Track lua event listener count, and let the C++ host know.

This allows completely avoiding the call overhead if there
are none. The downside is that the event object now has to
be a userdata with lots of metamethods.

LUA_API.rst

@@ -1448,8 +1448,8 @@ Core context specific functions:
 Event type
 ----------
 
-An event is just a lua table with a predefined metatable that
-contains a __call metamethod. When it is invoked, it loops
+An event is a native object transparently wrapping a lua table,
+and implementing a __call metamethod. When it is invoked, it loops
 through the table with next and calls all contained values.
 This is intended as an extensible way to add listeners.
 
@@ -1464,10 +1464,18 @@ Features:
 
 * ``event[key] = function``
 
-  Sets the function as one of the listeners.
+  Sets the function as one of the listeners. Assign *nil* to remove it.
 
   **NOTE**: The ``df.NULL`` key is reserved for the use by
-  the C++ owner of the event, and has some special semantics.
+  the C++ owner of the event; it is an error to try setting it.
+
+* ``#event``
+
+  Returns the number of non-nil listeners.
+
+* ``pairs(event)``
+
+  Iterates over all listeners in the table.
 
 * ``event(args...)``
 

Lua API.html

@@ -1596,8 +1596,8 @@ Using <tt class="docutils literal">timeout_active(id,nil)</tt> cancels the timer
 </ul>
 <div class="section" id="event-type">
 <h3><a class="toc-backref" href="#id29">Event type</a></h3>
-<p>An event is just a lua table with a predefined metatable that
-contains a __call metamethod. When it is invoked, it loops
+<p>An event is a native object transparently wrapping a lua table,
+and implementing a __call metamethod. When it is invoked, it loops
 through the table with next and calls all contained values.
 This is intended as an extensible way to add listeners.</p>
 <p>This type itself is available in any context, but only the
@@ -1608,9 +1608,15 @@ core context has the actual events defined by C++ code.</p>
 <p>Creates a new instance of an event.</p>
 </li>
 <li><p class="first"><tt class="docutils literal">event[key] = function</tt></p>
-<p>Sets the function as one of the listeners.</p>
+<p>Sets the function as one of the listeners. Assign <em>nil</em> to remove it.</p>
 <p><strong>NOTE</strong>: The <tt class="docutils literal">df.NULL</tt> key is reserved for the use by
-the C++ owner of the event, and has some special semantics.</p>
+the C++ owner of the event; it is an error to try setting it.</p>
+</li>
+<li><p class="first"><tt class="docutils literal">#event</tt></p>
+<p>Returns the number of non-nil listeners.</p>
+</li>
+<li><p class="first"><tt class="docutils literal">pairs(event)</tt></p>
+<p>Iterates over all listeners in the table.</p>
 </li>
 <li><p class="first"><tt class="docutils literal"><span class="pre">event(args...)</span></tt></p>
 <p>Invokes all listeners contained in the event in an arbitrary

library/LuaTools.cpp

@@ -68,6 +68,11 @@ lua_State *DFHack::Lua::Core::State = NULL;
 
 void dfhack_printerr(lua_State *S, const std::string &str);
 
+inline bool is_null_userdata(lua_State *L, int idx)
+{
+    return lua_islightuserdata(L, idx) && !lua_touserdata(L, idx);
+}
+
 inline void AssertCoreSuspend(lua_State *state)
 {
     assert(!Lua::IsCoreContext(state) || DFHack::Core::getInstance().isSuspended());
@@ -1244,14 +1249,123 @@ static const luaL_Reg dfhack_coro_funcs[] = {
 
 static int DFHACK_EVENT_META_TOKEN = 0;
 
