Create the infrastructure to stop and restart with the pm_mesh switched on.

src/engine.c

@@ -6295,6 +6295,7 @@ void engine_struct_dump(struct engine *e, FILE *stream) {
   phys_const_struct_dump(e->physical_constants, stream);
   hydro_props_struct_dump(e->hydro_properties, stream);
   gravity_props_struct_dump(e->gravity_properties, stream);
+  pm_mesh_struct_dump(e->mesh, stream);
   potential_struct_dump(e->external_potential, stream);
   cooling_struct_dump(e->cooling_func, stream);
   chemistry_struct_dump(e->chemistry, stream);
@@ -6364,6 +6365,10 @@ void engine_struct_restore(struct engine *e, FILE *stream) {
   gravity_props_struct_restore(gravity_properties, stream);
   e->gravity_properties = gravity_properties;
 
+  struct pm_mesh *mesh = (struct pm_mesh *)malloc(sizeof(struct pm_mesh));
+  pm_mesh_struct_restore(mesh, stream);
+  e->mesh = mesh;
+
   struct external_potential *external_potential =
       (struct external_potential *)malloc(sizeof(struct external_potential));
   potential_struct_restore(external_potential, stream);

src/gravity_properties.c

@@ -190,7 +190,7 @@ void gravity_props_struct_dump(const struct gravity_props *p, FILE *stream) {
  * @param p the struct
  * @param stream the file stream
  */
-void gravity_props_struct_restore(const struct gravity_props *p, FILE *stream) {
+void gravity_props_struct_restore(struct gravity_props *p, FILE *stream) {
   restart_read_blocks((void *)p, sizeof(struct gravity_props), 1, stream, NULL,
                       "gravity props");
 }

src/gravity_properties.h

@@ -27,10 +27,12 @@
 #endif
 
 /* Local includes. */
-#include "cosmology.h"
-#include "parser.h"
 #include "restart.h"
 
+/* Forward declarations */
+struct cosmology;
+struct swift_params;
+
 /**
  * @brief Contains all the constants and parameters of the self-gravity scheme
  */
@@ -96,6 +98,6 @@ void gravity_props_print_snapshot(hid_t h_grpsph,
 
 /* Dump/restore. */
 void gravity_props_struct_dump(const struct gravity_props *p, FILE *stream);
-void gravity_props_struct_restore(const struct gravity_props *p, FILE *stream);
+void gravity_props_struct_restore(struct gravity_props *p, FILE *stream);
 
 #endif /* SWIFT_GRAVITY_PROPERTIES */

src/mesh_gravity.c

@@ -509,3 +509,39 @@ void pm_mesh_clean(struct pm_mesh* mesh) {
   if (mesh->potential) free(mesh->potential);
   mesh->potential = 0;
 }
+
+/**
+ * @brief Write a #pm_mesh struct to the given FILE as a stream of bytes.
+ *
+ * @param p the struct
+ * @param stream the file stream
+ */
+void pm_mesh_struct_dump(const struct pm_mesh* mesh, FILE* stream) {
+  restart_write_blocks((void*)mesh, sizeof(struct pm_mesh), 1, stream,
+                       "gravity", "gravity props");
+}
+
+/**
+ * @brief Restore a #pm_mesh struct from the given FILE as a stream of
+ * bytes.
+ *
+ * @param p the struct
+ * @param stream the file stream
+ */
+void pm_mesh_struct_restore(struct pm_mesh* mesh, FILE* stream) {
+
+  restart_read_blocks((void*)mesh, sizeof(struct pm_mesh), 1, stream, NULL,
+                      "gravity props");
+
+  const int N = mesh->N;
+
+  /* Allocate the memory for the combined density and potential array */
+  mesh->potential = (double*)fftw_malloc(sizeof(double) * N * N * N);
+  if (mesh->potential == NULL)
+    error("Error allocating memory for the long-range gravity mesh.");
+
+#ifdef HAVE_FFTW
+#else
+  error("No FFTW library found. Cannot compute periodic long-range forces.");
+#endif
+}

src/mesh_gravity.h

@@ -24,6 +24,7 @@
 
 /* Local headers */
 #include "gravity_properties.h"
+#include "restart.h"
 
 /* Forward declarations */
 struct engine;
@@ -71,4 +72,8 @@ void pm_mesh_interpolate_forces(const struct pm_mesh *mesh,
                                 int gcount);
 void pm_mesh_clean(struct pm_mesh *mesh);
 
+/* Dump/restore. */
+void pm_mesh_struct_dump(const struct pm_mesh *p, FILE *stream);
+void pm_mesh_struct_restore(struct pm_mesh *p, FILE *stream);
+
 #endif /* SWIFT_MESH_GRAVITY_H */