GeometrySplit functor isn't threadsafe (#1607)

### Briefly, what does this PR introduce?
Fixes issues with the initialization of the GeometrySplit functor across
threads.

Currently only the std::once_flag is a shared pointer and none of the
other variables which are being initialized. This meant that when copied
to other threads the is_init flag was set but the decoder and geometry
divisions had not been so the loop over the detector ids was always
empty. This resulted in an apparent variable decrease in the efficiency
of the Low-Q2 tagger depending on how many threads were asked for.

Unit tests for all of the meta factories and functors should be added as
an issue to try head this off in the future but outwith this fix.

### What kind of change does this PR introduce?
- [x] Bug fix (issue #__)
- [ ] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No
### Does this PR change default behavior?
All of the Low-Q2 Tagger hits will be converted into clusters rather
than just those run on the first thread to call the functor.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Dmitry Kalinkin <[email protected]>

src/algorithms/meta/CollectionCollector.h

@@ -42,6 +42,8 @@ namespace eicrecon {
           out_collection->push_back(hit);
         }
       }
+      //Log how many hits were collected from N input collections
+      this->debug("Collected {} hits from {} input collections", out_collection->size(), in_collections.size());
     }
 
   };

src/algorithms/meta/SubDivideCollection.h

@@ -56,6 +56,13 @@ namespace eicrecon {
 
           }
 
+          //Log how the hits were divided between the collection names
+          this->debug("Divided {} hits between {} collections", entries->size(), subdivided_entries.size());
+          //How many hits in each output collection
+          for (size_t i = 0; i < subdivided_entries.size(); i++) {
+            this->debug("Collection {} takes {} hits", i, subdivided_entries[i]->size());
+          }
+
         };
 
   };

src/algorithms/meta/SubDivideFunctors.h

@@ -40,7 +40,10 @@ class GeometrySplit {
 public:
 
     GeometrySplit(std::vector<std::vector<long int>> ids, std::string readout, std::vector<std::string> divisions)
-    : m_ids(ids), m_readout(readout), m_divisions(divisions){};
+    : m_ids(ids), m_readout(readout), m_divisions(divisions),
+      is_init(std::make_shared<std::once_flag>()),
+      m_id_dec(std::make_shared<dd4hep::DDSegmentation::BitFieldCoder*>()),
+      m_div_ids(std::make_shared<std::vector<size_t>>()) {};
 
     template <typename T>
     std::vector<int> operator()(T& instance) const {
@@ -51,9 +54,10 @@ class GeometrySplit {
         //Check which detector division to put the hit into
         auto cellID = instance.getCellID();
         std::vector<long int> det_ids;
-        for(auto d : m_div_ids){
-            det_ids.push_back(m_id_dec->get(cellID, d));
+        for(auto d : *m_div_ids){
+            det_ids.push_back((*m_id_dec)->get(cellID, d));
         }
+
         auto index = std::find(m_ids.begin(),m_ids.end(),det_ids);
 
         std::vector<int> ids;
@@ -66,19 +70,19 @@ class GeometrySplit {
 private:
 
     void init() const {
-        m_id_dec = algorithms::GeoSvc::instance().detector()->readout(m_readout).idSpec().decoder();
+        *m_id_dec = algorithms::GeoSvc::instance().detector()->readout(m_readout).idSpec().decoder();
         for (auto d : m_divisions){
-            m_div_ids.push_back(m_id_dec->index(d));
+            m_div_ids->push_back((*m_id_dec)->index(d));
         }
     }
 
     std::vector<std::vector<long int>> m_ids;
     std::vector<std::string> m_divisions;
     std::string m_readout;
 
-    mutable std::shared_ptr<std::once_flag> is_init = std::make_shared<std::once_flag>();
-    mutable dd4hep::DDSegmentation::BitFieldCoder* m_id_dec;
-    mutable std::vector<size_t> m_div_ids;
+    std::shared_ptr<std::once_flag> is_init;
+    std::shared_ptr<dd4hep::DDSegmentation::BitFieldCoder*> m_id_dec;
+    std::shared_ptr<std::vector<size_t>> m_div_ids;
 
 };
 

src/factories/meta/CollectionCollector_factory.h

@@ -35,6 +35,7 @@ namespace eicrecon {
         }
         typename T::collection_type* merged_collection = m_output().get();
         m_algo->process(in_collections, merged_collection);
+
     };
 
     };