-int DFHack::Lua::NewEvent(lua_State *state)
+namespace {
+    struct EventObject {
+        int item_count;
+        Lua::Event::Owner *owner;
+    };
+}
+
+void DFHack::Lua::Event::New(lua_State *state, Owner *owner)
 {
-    lua_newtable(state);
+    auto obj = (EventObject *)lua_newuserdata(state, sizeof(EventObject));
+    obj->item_count = 0;
+    obj->owner = owner;
+
     lua_rawgetp(state, LUA_REGISTRYINDEX, &DFHACK_EVENT_META_TOKEN);
     lua_setmetatable(state, -2);
+    lua_newtable(state);
+    lua_setuservalue(state, -2);
+}
+
+void DFHack::Lua::Event::SetPrivateCallback(lua_State *L, int event)
+{
+    lua_getuservalue(L, event);
+    lua_swap(L);
+    lua_rawsetp(L, -2, NULL);
+    lua_pop(L, 1);
+}
+
+static int dfhack_event_new(lua_State *L)
+{
+    Lua::Event::New(L);
+    return 1;
+}
+
+static int dfhack_event_len(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    auto obj = (EventObject *)lua_touserdata(L, 1);
+    lua_pushinteger(L, obj->item_count);
+    return 1;
+}
+
+static int dfhack_event_tostring(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    auto obj = (EventObject *)lua_touserdata(L, 1);
+    lua_pushfstring(L, "<event: %d listeners>", obj->item_count);
     return 1;
 }
 
+static int dfhack_event_index(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    lua_getuservalue(L, 1);
+    lua_pushvalue(L, 2);
+    lua_rawget(L, -2);
+    return 1;
+}
+
+static int dfhack_event_next(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    lua_getuservalue(L, 1);
+    lua_pushvalue(L, 2);
+    while (lua_next(L, -2))
+    {
+        if (is_null_userdata(L, -2))
+            lua_pop(L, 1);
+        else
+            return 2;
+    }
+    lua_pushnil(L);
+    return 1;
+}
+
+static int dfhack_event_pairs(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    lua_pushcfunction(L, dfhack_event_next);
+    lua_pushvalue(L, 1);
+    lua_pushnil(L);
+    return 3;
+}
+
+static int dfhack_event_newindex(lua_State *L)
+{
+    luaL_checktype(L, 1, LUA_TUSERDATA);
+    if (is_null_userdata(L, 2))
+        luaL_argerror(L, 2, "Key NULL is reserved in events.");
+
+    lua_settop(L, 3);
+    lua_getuservalue(L, 1);
+    bool new_nil = lua_isnil(L, 3);
+
+    lua_pushvalue(L, 2);
+    lua_rawget(L, 4);
+    bool old_nil = lua_isnil(L, -1);
+    lua_settop(L, 4);
+
+    lua_pushvalue(L, 2);
+    lua_pushvalue(L, 3);
+    lua_rawset(L, 4);
+
+    int delta = 0;
+    if (old_nil && !new_nil) delta = 1;
+    else if (new_nil && !old_nil) delta = -1;
+
+    if (delta != 0)
+    {
+        auto obj = (EventObject *)lua_touserdata(L, 1);
+        obj->item_count += delta;
+        if (obj->owner)
+            obj->owner->on_count_changed(obj->item_count, delta);
+    }
+
+    return 0;
+}
+
 static void do_invoke_event(lua_State *L, int argbase, int num_args, int errorfun)
 {
     for (int i = 0; i < num_args; i++)
@@ -1292,7 +1406,7 @@ static void dfhack_event_invoke(lua_State *L, int base, bool from_c)
     while (lua_next(L, event))
     {
         // Skip the NULL key in the main loop
-        if (lua_islightuserdata(L, -2) && !lua_touserdata(L, -2))
+        if (is_null_userdata(L, -2))
             lua_pop(L, 1);
         else
             do_invoke_event(L, argbase, num_args, errorfun);
@@ -1303,14 +1417,20 @@ static void dfhack_event_invoke(lua_State *L, int base, bool from_c)
 
 static int dfhack_event_call(lua_State *state)
 {
-    luaL_checktype(state, 1, LUA_TTABLE);
+    luaL_checktype(state, 1, LUA_TUSERDATA);
     luaL_checkstack(state, lua_gettop(state)+2, "stack overflow in event dispatch");
 
+    auto obj = (EventObject *)lua_touserdata(state, 1);
+    if (obj->owner)
+        obj->owner->on_invoked(state, lua_gettop(state)-1, false);
+
+    lua_getuservalue(state, 1);
+    lua_replace(state, 1);
     dfhack_event_invoke(state, 0, false);
     return 0;
 }
 
