fix: validate passive boundary ownership and expose Jet exchange

src/CelerisLab/body/jets.py

@@ -216,11 +216,8 @@ def surface_link_velocity(objects, cfg, flags, fluid_idx, dir, body_id, rx, ry,
     enabled = [obj for obj in objects if obj.surface_actuation is not None]
     if not enabled and not jets:
         return None, None
-    from ..lbm.descriptors import FLUID, SOLID
+    from .preprocess.flag_overlay import validate_cut_link_ownership
-    if np.any((flags[fluid_idx] & FLUID) == 0) or np.any(flags[fluid_idx] & SOLID):
+    validate_cut_link_ownership(flags, fluid_idx, dir, cfg.nq)
-        raise ValueError("Surface cut links must have active fluid receivers")
-    if len(np.unique(fluid_idx.astype(np.uint64)*cfg.nq+dir)) != len(dir):
-        raise ValueError("Surface boundary requires unique cut-link writers")
     surface_velocity = np.zeros((len(dir), 2), np.float32)
     surface_aperture = np.zeros(len(dir), bool)
     for obj in enabled:

src/CelerisLab/body/manager.py

@@ -27,7 +27,7 @@ import pycuda.driver as cuda
 
 from .objects import SimObject
 from .registry import BodyRegistry
-from .preprocess.flag_overlay import merge_flag_masks
+from .preprocess.flag_overlay import merge_flag_masks, validate_cut_link_ownership
 from ..cuda.compiler_v2 import obs_layout
 from ..cuda.obs_layout import ObsLayout
 from ..lbm.descriptors import (
@@ -369,9 +369,9 @@ class ObjectManager:
 
         Args:
             objects: List of SimObject instances to process.
-            domain_flags: Optional uint16 flags for donor-cell classification.
+            domain_flags: Optional uint16 flags for receiver/donor classification.
             sensor_cell_counts_out: Int array to fill with per-object sensor
-                cell counts.  If provided, ``fill(0)`` is called first.
+                cell counts after the full geometry has passed validation.
             hits_out: List that receives one FP64 (n, 2) array of true hit
                 points row-aligned with the cut-link columns (for Jets).
 
@@ -394,8 +394,8 @@ class ObjectManager:
         active_fluid = ((domain_flags & FLUID) != 0) & ((domain_flags & SOLID) == 0)
         force_owners: set[int] = set()
 
-        if sensor_cell_counts_out is not None:
+        sensor_counts = (np.zeros_like(sensor_cell_counts_out)
-            sensor_cell_counts_out.fill(0)
+                         if sensor_cell_counts_out is not None else None)
 
         ez = None
         if self.cfg and self.cfg.is_d3q19:
@@ -430,9 +430,9 @@ class ObjectManager:
                 if len(cells) > 0:
                     s_cells.append(cells)
                     s_ids.append(ids)
-                    if (sensor_cell_counts_out is not None
+                    if (sensor_counts is not None
-                            and 0 <= obj.obj_id < sensor_cell_counts_out.size):
+                            and 0 <= obj.obj_id < sensor_counts.size):
-                        sensor_cell_counts_out[obj.obj_id] = int(len(cells))
+                        sensor_counts[obj.obj_id] = int(len(cells))
             if obj.is_force_region and hasattr(obj, 'get_sensor_list'):
                 cells, ids = obj.get_sensor_list(self.nx, self.ny, self.nz)
                 keep = active_fluid[cells]
@@ -461,8 +461,9 @@ class ObjectManager:
         r_fr_ids = np.concatenate(fr_ids) if fr_ids else np.zeros(0, dtype=np.int32)
 
         r_hit = np.concatenate(cl_hit) if cl_hit else np.zeros((0, 2))
-        if hits_out is not None:
+        # Passive walls need the same ownership guarantees as driven walls.
-            hits_out.append(r_hit)
+        # Overlapping solids are not a supported union-boundary operation.
+        validate_cut_link_ownership(domain_flags, r_idx, r_dir, self.nq)
         if self.jets or any(obj.surface_actuation is not None for obj in objects):
             # Reject unresolved apertures/ownership before sync or restart mutates state.
             from ..lbm.curved_links import CurvedLinkSoA
@@ -470,6 +471,11 @@ class ObjectManager:
             links.assign_host(r_idx, r_dir, r_q, r_bid, r_rx, r_ry, r_rz, r_fallback)
             links.assign_surface(objects, self.cfg, domain_flags, jets=self.jets, hits=r_hit)
 
