Fix swarm cache invalidation after parallel migration

src/underworld3/function/_function.pyx

@@ -502,6 +502,12 @@ def global_evaluate_nd(   expr,
     evaluation_swarm.dm.migrate(remove_sent_points=True)
     uw.mpi.barrier()
 
+    # Invalidate cached data after bare-bones dm.migrate —
+    # particle count and values changed but Swarm.migrate() was bypassed.
+    evaluation_swarm._particle_coordinates._canonical_data = None
+    for var in evaluation_swarm._vars.values():
+        if hasattr(var, "_canonical_data"):
+            var._canonical_data = None
 
     index = original_index.array[:,0,0]
 

src/underworld3/swarm.py

@@ -3315,14 +3315,13 @@ def migrate(
                 for index in indices:
                     self.dm.removePointAtIndex(index)
 
-                # CRITICAL FIX: Invalidate cached data after removing particles
-                # The _particle_coordinates variable caches data - must refresh after DM changes
-                self._particle_coordinates._canonical_data = None
-
-                # Also invalidate caches for all swarm variables
-                for var in self._vars.values():
-                    if hasattr(var, "_canonical_data"):
-                        var._canonical_data = None
+        # Invalidate all cached data after migration.
+        # Any particle movement (send, receive, or balanced swap) makes
+        # cached arrays stale — both size and values may have changed.
+        self._particle_coordinates._canonical_data = None
+        for var in self._vars.values():
+            if hasattr(var, "_canonical_data"):
+                var._canonical_data = None
 
         return
 

tests/parallel/test_0760_swarm_cache_migration.py

@@ -0,0 +1,103 @@
+"""
+Regression test for swarm cache invalidation after migration.
+
+Verifies that SwarmVariable._canonical_data caches are properly invalidated
+after Swarm.migrate() and after bare-bones dm.migrate() in global_evaluate.
+
+Bug: SwarmVariable caches were only invalidated inside the delete_lost_points
+branch of Swarm.migrate(), so caches became stale when particles moved between
+ranks without deletion. This caused shape mismatches in global_evaluate.
+
+See: https://github.com/underworldcode/underworld3/issues/64
+
+Run with:
+    mpirun -n 2 python -m pytest --with-mpi tests/parallel/test_0760_swarm_cache_migration.py
+"""
+
+import pytest
+import numpy as np
+import underworld3 as uw
+from mpi4py import MPI
+
+pytestmark = [pytest.mark.mpi(min_size=2), pytest.mark.timeout(60)]
+
+
+@pytest.mark.mpi(min_size=2)
+@pytest.mark.level_1
+@pytest.mark.tier_a
+def test_swarm_cache_valid_after_migration():
+    """Swarm variable caches must reflect actual particle count after migration."""
+    mesh = uw.meshing.UnstructuredSimplexBox(minCoords=(0.0, 0.0), maxCoords=(1.0, 1.0))
+    swarm = uw.swarm.Swarm(mesh)
+    var = uw.swarm.SwarmVariable("test_var", swarm, vtype=uw.VarType.SCALAR, _proxy=False)
+
+    # Add particles at random positions — distribution will be uneven across ranks
+    np.random.seed(42 + uw.mpi.rank)
+    coords = np.random.random((200, mesh.dim))
+    swarm.add_particles_with_global_coordinates(coords, migrate=False)
+    var.data[...] = uw.mpi.rank
+
+    pre_count = swarm.dm.getLocalSize()
+
+    # Migrate — particles move to owning rank
+    swarm.migrate(remove_sent_points=True, delete_lost_points=False)
+
+    post_count = swarm.dm.getLocalSize()
+    coords_cached = swarm._particle_coordinates.data.shape[0]
+    var_cached = var.data.shape[0]
+
+    # Cached sizes must match the actual DM particle count
+    assert coords_cached == post_count, (
+        f"Rank {uw.mpi.rank}: coordinate cache ({coords_cached}) != "
+        f"DM count ({post_count}) after migration"
+    )
+    assert var_cached == post_count, (
+        f"Rank {uw.mpi.rank}: variable cache ({var_cached}) != "
+        f"DM count ({post_count}) after migration"
+    )
+
+
+@pytest.mark.mpi(min_size=2)
+@pytest.mark.level_1
+@pytest.mark.tier_a
+def test_global_evaluate_after_migration():
+    """global_evaluate must succeed with coordinates that force heavy migration."""
+    mesh = uw.meshing.UnstructuredSimplexBox(minCoords=(0.0, 0.0), maxCoords=(1.0, 1.0))
+    v = uw.discretisation.MeshVariable("u", mesh, mesh.dim, degree=2)
+
+    # Bias coordinates to one side — forces cross-rank particle movement
+    np.random.seed(42)
+    N = 300
+    coords = np.random.random((N, mesh.dim))
+    coords[:, 0] = 0.5 + 0.5 * coords[:, 0]  # all in right half
+
+    result = uw.function.global_evaluate(v.sym, coords)
+
+    assert result.shape[0] == N, (
+        f"Rank {uw.mpi.rank}: expected {N} results, got {result.shape[0]}"
+    )
+
+
+@pytest.mark.mpi(min_size=2)
+@pytest.mark.level_1
+@pytest.mark.tier_a
+def test_global_evaluate_displaced_nodes():
+    """global_evaluate with displaced node coordinates (DDt/SemiLagrangian path)."""
+    mesh = uw.meshing.UnstructuredSimplexBox(minCoords=(0.0, 0.0), maxCoords=(1.0, 1.0))
+    v = uw.discretisation.MeshVariable("u", mesh, mesh.dim, degree=2)
+
+    # Displace node coordinates — simulates semi-Lagrangian departure points
+    node_coords = mesh.X.coords
+    np.random.seed(7)
+    displacement = np.random.random(node_coords.shape) * 0.3
+    mid_pt_coords = node_coords - displacement
+
+    # Clamp to domain
+    mid_pt_coords = np.clip(mid_pt_coords, 0.0, 1.0)
+
+    result = uw.function.evaluate(v.sym, mid_pt_coords)
+
+    assert result.shape[0] == node_coords.shape[0], (
+        f"Rank {uw.mpi.rank}: expected {node_coords.shape[0]} results, "
+        f"got {result.shape[0]}"
+    )