-void DFHack::Lua::InvokeEvent(color_ostream &out, lua_State *state, void *key, int num_args)
+void DFHack::Lua::Event::Invoke(color_ostream &out, lua_State *state, void *key, int num_args)
 {
     AssertCoreSuspend(state);
 
@@ -1325,7 +1445,7 @@ void DFHack::Lua::InvokeEvent(color_ostream &out, lua_State *state, void *key, i
 
     lua_rawgetp(state, LUA_REGISTRYINDEX, key);
 
-    if (!lua_istable(state, -1))
+    if (!lua_isuserdata(state, -1))
     {
         if (!lua_isnil(state, -1))
             out.printerr("Invalid event object in Lua::InvokeEvent");
@@ -1333,22 +1453,29 @@ void DFHack::Lua::InvokeEvent(color_ostream &out, lua_State *state, void *key, i
         return;
     }
 
+    auto obj = (EventObject *)lua_touserdata(state, -1);
     lua_insert(state, base+1);
 
+    if (obj->owner)
+        obj->owner->on_invoked(state, num_args, true);
+
+    lua_getuservalue(state, base+1);
+    lua_replace(state, base+1);
+
     color_ostream *cur_out = Lua::GetOutput(state);
     set_dfhack_output(state, &out);
     dfhack_event_invoke(state, base, true);
     set_dfhack_output(state, cur_out);
 }
 
-void DFHack::Lua::MakeEvent(lua_State *state, void *key)
+void DFHack::Lua::Event::Make(lua_State *state, void *key, Owner *owner)
 {
     lua_rawgetp(state, LUA_REGISTRYINDEX, key);
 
     if (lua_isnil(state, -1))
     {
         lua_pop(state, 1);
-        NewEvent(state);
+        New(state, owner);
     }
 
     lua_dup(state);
@@ -1358,7 +1485,7 @@ void DFHack::Lua::MakeEvent(lua_State *state, void *key)
 void DFHack::Lua::Notification::invoke(color_ostream &out, int nargs)
 {
     assert(state);
-    InvokeEvent(out, state, key, nargs);
+    Event::Invoke(out, state, key, nargs);
 }
 
 void DFHack::Lua::Notification::bind(lua_State *state, void *key)
@@ -1369,12 +1496,12 @@ void DFHack::Lua::Notification::bind(lua_State *state, void *key)
 
 void DFHack::Lua::Notification::bind(lua_State *state, const char *name)
 {
-    MakeEvent(state, this);
+    Event::Make(state, this);
 
     if (handler)
     {
         PushFunctionWrapper(state, 0, name, handler);
-        lua_rawsetp(state, -2, NULL);
+        Event::SetPrivateCallback(state, -2);
     }
 
     this->state = state;
@@ -1435,11 +1562,26 @@ lua_State *DFHack::Lua::Open(color_ostream &out, lua_State *state)
     lua_newtable(state);
     lua_pushcfunction(state, dfhack_event_call);
     lua_setfield(state, -2, "__call");
-    lua_pushcfunction(state, Lua::NewEvent);
-    lua_setfield(state, -2, "new");
+    lua_pushcfunction(state, dfhack_event_len);
+    lua_setfield(state, -2, "__len");
+    lua_pushcfunction(state, dfhack_event_tostring);
+    lua_setfield(state, -2, "__tostring");
+    lua_pushcfunction(state, dfhack_event_index);
+    lua_setfield(state, -2, "__index");
+    lua_pushcfunction(state, dfhack_event_newindex);
+    lua_setfield(state, -2, "__newindex");
+    lua_pushcfunction(state, dfhack_event_pairs);
+    lua_setfield(state, -2, "__pairs");
     lua_dup(state);
     lua_rawsetp(state, LUA_REGISTRYINDEX, &DFHACK_EVENT_META_TOKEN);
-    lua_setfield(state, -2, "event");
+
+    lua_newtable(state);
+    lua_pushcfunction(state, dfhack_event_new);
+    lua_setfield(state, -2, "new");
+    lua_dup(state);
+    lua_setfield(state, -3, "__metatable");
+    lua_setfield(state, -3, "event");
+    lua_pop(state, 1);
 
