Check only one component can make final write

Author: cmacmackin

src/component_scheduler.cxx

@@ -203,6 +203,10 @@ sortComponents(std::vector<std::unique_ptr<Component>>&& components) {
   // Components which do a final write on a variable depend on all
   // components which do non-final writes on that variable
   for (const auto& [var, comp_indices] : final_writes) {
+    if (comp_indices.size() > 1) {
+      throw BoutException(
+          "Multiple components have permission to make final write to variable {}", var);
+    }
     for (size_t i : comp_indices) {
       const auto item = nonfinal_writes.find(var);
       if (item != nonfinal_writes.end()) {

tests/unit/test_component_scheduler.cxx

@@ -276,6 +276,14 @@ INSTANTIATE_TEST_SUITE_P(
                  {"b",
                   {{"type", "orderchecker"},
                    {"permissions", toString(Permissions({readWrite("3")}))}}}}),
+        // Multiple final writes
+        Options({{"components", "a,b"},
+                 {"a",
+                  {{"type", "orderchecker"},
+                   {"permissions", toString(Permissions({writeFinal("1")}))}}},
+                 {"b",
+                  {{"type", "orderchecker"},
+                   {"permissions", toString(Permissions({writeFinal("1")}))}}}}),
         // Circular dependency
         Options({{"components", "a,b"},
                  {"a",