[BUG] - Fix three stale-cache bugs after swarm particle addition

src/underworld3/checkpoint/disk_snapshot.py

@@ -689,11 +689,7 @@ def _read_swarm_from_sidecar(swarm, sidecar_path: str) -> None:
 
     # Invalidate canonical-data caches so subsequent var.data reads
     # re-resolve to the rebuilt PETSc fields.
-    if hasattr(swarm._particle_coordinates, "_canonical_data"):
-        swarm._particle_coordinates._canonical_data = None
-    for var in swarm._vars.values():
-        if hasattr(var, "_canonical_data"):
-            var._canonical_data = None
+    swarm._invalidate_canonical_data()
 
     # Restore counted as a population change (matches in-memory path).
     swarm._population_generation += 1

src/underworld3/function/_function.pyx

@@ -610,7 +610,9 @@ def _project_to_work_variable(expr, mesh, smoothing=1e-6):
         )
         projector.uw_function = flat_source
         projector.smoothing = smoothing
-        projector.solve(zero_init_guess=False)
+        # _force_setup=True: rebuild solver state to avoid stale cached
+        # projector after Stokes/DM modifications (issue #215, Bug 2).
+        projector.solve(zero_init_guess=False, _force_setup=True)
 
         # Fan flat result back to the tensor work variable
         for idx in range(n_components):
@@ -637,7 +639,9 @@ def _project_to_work_variable(expr, mesh, smoothing=1e-6):
 
     projector.uw_function = scalar_expr
     projector.smoothing = smoothing
-    projector.solve(zero_init_guess=False)
+    # _force_setup=True: rebuild solver state to avoid stale cached
+    # projector after Stokes/DM modifications (issue #215, Bug 2).
+    projector.solve(zero_init_guess=False, _force_setup=True)
 
     return work_var
 

src/underworld3/swarm.py

@@ -1031,6 +1031,12 @@ def _update(self):
 
         return
 
+    # TODO(BUG): Stale proxy DM after swarm data write
+    # _update() marks proxy as stale, but _update_proxy_if_stale() (lazy
+    # re-interpolation) only fires when material.sym is accessed. Code that
+    # reads the proxy MeshVariable DM directly (e.g. a Projection solver
+    # evaluating its uw_function at quadrature points) gets stale data.
+    # See GitHub issue #215 (Bug 3).
     def _update_proxy_if_stale(self):
         """
         Actually update the proxy mesh variable if it's marked as stale.
@@ -2643,6 +2649,9 @@ def _invalidate_canonical_data(self):
         for var in list(self._vars.values()):
             if hasattr(var, "_canonical_data"):
                 var._canonical_data = None
+            # Mark proxy as stale — particle positions or values changed
+            if hasattr(var, "_update"):
+                var._update()
 
         # Invalidate cached spatial index
         self._kdtree = None
@@ -3571,10 +3580,7 @@ def add_particles_with_coordinates(self, coordinatesArray) -> int:
         self.dm.migrate(remove_sent_points=True)
 
         # Invalidate cached data — particle count changed after addNPoints + migrate
-        self._particle_coordinates._canonical_data = None
-        for var in self._vars.values():
-            if hasattr(var, "_canonical_data"):
-                var._canonical_data = None
+        self._invalidate_canonical_data()
 
         # Informational: addNPoints + direct dm.migrate path doesn't go
         # through Swarm.migrate, so bump explicitly.
@@ -4204,11 +4210,7 @@ def apply_snapshot_payload(self, payload: dict) -> None:
 
         # Invalidate canonical-data caches — the underlying arrays
         # have been reallocated by the addNPoints path.
-        if hasattr(self._particle_coordinates, "_canonical_data"):
-            self._particle_coordinates._canonical_data = None
-        for var in self._vars.values():
-            if hasattr(var, "_canonical_data"):
-                var._canonical_data = None
+        self._invalidate_canonical_data()
 
         # The raw PETSc primitives used above (removePoint loop +
         # addNPoints + direct field writes) deliberately bypass