Protect against empty weight container from pythia8

[mkirsano]

PR description:

Fix GeneratorInterface crash at the upgrade of pythia8 to 303

PR validation:

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[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-32077/19721

• This PR adds an extra 20KB to repository

[cmsbuild]

A new Pull Request was created by @mkirsano (Mikhail Kirsanov) for master.

It involves the following packages:

GeneratorInterface/Pythia8Interface

@SiewYan, @mkirsano, @cmsbuild, @GurpreetSinghChahal, @agrohsje, @alberto-sanchez can you please review it and eventually sign? Thanks.
@alberto-sanchez this is something you requested to watch as well.
@silviodonato, @dpiparo, @qliphy you are the release manager for this.

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[cmsbuild]

The tests are being triggered in jenkins.
Tested with other pull request(s) cms-sw/cmsdist#6286,#31816,cms-data/GeneratorInterface-EvtGenInterface#15

• CMSSW_11_2_X_2020-11-09-2300/slc7_amd64_gcc820: https://cmssdt.cern.ch/jenkins/job/ib-run-pr-tests/10612/console Started: 2020/11/10 13:29

[cmsbuild]

+1
Tested at: 8335138
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-44598f/10612/summary.html
CMSSW: CMSSW_11_2_X_2020-11-09-2300
SCRAM_ARCH: slc7_amd64_gcc820

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-44598f/10612/summary.html

Comparison Summary:

• No significant changes to the logs found
• ROOTFileChecks: Some differences in event products or their sizes found
• Reco comparison results: 43268 differences found in the comparisons
• DQMHistoTests: Total files compared: 35
• DQMHistoTests: Total histograms compared: 2527080
• DQMHistoTests: Total failures: 476269
• DQMHistoTests: Total nulls: 172
• DQMHistoTests: Total successes: 2050617
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: -1109.497 KiB( 34 files compared)
• DQMHistoSizes: changed ( 10224.0 ): 0.625 KiB SiStrip/MechanicalView
• DQMHistoSizes: changed ( 11634.0,... ): -555.274 KiB HLT/HLTEgammaValidation
• DQMHistoSizes: changed ( 250202.181 ): 0.543 KiB SiStrip/MechanicalView
• DQMHistoSizes: changed ( 25202.0 ): -0.117 KiB SiStrip/MechanicalView
• Checked 148 log files, 22 edm output root files, 35 DQM output files

[agrohsje]

Hi @mkirsano . Thanks for preparing this PR. Let me try to understand a little better. What was the example that crashed and why didn't this happen before? I am wondering if just filling the weight vector might cause problems upstream in production, hence my question.

[mkirsano]

With pythia8 303 any hadronization crashed if the pythia parameter nAttemps with value > 1 was used because if pythia8 works in the mode with only one weight, the weight container in HepMC record is empty. It was never empty in 302. May be a result of some optimization in pythia8. In principle, if there no multiple weights, this container is not needed, but this was not protected in CMSSW. I will ask the authors if this this a bug or a feature.

[silviodonato]

@cms-sw/generators-l2 is this PR ready?

[silviodonato]

(to be merged with cms-sw/cmsdist#6286, #31816)

[mkirsano]

For me yes. Steve reproduced the behaviour in a standalone pythia8. If it is fixed we can reconsider this protection for the next version.

[agrohsje]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[qliphy]

+1