+        if sensor_cell_counts_out is not None:
+            sensor_cell_counts_out[:] = sensor_counts
+        if hits_out is not None:
+            hits_out.append(r_hit)
+
         return (
             r_idx, r_dir, r_q, r_bid,
             r_rx, r_ry, r_rz, r_fallback,
@@ -487,8 +493,6 @@ class ObjectManager:
                 the caller has already prepared and uploaded the final flag field
                 for this geometry, such as during initialization.
         """
-        self._telemetry_field = field
-
         next_flags = self.build_flags(field.build_channel_flags()) if rebuild_flags else field.flag
         hits = []
 
@@ -499,6 +503,7 @@ class ObjectManager:
             fr_cells, fr_obj_id,
         ) = self.build_compact_lists(domain_flags=next_flags, hits_out=hits)
 
+        self._telemetry_field = field
         if rebuild_flags:
             field.flag = next_flags
             field.upload_flags()

src/CelerisLab/body/preprocess/flag_overlay.py

@@ -1,6 +1,6 @@
 # CelerisLab/body/preprocess/flag_overlay.py
 """
-Flag overlay: merge per-object flag masks onto a clean domain base.
+Flag overlay and cut-link ownership checks against the merged domain.
 """
 
 from __future__ import annotations
@@ -12,6 +12,15 @@ import numpy as np
 from ...lbm.descriptors import FLUID, SOLID, SENSOR_FLAG, FRC_REGION
 
 
+def validate_cut_link_ownership(flags, fluid_idx, directions, nq):
+    """Reject solid receivers and multiple writers to one population."""
+    if np.any((flags[fluid_idx] & FLUID) == 0) or np.any(flags[fluid_idx] & SOLID):
+        raise ValueError("Cut links must have active fluid receivers")
+    writers = fluid_idx.astype(np.uint64) * nq + directions
+    if len(np.unique(writers)) != len(directions):
+        raise ValueError("Boundary requires unique cut-link writers")
+
+
 def merge_flag_masks(
     base: np.ndarray,
     masks: List[np.ndarray],

src/CelerisLab/simulation.py

@@ -336,13 +336,19 @@ class Simulation:
     def read_surface_exchange(self, id: int):
         """Latest step's wall/aperture (Fx,Fy,Tz) exchange, not total nozzle force.
 
+        Supports SurfaceActuation and Jet-driven bodies, including zero-amplitude
+        apertures. Passive bodies without an aperture are rejected.
         Unlike read_body this is not accumulated/time-averaged. It resets after
         geometry sync or checkpoint load; advance before interpreting it.
         """
-        if self.bodies.get(id).surface_actuation is None:
+        obj = self.bodies.get(id)
+        curved = self.field.curved
+        has_aperture = (curved.aperture is not None
+                        and np.any(curved.aperture & (curved.body_id == id)))
+        if obj.surface_actuation is None and not has_aperture:
             raise ValueError("Body has no surface actuation")
         self.stream.synchronize()
-        return self.field.curved.surface_exchange(id)
+        return curved.surface_exchange(id)
 
     @context_bound
     def read_body(self, id: int, *,
@@ -451,18 +457,21 @@ class Simulation:
     @context_bound
     def initialize(self):
         """Initialize flow field and sync objects to GPU."""
-        # Recompile if objects were added after construction.
         self._assert_allocation_contract()
+        # Validate the complete geometry before flags, populations or modules
+        # change. Compact-list construction publishes no outputs on failure.
+        next_flags = self.bodies.build_flags(self.field.build_channel_flags())
+        if self.bodies.jets and self.bodies.count == 0:
+            raise ValueError("Jets need bodies with cut links")
+        self.bodies.build_compact_lists_for(self.bodies.objects, next_flags)
+
+        # Recompile if objects were added after construction.
         if self._read_generated_n_objects() != self.bodies.count:
             self.recompile()
 
