libcc1: use variadic templates for callbacks

This patch completes the transition of libcc1 from the use of separate
template functions for different arities to the use of variadic
functions.  This is how I had wanted it to work from the very
beginning, and is possible now with C++11.

I had thought that variadic callbacks required C++17, but it turns out
that the approach taken here is basically equivalent to std::apply --
just a bit wordier.

libcc1

	* rpc.hh (argument_wrapper) <get>: Replace cast operator.
	(argument_wrapper<T *>) <get>: Likewise.
	(unmarshall): Add std::tuple overloads.
	(callback): Remove.
	(class invoker): New.
	* libcp1plugin.cc (plugin_init): Update.
	* libcp1.cc (libcp1::add_callbacks): Update.
	* libcc1plugin.cc (plugin_init): Update.
	* libcc1.cc (libcc1::add_callbacks): Update.
	* connection.cc (cc1_plugin::connection::do_wait): Update.

1 files changed, 56 insertions, 163 deletions

diff --git a/libcc1/rpc.hh b/libcc1/rpc.hh
index 09cd7bd..8e43fa1 100644
--- a/libcc1/rpc.hh
+++ b/libcc1/rpc.hh
@@ -43,7 +43,7 @@ namespace cc1_plugin
     argument_wrapper (const argument_wrapper &) = delete;
     argument_wrapper &operator= (const argument_wrapper &) = delete;
 
-    operator T () const { return m_object; }
+    T get () const { return m_object; }
 
     status unmarshall (connection *conn)
     {
@@ -68,7 +68,7 @@ namespace cc1_plugin
 
     typedef typename std::remove_const<T>::type type;
 
-    operator const type * () const
+    const type *get () const
     {
       return m_object.get ();
     }
@@ -88,17 +88,14 @@ namespace cc1_plugin
   };
 
   // There are two kinds of template functions here: "call" and
-  // "callback".  "call" is implemented with variadic templates, but
-  // "callback" is repeated multiple times to handle different numbers
-  // of arguments.  (This could be improved with C++17 and
-  // std::apply.)
+  // "invoker".
 
   // The "call" template is used for making a remote procedure call.
   // It starts a query ('Q') packet, marshalls its arguments, waits
   // for a result, and finally reads and returns the result via an
   // "out" parameter.
 
-  // The "callback" template is used when receiving a remote procedure
+  // The "invoker" template is used when receiving a remote procedure
   // call.  This template function is suitable for use with the
   // "callbacks" and "connection" classes.  It decodes incoming
   // arguments, passes them to the wrapped function, and finally
@@ -123,175 +120,71 @@ namespace cc1_plugin
     return OK;
   }
 
-  template<typename R, R (*func) (connection *)>
-  status
-  callback (connection *conn)
+  // The base case -- just return OK.
+  template<int I, typename... T>
+  typename std::enable_if<I == sizeof... (T), status>::type
+  unmarshall (connection *, std::tuple<T...> &)
   {
-    R result;
-
-    if (!unmarshall_check (conn, 0))
-      return FAIL;
-    result = func (conn);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
-
-  template<typename R, typename A, R (*func) (connection *, A)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A> arg;
-    R result;
-
-    if (!unmarshall_check (conn, 1))
-      return FAIL;
-    if (!arg.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
+    return OK;
   }
 
-  template<typename R, typename A1, typename A2, R (*func) (connection *,
-							    A1, A2)>
-  status
-  callback (connection *conn)
+  // Unmarshall this argument, then unmarshall all subsequent args.
+  template<int I, typename... T>
+  typename std::enable_if<I < sizeof... (T), status>::type
+  unmarshall (connection *conn, std::tuple<T...> &value)
   {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    R result;
-
-    if (!unmarshall_check (conn, 2))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
+    if (!std::get<I> (value).unmarshall (conn))
       return FAIL;
-    result = func (conn, arg1, arg2);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
+    return unmarshall<I + 1, T...> (conn, value);
   }
 
-  template<typename R, typename A1, typename A2, typename A3,
-	   R (*func) (connection *, A1, A2, A3)>
-  status
-  callback (connection *conn)
+  // Wrap a static function that is suitable for use as a callback.
+  // This is a template function inside a template class to work
+  // around limitations with multiple variadic packs.
+  template<typename R, typename... Arg>
+  class invoker
   {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    R result;
+    // Base case -- we can call the function.
+    template<int I, R func (connection *, Arg...), typename... T>
+    static typename std::enable_if<I == sizeof... (Arg), R>::type
+    call (connection *conn, const std::tuple<argument_wrapper<Arg>...> &,
+	  T... args)
+    {
+      return func (conn, args...);
+    }
 
-    if (!unmarshall_check (conn, 3))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+    // Unpack one argument and continue the recursion.
+    template<int I, R func (connection *, Arg...), typename... T>
+    static typename std::enable_if<I < sizeof... (Arg), R>::type
+    call (connection *conn, const std::tuple<argument_wrapper<Arg>...> &value,
+	  T... args)
+    {
+      return call<I + 1, func> (conn, value, args...,
+				std::get<I> (value).get ());
+    }
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   R (*func) (connection *, A1, A2, A3, A4)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    R result;
+  public:
 
-    if (!unmarshall_check (conn, 4))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+    // A callback function that reads arguments from the connection,
+    // calls the wrapped function, and then sends the result back on
+    // the connection.
+    template<R func (connection *, Arg...)>
+    static status
+    invoke (connection *conn)
+    {
+      if (!unmarshall_check (conn, sizeof... (Arg)))
+	return FAIL;
+      std::tuple<argument_wrapper<Arg>...> wrapped;
+      if (!unmarshall<0> (conn, wrapped))
+	return FAIL;
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   typename A5, R (*func) (connection *, A1, A2, A3, A4, A5)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    argument_wrapper<A5> arg5;
-    R result;
-
-    if (!unmarshall_check (conn, 5))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    if (!arg5.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4, arg5);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+      R result = call<0, func> (conn, wrapped);
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   typename A5, typename A6, typename A7,
-	   R (*func) (connection *, A1, A2, A3, A4, A5, A6, A7)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    argument_wrapper<A5> arg5;
-    argument_wrapper<A6> arg6;
-    argument_wrapper<A7> arg7;
-    R result;
-
-    if (!unmarshall_check (conn, 7))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    if (!arg5.unmarshall (conn))
-      return FAIL;
-    if (!arg6.unmarshall (conn))
-      return FAIL;
-    if (!arg7.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4, arg5, arg6, arg7);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+      if (!conn->send ('R'))
+	return FAIL;
+      return marshall (conn, result);
+    }
+  };
 };
 
 #endif // CC1_PLUGIN_RPC_HH