     // Initialize the dfhack global
     luaL_setfuncs(state, dfhack_funcs, 0);
@@ -1599,7 +1741,7 @@ void DFHack::Lua::Core::onStateChange(color_ostream &out, int code) {
     }
 
     Lua::Push(State, code);
-    Lua::InvokeEvent(out, State, (void*)onStateChange, 1);
+    Lua::Event::Invoke(out, State, (void*)onStateChange, 1);
 }
 
 static void run_timers(color_ostream &out, lua_State *L,
@@ -1653,7 +1795,7 @@ void DFHack::Lua::Core::Init(color_ostream &out)
     // Register events
     lua_rawgetp(State, LUA_REGISTRYINDEX, &DFHACK_DFHACK_TOKEN);
 
-    MakeEvent(State, (void*)onStateChange);
+    Event::Make(State, (void*)onStateChange);
     lua_setfield(State, -2, "onStateChange");
 
     lua_pushcfunction(State, dfhack_timeout);

library/PluginManager.cpp

@@ -108,6 +108,50 @@ struct Plugin::RefAutoinc
     ~RefAutoinc(){ lock->lock_sub(); };
 };
 
+struct Plugin::LuaCommand {
+    Plugin *owner;
+    std::string name;
+    int (*command)(lua_State *state);
+
+    LuaCommand(Plugin *owner, std::string name)
+      : owner(owner), name(name), command(NULL) {}
+};
+
+struct Plugin::LuaFunction {
+    Plugin *owner;
+    std::string name;
+    function_identity_base *identity;
+    bool silent;
+
+    LuaFunction(Plugin *owner, std::string name)
+      : owner(owner), name(name), identity(NULL), silent(false) {}
+};
+
+struct Plugin::LuaEvent : public Lua::Event::Owner {
+    LuaFunction handler;
+    Lua::Notification *event;
+    bool active;
+    int count;
+
+    LuaEvent(Plugin *owner, std::string name)
+      : handler(owner,name), event(NULL), active(false), count(0)
+    {
+        handler.silent = true;
+    }
+
+    void on_count_changed(int new_cnt, int delta) {
+        RefAutoinc lock(handler.owner->access);
+        count = new_cnt;
+        if (event)
+            event->on_count_changed(new_cnt, delta);
+    }
+    void on_invoked(lua_State *state, int nargs, bool from_c) {
+        RefAutoinc lock(handler.owner->access);
+        if (event)
+            event->on_invoked(state, nargs, from_c);
+    }
+};
+
 Plugin::Plugin(Core * core, const std::string & filepath, const std::string & _filename, PluginManager * pm)
 {
     filename = filepath;
@@ -439,7 +483,11 @@ void Plugin::index_lua(DFLibrary *lib)
             cmd->handler.identity = evlist->event->get_handler();
             cmd->event = evlist->event;
             if (cmd->active)
+            {
                 cmd->event->bind(Lua::Core::State, cmd);
+                if (cmd->count > 0)
+                    cmd->event->on_count_changed(cmd->count, 0);
+            }
         }
     }
 }
@@ -477,8 +525,13 @@ int Plugin::lua_fun_wrapper(lua_State *state)
     RefAutoinc lock(cmd->owner->access);
 
     if (!cmd->identity)
+    {
+        if (cmd->silent)
+            return 0;
+
         luaL_error(state, "plugin function %s() has been unloaded",
                    (cmd->owner->name+"."+cmd->name).c_str());
+    }
 
     return LuaWrapper::method_wrapper_core(state, cmd->identity);
 }
@@ -506,14 +559,14 @@ void Plugin::open_lua(lua_State *state, int table)
     {
         for (auto it = lua_events.begin(); it != lua_events.end(); ++it)
         {
-            Lua::MakeEvent(state, it->second);
+            Lua::Event::Make(state, it->second, it->second);
 
             push_function(state, &it->second->handler);
-            lua_rawsetp(state, -2, NULL);
+            Lua::Event::SetPrivateCallback(state, -2);
 
             it->second->active = true;
             if (it->second->event)
-                it->second->event->bind(state, it->second);
+                it->second->event->bind(Lua::Core::State, it->second);
 
             lua_setfield(state, table, it->first.c_str());
         }

library/include/LuaTools.h

@@ -310,9 +310,18 @@ namespace DFHack {namespace Lua {
 
     DFHACK_EXPORT bool IsCoreContext(lua_State *state);
 
