Bug: _vert_id tag type mismatch in deriveMdlFromManifold/derive2DMdlFromManifold (mds/apfMDS.cc)

[cekees]

Both deriveMdlFromManifold (3D) and derive2DMdlFromManifold (2D) create the internal _vert_id mesh tag as a long tag and populate it with global vertex IDs:

apf::MeshTag* vIDTag = mesh->createLongTag("_vert_id", 1);
mesh->setLongTag(vit->second, vIDTag, &(vit->first));

Each of these functions then calls a private helper — getFaceIdInRegion (from deriveMdlFromManifold) or getEdgeIdInFace (from derive2DMdlFromManifold) — to identify which face/edge of a boundary element is being classified. Both helpers read the same _vert_id tag back, but declare the destination as int and call getIntTag:

int vID;
mesh->getIntTag(verts[0], vIDTag, &vID);

On any platform where long and int have different widths (i.e. every common 64-bit LP64 target), getIntTag reads a mismatched-width value out of a tag that was stored as long, so vID comes back as garbage. The vertex-ID comparisons that follow (if (vID != bface_data[2] && ...)) then never match for any vertex, and both helpers fall through to their explicit last-resort return value — which the source itself labels as a bug:

return 12; // Should give segmentation fault

That sentinel (12 for the face case, out-of-range for the up-to-4-entry apf::Downward array it indexes into afterward) produces a garbage MeshEntity*, which crashes on the subsequent setIntTag() call. This reproduces reliably and identically on both the 2.2.9 and 4.1.0 tags — the two getIntTag/createLongTag sites have been present unchanged across that entire range.

Fix: read the tag with the same type it was written with (getLongTag instead of getIntTag, long locals instead of int).

--- a/mds/apfMDS.cc
+++ b/mds/apfMDS.cc
@@ -56,20 +56,20 @@ static int getFaceIdInRegion(apf::Mesh* mesh, apf::MeshEntity* region,
 {
   apf::Downward verts;
   apf::MeshTag* vIDTag = mesh->findTag("_vert_id");
-  int vID;
+  long vID;
   mesh->getDownward(region, 0, verts);
   // Go through all vertices. What vertex is not on the face can be used to determine the face id.
   // TODO: Good way to assert that the rest of the 3 actually exist?
-  mesh->getIntTag(verts[0], vIDTag, &vID);
+  mesh->getLongTag(verts[0], vIDTag, &vID);
   if (vID != bface_data[2] && vID != bface_data[3] && vID != bface_data[4])
     return 2;
-  mesh->getIntTag(verts[1], vIDTag, &vID);
+  mesh->getLongTag(verts[1], vIDTag, &vID);
   if (vID != bface_data[2] && vID != bface_data[3] && vID != bface_data[4])
     return 3;
-  mesh->getIntTag(verts[2], vIDTag, &vID);
+  mesh->getLongTag(verts[2], vIDTag, &vID);
   if (vID != bface_data[2] && vID != bface_data[3] && vID != bface_data[4])
     return 1;
-  mesh->getIntTag(verts[3], vIDTag, &vID);
+  mesh->getLongTag(verts[3], vIDTag, &vID);
   if (vID != bface_data[2] && vID != bface_data[3] && vID != bface_data[4])
     return 0;
   return 12; // Should give segmentation fault
@@ -80,12 +80,13 @@ static int getEdgeIdInFace(apf::Mesh* mesh, apf::MeshEntity* face,
 {
   apf::Downward verts, edges;
   apf::MeshTag* vIDTag = mesh->findTag("_vert_id");
-  int vID[2], eID;
+  long vID[2];
+  int eID;
   mesh->getDownward(face, 1, edges);
   for (eID = 0; eID < 3; ++eID) {
     mesh->getDownward(edges[eID], 0, verts);
-    mesh->getIntTag(verts[0], vIDTag, &vID[0]);
-    mesh->getIntTag(verts[1], vIDTag, &vID[1]);
+    mesh->getLongTag(verts[0], vIDTag, &vID[0]);
+    mesh->getLongTag(verts[1], vIDTag, &vID[1]);
     if((vID[0] == bedge_data[2] && vID[1] == bedge_data[3]) ||
        (vID[0] == bedge_data[3] && vID[1] == bedge_data[2])) {
       return eID;

Confirmed present identically on tags 2.2.9 and 4.1.0; likely affects everything in between.