fix: Prevent unphysical number of phi bins (#1161) (#1167)

The bug described in issue #1161 is caused by the creation of negative or (near) infinite number of phi bins, which cause the internal mechanisms of `std::vector` to crash. This effect is caused by the fact that the number of phi neighbours is not properly initialized, causing it to take on uninitialized values.

This commit does two things to prevent this issue. First of all, it adds a sensible default value for the number of phi neighbours, ensuring that it never takes on uninitialized values. Secondly, a sanity check is added to the space point grid to ensure that the phi delta value has a physically meaningful value; if this is not the case, an exception is thrown.

Resolves #1161.

Core/include/Acts/Seeding/SpacePointGrid.hpp

@@ -38,7 +38,7 @@ struct SpacePointGridConfig {
   // maximum impact parameter in mm
   float impactMax;
   // sets of consecutive phi bins in the seed making step
-  int numPhiNeighbors;
+  int numPhiNeighbors = 0;
   // enable non equidistant binning in z
   std::vector<float> zBinEdges;
 

Core/include/Acts/Seeding/SpacePointGrid.ipp

@@ -58,6 +58,14 @@ Acts::SpacePointGridCreator::createGrid(
     float deltaPhi = (outerAngle - innerAngle + deltaAngleWithMaxD0) /
                      (2 * config.numPhiNeighbors + 1);
 
+    // sanity check: if the delta phi is equal to or less than zero, we'll be
+    // creating an infinite or a negative number of bins, which would be bad!
+    if (deltaPhi <= 0.f) {
+      throw std::domain_error(
+          "Delta phi value is equal to or less than zero, leading to an "
+          "impossible number of bins (negative or infinite)");
+    }
+
     // divide 2pi by angle delta to get number of phi-bins
     // size is always 2pi even for regions of interest
     phiBins = std::ceil(2 * M_PI / deltaPhi);