ircd::util: Add an unwind::defer device.

include/ircd/util/unwind.h

@@ -24,6 +24,7 @@ namespace ircd::util
 ///
 struct ircd::util::unwind
 {
+	struct defer;
 	struct nominal;
 	struct exceptional;
 
@@ -94,6 +95,22 @@ struct ircd::util::unwind::exceptional
 	exceptional(const exceptional &) = delete;
 };
 
+/// Posts function to the event loop at the unwind rather than executing it
+/// as with nominal/exceptional.
+///
+struct ircd::util::unwind::defer
+{
+	std::function<void ()> func;
+
+	template<class F>
+	defer(F&& func)
+	:func{std::forward<F>(func)}
+	{}
+
+	defer(const defer &) = delete;
+	~defer() noexcept;
+};
+
 /// Asserts that unwind is occurring without any exception. This allows some
 /// section of code, for example, the latter half of a function, to assert that
 /// it is not throwing and disrupting the other half. This is useful when no

ircd/util.cc

@@ -582,6 +582,20 @@ ircd::util::a2u(const mutable_buffer &out,
 	return { data(out), len };
 }
 
+///////////////////////////////////////////////////////////////////////////////
+//
+// util/unwind.h
+//
+
+ircd::util::unwind::defer::~defer()
+noexcept
+{
+	ircd::defer
+	{
+		std::move(func)
+	};
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 //
 // util/what.h