Merge pull request #43247 from rzarzynski/wip-crimson-ertr-safe_then_unpack

crimson/common: add safe_then_unpack() to errorated futures

Reviewed-by: Samuel Just <sjust@redhat.com>
Reviewed-by: Chunmei Liu <chunmei.liu@intel.com>

src/crimson/common/errorator.h

@@ -8,6 +8,7 @@
 
 #include <seastar/core/future-util.hh>
 
+#include "crimson/common/utility.h"
 #include "include/ceph_assert.h"
 
 namespace crimson::interruptible {
@@ -667,6 +668,19 @@ private:
                        errorator_type::pass_further{});
     }
 
+    template <class ValueFunc,
+              class... ErrorFuncs>
+    auto safe_then_unpack(ValueFunc&& value_func,
+                          ErrorFuncs&&... error_funcs) {
+      return safe_then(
+        [value_func=std::move(value_func)] (ValueT&& tuple) mutable {
+          assert_moveable(value_func);
+          return std::apply(std::move(value_func), std::move(tuple));
+        },
+        std::forward<ErrorFuncs>(error_funcs)...
+      );
+    }
+
     template <class Func>
     void then(Func&&) = delete;
 

src/crimson/common/utility.h

@@ -5,12 +5,16 @@
 
 #include <type_traits>
 
+namespace _impl {
+  template <class T> struct always_false : std::false_type {};
+};
+
 template <class T>
 void assert_moveable(T& t) {
     // It's fine
 }
 template <class T>
 void assert_moveable(const T& t) {
-    static_assert(always_false<T>::value, "unable to move-out from T");
+    static_assert(_impl::always_false<T>::value, "unable to move-out from T");
 }