fix: Make SeedingPerformanceWriter thread-safe (acts-project#3850)

Not sure if the lock got lost by accident at some point or if it was never there. But the lock is definitely necessary to make the performance writer thread-safe.

Examples/Io/Root/src/SeedingPerformanceWriter.cpp

@@ -13,7 +13,6 @@
 #include "ActsExamples/Utilities/EventDataTransforms.hpp"
 #include "ActsExamples/Validation/TrackClassification.hpp"
 #include "ActsFatras/EventData/Barcode.hpp"
-#include "ActsFatras/EventData/Particle.hpp"
 
 #include <cstddef>
 #include <ostream>
@@ -28,10 +27,6 @@
 using Acts::VectorHelpers::eta;
 using Acts::VectorHelpers::phi;
 
-namespace ActsExamples {
-struct AlgorithmContext;
-}  // namespace ActsExamples
-
 ActsExamples::SeedingPerformanceWriter::SeedingPerformanceWriter(
     ActsExamples::SeedingPerformanceWriter::Config config,
     Acts::Logging::Level level)
@@ -127,6 +122,9 @@ ActsExamples::ProcessCode ActsExamples::SeedingPerformanceWriter::writeT(
   const auto& particles = m_inputParticles(ctx);
   const auto& hitParticlesMap = m_inputMeasurementParticlesMap(ctx);
 
+  // Exclusive access to the tree while writing
+  std::lock_guard<std::mutex> lock(m_writeMutex);
+
   std::size_t nSeeds = seeds.size();
   std::size_t nMatchedSeeds = 0;
   // Map from particles to how many times they were successfully found by a seed