Fix LBWalberla ghost communication

src/core/lb/LBWalberla.cpp

@@ -59,7 +59,7 @@ void LBWalberla::propagate() { lb_fluid->integrate(); }
 void LBWalberla::ghost_communication() { lb_fluid->ghost_communication(); }
 
 void LBWalberla::ghost_communication_pdf() {
-  lb_fluid->ghost_communication_vel();
+  lb_fluid->ghost_communication_pdf();
 }
 
 void LBWalberla::ghost_communication_vel() {

src/core/unit_tests/lb_particle_coupling_test.cpp

@@ -521,6 +521,61 @@ BOOST_DATA_TEST_CASE_F(CleanupActorLB, coupling_particle_lattice_ia,
   }
 }
 
+BOOST_FIXTURE_TEST_CASE(ghost_communication_pdf_flushes_density,
+                        CleanupActorLB) {
+  auto &lb = espresso::system->lb;
+  auto const &lattice = espresso::lb_fluid->get_lattice();
+  auto const &grid = params.grid_dimensions;
+  auto const gl = static_cast<int>(lattice.get_ghost_layers());
+
+  // distinct, node-dependent density (differs from the initial params.density)
+  auto fold = [](Utils::Vector3i n) {
+    for (auto i = 0u; i < 3u; ++i) {
+      if (n[i] < 0)
+        n[i] += grid[i];
+      else if (n[i] >= grid[i])
+        n[i] -= grid[i];
+    }
+    return n;
+  };
+  auto node_density = [&fold](Utils::Vector3i const &node) {
+    auto const f = fold(node);
+    return 2. + 1e-3 * static_cast<double>(f[0] + 17 * f[1] + 311 * f[2]);
+  };
+
+  // Write a distinct density on every locally-owned node. This sets ONLY the
+  // GhostComm::PDF pending flag; GhostComm::VEL stays clear.
+  for (auto x = -gl; x < grid[0] + gl; ++x) {
+    for (auto y = -gl; y < grid[1] + gl; ++y) {
+      for (auto z = -gl; z < grid[2] + gl; ++z) {
+        auto const node = Utils::Vector3i{x, y, z};
+        if (lattice.node_in_local_domain(node)) {
+          BOOST_REQUIRE(
+              espresso::lb_fluid->set_node_density(node, node_density(node)));
+        }
+      }
+    }
+  }
+
+  // synchronize the PDF ghost layer through the core wrapper
+  lb.ghost_communication_pdf();
+
+  // read local halo and check the PDF ghost layer was correctly updated
+  for (auto x = -gl; x < grid[0] + gl; ++x) {
+    for (auto y = -gl; y < grid[1] + gl; ++y) {
+      for (auto z = -gl; z < grid[2] + gl; ++z) {
+        auto const node = Utils::Vector3i{x, y, z};
+        if (lattice.node_in_local_halo(node) and
+            not lattice.node_in_local_domain(node)) {
+          auto const res = espresso::lb_fluid->get_node_density(node, true);
+          BOOST_REQUIRE(res);
+          BOOST_CHECK_CLOSE(*res, node_density(node), 1e-9);
+        }
+      }
+    }
+  }
+}
+
 BOOST_FIXTURE_TEST_CASE(runtime_exceptions, CleanupActorLB) {
   boost::mpi::communicator world;
   auto &lb = espresso::system->lb;