-        self.field.flag = self.field.build_channel_flags()
+        self.field.flag = next_flags
-        if self.bodies.count > 0:
-            self.field.flag = self.bodies.build_flags(self.field.flag)
         self.field.upload_flags()
 
-        if self.bodies.jets and self.bodies.count == 0:
-            raise ValueError("Jets need bodies with cut links")
         self.stepper.initialize()
         if self.bodies.count > 0:
             self.bodies.sync_to_gpu(self.field, rebuild_flags=False)

tests/unit/test_jets.py

@@ -188,6 +188,55 @@ def jet_case(device, jets):
     return field, bodies
 
 
+def test_public_surface_exchange_for_jet_preserves_aperture_at_zero_and_reverse(device):
+    from CelerisLab.simulation import Simulation
+    jet = Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER, inward=False), "quartic", 2e-3)
+    field, bodies = jet_case(device, [jet])
+    sim = SimpleNamespace(bodies=bodies, field=field, _initialized=True,
+                          stream=SimpleNamespace(synchronize=device.synchronize))
+    cl = field.curved
+    assert bodies.get(0).surface_actuation is None
+    aperture, velocity = cl.aperture, cl.surface_velocity.copy()
+    assert aperture.any() and (~aperture).any()
+    # Synthetic per-link exchanges test readback/splitting, not kernel physics.
+    values = np.arange(cl.count * 3, dtype=np.float32).reshape(-1, 3)
+    device.htod(cl.link_exchange_gpu, values)
+    for amplitude in (2e-3, 0., -1e-3):
+        Simulation.set_jet_amplitude(sim, 0, amplitude)
+        result = Simulation.read_surface_exchange(sim, 0)
+        assert cl.aperture is aperture
+        np.testing.assert_array_equal(aperture, cl.jet_id >= 0)
+        np.testing.assert_allclose(cl.surface_velocity, velocity * (amplitude / 2e-3))
+        for name, mask in (("wall", ~aperture), ("aperture", aperture)):
+            np.testing.assert_array_equal(result[name], values[mask].sum(axis=0, dtype=np.float64))
+
+
+def test_public_surface_exchange_keeps_legacy_and_passive_guards(device):
+    from CelerisLab.simulation import Simulation
+    field = make_field(device)
+    bodies = manager(field, [circle(SurfaceActuation(0., .6, .002), radius=2.)])
+    bodies.sync_to_gpu(field)
+    sim = SimpleNamespace(bodies=bodies, field=field,
+                          stream=SimpleNamespace(synchronize=device.synchronize))
+    for value in Simulation.read_surface_exchange(sim, 0).values():
+        np.testing.assert_array_equal(value, 0.)
+    field.curved.free_gpu_columns()
+    with pytest.raises(RuntimeError, match="diagnostic is allocated"):
+        Simulation.read_surface_exchange(sim, 0)
+
+    # An exchange buffer elsewhere in the scene must not admit a passive body.
+    for jets in ([], [Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER))]):
+        field, bodies = jet_case(device, jets)
+        passive = bodies.add(circle(center=(2., 3.), radius=.6))
+        bodies.sync_to_gpu(field)
+        sim.bodies, sim.field = bodies, field
+        with pytest.raises(ValueError, match="Body has no surface actuation"):
+            Simulation.read_surface_exchange(sim, passive)
+        if not jets:
+            with pytest.raises(ValueError, match="Body has no surface actuation"):
+                Simulation.read_surface_exchange(sim, 0)
+
+
 def test_manager_passes_true_hits_and_reuploads_only_velocity(device):
     # CPU memory double only: checks host/device bookkeeping, not GPU numerics.
     jet = Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER, inward=False), "quartic", 2e-3)

tests/unit/test_state_regressions.py

@@ -238,6 +238,81 @@ def test_overlapping_force_regions_rejected(device):
         bodies.build_compact_lists(bodies.build_flags(field.flag))
 
