Generalize transactor callable (#279)

* Generalize transactor callable

This permits calling pqxx::perform on a more general set of callable[1] objects by using std::invoke[2] for perfect forwarding.

[1] https://en.cppreference.com/w/cpp/named_req/Callable
[2] https://en.cppreference.com/w/cpp/utility/functional/invoke

* Mark transactor unit tests noexcept as necessary

Tests are run with -Werror=noexcept, so we need to mark these lambdas as noexcept in this case.

include/pqxx/transactor.hxx

@@ -14,6 +14,8 @@
 #include "pqxx/compiler-public.hxx"
 #include "pqxx/internal/compiler-internal-pre.hxx"
 
+#include <functional>
+
 #include "pqxx/connection.hxx"
 #include "pqxx/transaction.hxx"
 
@@ -94,8 +96,8 @@ namespace pqxx
  * @return Whatever your callback returns.
  */
 template<typename TRANSACTION_CALLBACK>
-inline auto perform(TRANSACTION_CALLBACK const &callback, int attempts = 3)
-  -> decltype(callback())
+inline auto perform(TRANSACTION_CALLBACK &&callback, int attempts = 3)
+  -> decltype(std::invoke(callback))
 {
   if (attempts <= 0)
     throw std::invalid_argument{
@@ -105,7 +107,7 @@ inline auto perform(TRANSACTION_CALLBACK const &callback, int attempts = 3)
   {
     try
     {
-      return callback();
+      return std::invoke(callback);
     }
     catch (in_doubt_error const &)
     {

test/unit/test_transactor.cxx

@@ -19,15 +19,15 @@ void test_transactor_newstyle_executes_simple_query()
 void test_transactor_newstyle_can_return_void()
 {
   bool done{false};
-  pqxx::perform([&done] { done = true; });
+  pqxx::perform([&done]() noexcept { done = true; });
   PQXX_CHECK(done, "Callback was not executed.");
 }
 
 
 void test_transactor_newstyle_completes_upon_success()
 {
   int attempts{0};
-  pqxx::perform([&attempts]() { attempts++; });
+  pqxx::perform([&attempts]() noexcept { attempts++; });
   PQXX_CHECK_EQUAL(attempts, 1, "Successful transactor didn't run 1 time.");
 }