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FakeBkgTools: bug fix for imported efficiency systematic uncertainties

Julien Maurer requested to merge jmaurer/athena:b21_2-TrigGlobEffCorrTool-fix-210420 into 21.2

In the recent !31259 (merged), a small bug was introduced (credits to @evarnes for identifying it) resulting in unwanted behaviour when loading systematic uncertainties in the efficiencies from ROOT files.

In principle this can lead in some cases to incorrect values of uncertainties being used; a non-exhaustive list of such circumstances being:

  • when setting bin content = desired systematic, and error bar != 0, and error bar != nominal histogram's error bar;
  • when setting bin error = desired systematic;

the former is not very likely to happen (no reason to do that), while the latter should only concern a small fraction of users as this possibility is a recent addition (it was in fact the purpose of !31259 (merged)).

For the record, the logic proposed in !31359 (merged) is correct and had been tested; the bug was in fact accidentally introduced at the end of the MR review, when replacing the strict comparisons by looser comparisons of floating-point numbers as suggested, explaining why this escaped scrutiny.

Tagging @lcorpe as he's probably among those affected!

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