core: Fix broken Utils::Mpi::gather_buffer()

The function must move the first n_elem of the buffer on rank root
to the correct place, otherwise the first n_elem of the buffer on
rank 0 will overwrite them, and a hole of size n_elem can be found
somewhere in the final buffer. This bug doesn't affect code where
root is 0.

src/utils/include/utils/mpi/gather_buffer.hpp

@@ -39,14 +39,16 @@ namespace Mpi {
  *
  * Gathers buffers with different lengths from all nodes to root.
  * The buffer is assumed to be large enough to hold the data from
- * all the nodes and is owned by the caller. On the root node no
- * data is copied, and the first n_elem elements of buffer are not
- * touched. This combines a common combination of MPI_Gather and
- * MPI_{Send,Recv}.
+ * all the nodes and is owned by the caller. On the @p root node,
+ * the first @p n_elem elements of @p buffer are moved, if need
+ * be. On the other nodes, @p buffer is not touched.
+ *
+ * This encapsulates a common combination of <tt>MPI_Gather()</tt>
+ * and <tt>MPI_{Send,Recv}()</tt>.
  *
  * @param buffer On the master the target buffer that has to be
-          large enough to hold all elements and has the local
-          part in the beginning. On the slaves the local buffer.
+ *        large enough to hold all elements and has the local
+ *        part in the beginning. On the slaves the local buffer.
  * @param n_elem The number of elements in the local buffer.
  * @param comm The MPI communicator.
  * @param root The rank where the data should be gathered.
@@ -63,11 +65,19 @@ int gather_buffer(T *buffer, int n_elem, boost::mpi::communicator comm,
     auto const total_size =
         detail::size_and_offset<T>(sizes, displ, n_elem, comm, root);
 
+    /* Move the original data to its new location */
+    if (sizes[root] && displ[root]) {
+      for (int i = sizes[root]; i >= 0; --i) {
+        buffer[i + displ[root]] = buffer[i];
+      }
+    }
+
     /* Gather data */
     gatherv(comm, buffer, 0, buffer, sizes.data(), displ.data(), root);
 
     return total_size;
   }
+  /* Send local size */
   detail::size_and_offset(n_elem, comm, root);
   /* Send data */
   gatherv(comm, buffer, n_elem, static_cast<T *>(nullptr), nullptr, nullptr,
@@ -80,12 +90,15 @@ int gather_buffer(T *buffer, int n_elem, boost::mpi::communicator comm,
  * @brief Gather buffer with different size on each node.
  *
  * Gathers buffers with different lengths from all nodes to root.
- * The buffer is resized to the total size. On the root node no
- * data is copied, and the first n_elem elements of buffer are not
- * touched. On the slaves, the buffer is not touched.
+ * The buffer is resized to the total size. On the @p root node,
+ * the first @p n_elem elements of @p buffer are moved, if need
+ * be. On the other nodes, @p buffer is not touched.
+ *
+ * This encapsulates a common combination of <tt>MPI_Gather()</tt>
+ * and <tt>MPI_{Send,Recv}()</tt>.
  *
  * @param buffer On the master the target buffer that has the local
-          part in the beginning. On the slaves the local buffer.
+ *        part in the beginning. On the slaves the local buffer.
  * @param comm The MPI communicator.
  * @param root The rank where the data should be gathered.
  */
@@ -104,6 +117,13 @@ void gather_buffer(std::vector<T, Allocator> &buffer,
     /* Resize the buffer */
     buffer.resize(tot_size);
 
+    /* Move the original data to its new location */
+    if (sizes[root] && displ[root]) {
+      for (int i = sizes[root]; i >= 0; --i) {
+        buffer[i + displ[root]] = buffer[i];
+      }
+    }
+
     /* Gather data */
     gatherv(comm, buffer.data(), buffer.size(), buffer.data(), sizes.data(),
             displ.data(), root);

src/utils/tests/gather_buffer_test.cpp

@@ -149,6 +149,12 @@ BOOST_AUTO_TEST_CASE(pointer) {
   check_pointer(world, 0);
 }
 
+BOOST_AUTO_TEST_CASE(pointer_overlap) {
+  mpi::communicator world;
+  if (world.size() >= 2)
+    check_pointer(world, 1);
+}
+
 BOOST_AUTO_TEST_CASE(pointer_root) {
   mpi::communicator world;
 
@@ -161,6 +167,12 @@ BOOST_AUTO_TEST_CASE(vector) {
   check_vector(world, 0);
 }
 
+BOOST_AUTO_TEST_CASE(vector_overlap) {
+  mpi::communicator world;
+  if (world.size() >= 2)
+    check_vector(world, 1);
+}
+
 BOOST_AUTO_TEST_CASE(vector_root) {
   mpi::communicator world;