Fail read calls after exception. (same as #12590)

[slava77]

If a read call was performed after XrdFile threw an exception, it
is possible there are no active sources. However, not all paths
check for the case of zero sources and may use-after-free the last
source.

This caused a deadlock at the Tier-0.

[slava77]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10005/console

[cmsbuild]

A new Pull Request was created by @slava77 (Slava Krutelyov) for CMSSW_7_5_X.

It involves the following packages:

Utilities/XrdAdaptor

@cmsbuild, @smuzaffar, @Dr15Jones, @davidlange6 can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

Following commands in first line of a comment are recognized

• +1|approve[d]|sign[ed]: L1/L2's to approve it
• -1|reject[ed]: L1/L2's to reject it
• assign <category>[,<category>[,...]]: L1/L2's to request signatures from other categories
• unassign <category>[,<category>[,...]]: L1/L2's to remove signatures from other categories
• hold: L1/all L2's/release manager to mark it as on hold
• unhold: L1/user who put this PR on hold
• merge: L1/release managers to merge this request
• [@cmsbuild,] please test: L1/L2 and selected users to start jenkins tests
• [@cmsbuild,] please test with cms-sw/cmsdist#<PR>: L1/L2 and selected users to start jenkins tests using externals from cmsdist

[cmsbuild]

+1
Tested at: 6bdea49
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12592/10005/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12592/10005/summary.html

[slava77]

merge

[Dr15Jones]

+1
I'm not completely thrilled with how exposed the error logic is at the moment and that m_activeSources.size() == 0 is an implicit error condition. Brian and I had a conversation but didn't come to any immediate better ideas.

[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. @slava77, @davidlange6, @Degano, @smuzaffar