 
+@pytest.mark.parametrize("second_x,error", [(14., "unique cut-link writers"),
+                                           (17., "active fluid receivers")])
+def test_passive_overlap_rejected_before_publishing_lists(second_x, error):
+    cfg = LBMConfig(nx=32, ny=24)
+    cfg.validate()
+    bodies = ObjectManager(cfg.nx, cfg.ny, cfg.nz, cfg.nq, cfg)
+    for x in (14., second_x):
+        bodies.add(obj(x=x, y=12., r=4.3))
+    bodies.add(obj(x=4., y=4., r=2., is_sensor=True))
+    counts = np.full(bodies.count, 91, np.int32)
+    hits = []
+    flags = bodies.build_flags(np.full(cfg.nx * cfg.ny, FLUID, np.uint16))
+    for domain_flags in (flags, None):
+        with pytest.raises(ValueError, match=error):
+            bodies.build_compact_lists_for(bodies.objects, domain_flags,
+                                          sensor_cell_counts_out=counts, hits_out=hits)
+        np.testing.assert_array_equal(counts, 91)
+        assert hits == []
+
+
+@pytest.mark.parametrize("centers", [((10., 12.), (22., 12.)),
+                                    ((10.5, 12.), (19.5, 12.))])
+def test_separated_passive_bodies_keep_valid_writers(centers):
+    # The second case has a subcell physical gap with a resolved fluid column.
+    cfg = LBMConfig(nx=32, ny=24)
+    cfg.validate()
+    bodies = ObjectManager(cfg.nx, cfg.ny, cfg.nz, cfg.nq, cfg)
+    for x, y in centers:
+        bodies.add(obj(x=x, y=y, r=4.3))
+    flags = bodies.build_flags(np.full(cfg.nx * cfg.ny, FLUID, np.uint16))
+    indices, directions, _, ids, *_ = bodies.build_compact_lists(flags)
+    assert set(ids) == {0, 1}
+    assert np.all(flags[indices] & FLUID) and not np.any(flags[indices] & SOLID)
+    assert len(np.unique(indices.astype(np.uint64) * cfg.nq + directions)) == len(indices)
+
+
+@pytest.mark.parametrize("entry", ["sync", "initialize", "runtime_edit"])
+@pytest.mark.parametrize("second_x", [5., 6.])
+def test_passive_overlap_failure_keeps_live_state(device, entry, second_x):
+    from CelerisLab.simulation import Simulation
+    field = make_field(device)
+    bodies = manager(field, [obj(), obj(x=9., y=3., r=1., is_sensor=True)])
+    bodies.sync_to_gpu(field)
+    overlapping = obj(x=second_x)
+    if entry == "runtime_edit":
+        bodies.stage_add(overlapping)
+    else:
+        bodies.add(overlapping)
+    objects = bodies.objects
+    flags, curved, counts = field.flag.copy(), field.curved.fluid_idx.copy(), bodies.sensor_cell_counts.copy()
+    action = bodies.action.copy()
+    buffers = [(block, bytes(block.data), block.freed) for block in device.blocks]
+    writes = device.writes
+    sim = SimpleNamespace(bodies=bodies, field=field, lbm_cfg=field.cfg,
+                          _initialized=entry == "runtime_edit",
+                          _assert_allocation_contract=lambda: None,
+                          stream=SimpleNamespace(synchronize=device.synchronize))
+    with pytest.raises(ValueError, match="active fluid receivers|unique cut-link writers"):
+        if entry == "sync":
+            bodies.sync_to_gpu(field)
+        elif entry == "initialize":
+            Simulation.initialize(sim)
+        else:
+            Simulation.sync_bodies(sim)
+    assert bodies.objects == objects
+    assert bodies.has_pending_edit() == (entry == "runtime_edit")
+    assert sim._initialized == (entry == "runtime_edit")
+    assert device.writes == writes and len(device.blocks) == len(buffers)
+    assert all(bytes(block.data) == data and block.freed == freed for block, data, freed in buffers)
+    np.testing.assert_array_equal(field.flag, flags)
+    np.testing.assert_array_equal(field.curved.fluid_idx, curved)
+    np.testing.assert_array_equal(bodies.sensor_cell_counts, counts)
+    np.testing.assert_array_equal(bodies.action, action)
+
+
 def test_action_upload_and_scratch_are_pinned(device):
     field = make_field(device, streaming="esopull")
     bodies = manager(field, [obj(is_force_region=True)])