Restored the old behaviour of the external gravity tasks.

src/cell.c

@@ -1401,8 +1401,13 @@ void cell_activate_sorts(struct cell *c, int sid, struct scheduler *s) {
 }
 
 /**
- * @brief Traverse a sub-cell task and activate the drift and sort tasks along
- * the way.
+ * @brief Traverse a sub-cell task and activate the hydro drift tasks that are
+ * required
+ * by a hydro task
+ *
+ * @param ci The first #cell we recurse in.
+ * @param cj The second #cell we recurse in.
+ * @param s The task #scheduler.
  */
 void cell_activate_subcell_tasks(struct cell *ci, struct cell *cj,
                                  struct scheduler *s) {
@@ -1670,7 +1675,13 @@ void cell_activate_subcell_tasks(struct cell *ci, struct cell *cj,
 }
 
 /**
- * @brief Traverse a sub-cell task and activate the gravity drift tasks
+ * @brief Traverse a sub-cell task and activate the gravity drift tasks that are
+ * required
+ * by a self gravity task.
+ *
+ * @param ci The first #cell we recurse in.
+ * @param cj The second #cell we recurse in.
+ * @param s The task #scheduler.
  */
 void cell_activate_subcell_grav_tasks(struct cell *ci, struct cell *cj,
                                       struct scheduler *s) {
@@ -1795,6 +1806,40 @@ void cell_activate_subcell_grav_tasks(struct cell *ci, struct cell *cj,
   }
 }
 
+/**
+ * @brief Traverse a sub-cell task and activate the gravity drift tasks that are
+ * required
+ * by an external gravity task.
+ *
+ * @param ci The #cell we recurse in.
+ * @param s The task #scheduler.
+ */
+void cell_activate_subcell_external_grav_tasks(struct cell *ci,
+                                               struct scheduler *s) {
+
+  /* Some constants */
+  const struct space *sp = s->space;
+  const struct engine *e = sp->e;
+
+  /* Do anything? */
+  if (!cell_is_active(ci, e)) return;
+
+  /* Recurse? */
+  if (ci->split) {
+
+    /* Loop over all progenies (no need for pairs for self-gravity) */
+    for (int j = 0; j < 8; j++) {
+      if (ci->progeny[j] != NULL) {
+        cell_activate_subcell_external_grav_tasks(ci->progeny[j], s);
+      }
+    }
+  } else {
+
+    /* We have reached the bottom of the tree: activate gpart drift */
+    cell_activate_drift_gpart(ci, s);
+  }
+}
+
 /**
  * @brief Un-skips all the tasks associated with a given cell and checks
  * if the space needs to be rebuilt.
@@ -1987,7 +2032,10 @@ int cell_unskip_tasks(struct cell *c, struct scheduler *s) {
       scheduler_activate(s, t);
 
       /* Set the drifting flags */
-      if (t->type == task_type_self) {
+      if (t->type == task_type_self &&
+          t->subtype == task_subtype_external_grav) {
+        cell_activate_subcell_external_grav_tasks(t->ci, s);
+      } else if (t->type == task_type_self && t->subtype == task_subtype_grav) {
         cell_activate_subcell_grav_tasks(t->ci, NULL, s);
       } else if (t->type == task_type_pair) {
         cell_activate_subcell_grav_tasks(t->ci, t->cj, s);

src/cell.h

@@ -408,6 +408,8 @@ void cell_activate_subcell_tasks(struct cell *ci, struct cell *cj,
                                  struct scheduler *s);
 void cell_activate_subcell_grav_tasks(struct cell *ci, struct cell *cj,
                                       struct scheduler *s);
+void cell_activate_subcell_external_grav_tasks(struct cell *ci,
+                                               struct scheduler *s);
 void cell_activate_drift_part(struct cell *c, struct scheduler *s);
 void cell_activate_drift_gpart(struct cell *c, struct scheduler *s);
 void cell_activate_sorts(struct cell *c, int sid, struct scheduler *s);

src/engine.c

@@ -196,7 +196,7 @@ void engine_make_hierarchical_tasks(struct engine *e, struct cell *c) {
       scheduler_addunlock(s, c->kick2, c->timestep);
       scheduler_addunlock(s, c->timestep, c->kick1);
 
-      /* Add the gravity tasks */
+      /* Add the self-gravity tasks */
       if (is_self_gravity) {
 
         /* Initialisation of the multipoles */
@@ -217,8 +217,10 @@ void engine_make_hierarchical_tasks(struct engine *e, struct cell *c) {
         scheduler_addunlock(s, c->grav_down, c->kick2);
       }
 
-      /* Generate the ghost tasks. */
+      /* Add the hydrodynamics tasks */
       if (is_with_hydro) {
+
+        /* Generate the ghost tasks. */
         c->ghost_in =
             scheduler_addtask(s, task_type_ghost, task_subtype_none, 0,
                               /* implicit = */ 1, c, NULL);

src/scheduler.c

@@ -695,7 +695,7 @@ void scheduler_splittasks_mapper(void *map_data, int num_elements,
       scheduler_splittask_gravity(t, s);
     } else if (t->type == task_type_grav_top_level ||
                t->type == task_type_grav_ghost) {
-      // MATTHIEU: for the future
+      /* For future use */
     } else {
       error("Unexpected task sub-type");
     }