to open a pileup file at construction time.
Before this PR, it always opened the first file in the list of files, which caused contention. This PR randomizes the choice of the file to open at construction time. This PR does not affect which minbias events will be chosen for mixing.
This change can easily be ported back to 7_4_X or prior releases if needed.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_5_X.

Secondary input source chooses first file to open randomly

It involves the following packages:

IOPool/Input

@cmsbuild, @Dr15Jones, @nclopezo can you please review it and eventually sign? Thanks.
@wddgit this is something you requested to watch as well.
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[Dr15Jones]

please test

[Dr15Jones]

+1

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs unless changes or unless it breaks tests. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @nclopezo, @smuzaffar

[cmsbuild]

+1
Tested at: d377fb9
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8978/41/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8978/41/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8978/41/summary.html

[mark-grimes]

I've been back porting this to 6_2_X_SLHC (#8985) and just want to check something.
If it is a SecondarySource then the

factory->activateTimeout(fileIter_->fileName());
factory->stagein(fileIter_->fileName());

commands are not run, whereas they were in the old way. Is this okay?

[mark-grimes]

Sorry, I see in the comment that you intentionally don't stage in. Nevermind.