Fix corner case in processingOrderMerge

Author: wddgit

DataFormats/Provenance/interface/processingOrderMerge.h

@@ -19,4 +19,4 @@ namespace edm {
      */
   void processingOrderMerge(std::vector<std::string> const& iFrom, std::vector<std::string>& iTo);
 }  // namespace edm
-#endif
+#endif

DataFormats/Provenance/src/processingOrderMerge.cc

@@ -12,6 +12,7 @@ namespace edm {
     template <typename Iter>
     Iter foundLaterIn(Iter itBegin, Iter it, Iter itEnd, std::string const& name) {
       auto itFind = std::find(itBegin, itEnd, name);
+      // Sanity check, it should be impossible for this to happen
       if (itFind < it) {
         throwIncompatibleOrdering("process " + name + " found earlier in other list");
       }
@@ -55,28 +56,17 @@ namespace edm {
           auto itFindNew = foundLaterIn(iHistory.begin(), itNew, itNewEnd, *itOld);
           if (itFindOld != itOldEnd) {
             if (itFindNew != itNewEnd) {
-              throwIncompatibleOrdering("process " + *itNew + " and " + *itOld + " are out of order");
-            }
-            //found it, copy over everything up to and including it
-            while (itOld != itFindOld) {
-              tempNames.push_back(*itOld);
-              ++itOld;
+              throwIncompatibleOrdering("order of processes " + *itNew + " and " + *itOld +
+                                        " is not the same in all ProcessHistories");
             }
             tempNames.push_back(*itOld);
             ++itOld;
-            ++itNew;
           } else {
             if (itFindNew == itNewEnd) {
-              throwIncompatibleOrdering("process " + *itOld + " and " + *itNew + " are independent");
+              throwIncompatibleOrdering("order of processes " + *itOld + " and " + *itNew + " is ambiguous");
             }
-            while (itNew != itFindNew) {
-              tempNames.push_back(*itNew);
-              ++itNew;
-            }
-            //not found, add the new one
             tempNames.push_back(*itNew);
             ++itNew;
-            ++itOld;
           }
         }
       }

DataFormats/Provenance/test/processingOrderMerge_t.cpp

@@ -167,6 +167,13 @@ TEST_CASE("processingOrderMerge", "[processingOrderMerge]") {
         std::vector<std::string> names = {"D", "B", "C", "A"};
         REQUIRE_THROWS_AS(edm::processingOrderMerge(ph, names), cms::Exception);
       }
+      SECTION("reverse order with an extra one in front") {
+        edm::ProcessHistory ph;
+        ph.emplace_back("C", edm::ReleaseVersion(""), edm::HardwareResourcesDescription());
+        ph.emplace_back("A", edm::ReleaseVersion(""), edm::HardwareResourcesDescription());
+        std::vector<std::string> names = {"B", "C", "A"};
+        REQUIRE_THROWS_AS(edm::processingOrderMerge(ph, names), cms::Exception);
+      }
     }
   }
-}
+}