allow inplace task to be called in const contexts

lib/include/srslte/common/inplace_task.h

@@ -155,7 +155,7 @@ public:
     return *this;
   }
 
-  R operator()(Args&&... args) { return oper_ptr->call(&buffer, std::forward<Args>(args)...); }
+  R operator()(Args&&... args) const { return oper_ptr->call(&buffer, std::forward<Args>(args)...); }
 
   bool is_empty() const { return oper_ptr == oper_table_t::get_empty(); }
   bool is_in_small_buffer() const { return oper_ptr->is_in_buffer; }
@@ -188,7 +188,7 @@ public:
 
 private:
   union {
-    storage_t buffer;
+    mutable storage_t buffer;
     void*             ptr;
   };
   const oper_table_t* oper_ptr;

lib/test/common/queue_test.cc

@@ -390,6 +390,13 @@ int test_inplace_task()
   t2();
   TESTASSERT(v == 5);
 
+  // TEST: task works in const contexts
+  t       = l2;
+  auto l3 = [](const srslte::inplace_task<void()>& task) { task(); };
+  v       = 0;
+  l3(t);
+  TESTASSERT(v == 6);
+
   std::cout << "outcome: Success\n";
   std::cout << "========================================\n";
   return 0;