Correctly handle errors in merging histograms with kAverage (#15189)

knopers8 · web-flow · commit 2e6b8722d2d4 · 2026-03-19T08:16:45.000+01:00
As pointed out by Felix Schlepper, TH1::Add reports errors differently than Merge, see https://root.cern.ch/doc/master/classTH1.html#a6e3008f571628f0c9d17d754c8b88730 Fixes QC-1341.
diff --git a/Utilities/Mergers/src/MergerAlgorithm.cxx b/Utilities/Mergers/src/MergerAlgorithm.cxx
@@ -111,7 +111,7 @@ Long64_t mergeDefault(TObject* const target, TObject* const other)
       // Merge() does not support averages, we have to use Add()
       // this will break if collection.size != 1
       if (auto otherTH1 = dynamic_cast<TH1*>(otherCollection.First())) {
-        errorCode = targetTH1->Add(otherTH1);
+        errorCode = targetTH1->Add(otherTH1) == kFALSE ? -1 : 0;
       }
     } else {
       // Add() does not support histograms with labels, thus we resort to Merge() by default

Original file line number	Diff line number	Diff line change
`@@ -111,7 +111,7 @@ Long64_t mergeDefault(TObject* const target, TObject* const other)`
`111`	`111`	`// Merge() does not support averages, we have to use Add()`
`112`	`112`	`// this will break if collection.size != 1`
`113`	`113`	`if (auto otherTH1 = dynamic_cast<TH1*>(otherCollection.First())) {`
`114`		`- errorCode = targetTH1->Add(otherTH1);`
	`114`	`+ errorCode = targetTH1->Add(otherTH1) == kFALSE ? -1 : 0;`
`115`	`115`	`}`
`116`	`116`	`} else {`
`117`	`117`	`// Add() does not support histograms with labels, thus we resort to Merge() by default`