-    DFHACK_EXPORT int NewEvent(lua_State *state);
-    DFHACK_EXPORT void MakeEvent(lua_State *state, void *key);
-    DFHACK_EXPORT void InvokeEvent(color_ostream &out, lua_State *state, void *key, int num_args);
+    namespace Event {
+        struct DFHACK_EXPORT Owner {
+            virtual ~Owner() {}
+            virtual void on_count_changed(int new_cnt, int delta) {}
+            virtual void on_invoked(lua_State *state, int nargs, bool from_c) {}
+        };
+
+        DFHACK_EXPORT void New(lua_State *state, Owner *owner = NULL);
+        DFHACK_EXPORT void Make(lua_State *state, void *key, Owner *owner = NULL);
+        DFHACK_EXPORT void SetPrivateCallback(lua_State *state, int ev_idx);
+        DFHACK_EXPORT void Invoke(color_ostream &out, lua_State *state, void *key, int num_args);
+    }
 
     class StackUnwinder {
         lua_State *state;
@@ -365,18 +374,24 @@ namespace DFHack {namespace Lua {
         }
     }
 
-    class DFHACK_EXPORT Notification {
+    class DFHACK_EXPORT Notification : public Event::Owner {
         lua_State *state;
         void *key;
         function_identity_base *handler;
+        int count;
 
     public:
         Notification(function_identity_base *handler = NULL)
-            : state(NULL), key(NULL), handler(handler) {}
+            : state(NULL), key(NULL), handler(handler), count(0) {}
 
+        int get_listener_count() { return count; }
         lua_State *get_state() { return state; }
         function_identity_base *get_handler() { return handler; }
 
+        lua_State *state_if_count() { return (count > 0) ? state : NULL; }
+
+        void on_count_changed(int new_cnt, int) { count = new_cnt; }
+
         void invoke(color_ostream &out, int nargs);
 
         void bind(lua_State *state, const char *name);
@@ -388,7 +403,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out) { \
         handler(out); \
-        if (name##_event.get_state()) { \
+        if (name##_event.state_if_count()) { \
             name##_event.invoke(out, 0); \
         } \
     }
@@ -397,7 +412,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out, arg_type1 arg1) { \
         handler(out, arg1); \
-        if (auto state = name##_event.get_state()) { \
+        if (auto state = name##_event.state_if_count()) { \
             DFHack::Lua::Push(state, arg1); \
             name##_event.invoke(out, 1); \
         } \
@@ -407,7 +422,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out, arg_type1 arg1, arg_type2 arg2) { \
         handler(out, arg1, arg2); \
-        if (auto state = name##_event.get_state()) { \
+        if (auto state = name##_event.state_if_count()) { \
             DFHack::Lua::Push(state, arg1); \
             DFHack::Lua::Push(state, arg2); \
             name##_event.invoke(out, 2); \
@@ -418,7 +433,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out, arg_type1 arg1, arg_type2 arg2, arg_type3 arg3) { \
         handler(out, arg1, arg2, arg3); \
-        if (auto state = name##_event.get_state()) { \
+        if (auto state = name##_event.state_if_count()) { \
             DFHack::Lua::Push(state, arg1); \
             DFHack::Lua::Push(state, arg2); \
             DFHack::Lua::Push(state, arg3); \
@@ -430,7 +445,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out, arg_type1 arg1, arg_type2 arg2, arg_type3 arg3, arg_type4 arg4) { \
         handler(out, arg1, arg2, arg3, arg4); \
-        if (auto state = name##_event.get_state()) { \
+        if (auto state = name##_event.state_if_count()) { \
             DFHack::Lua::Push(state, arg1); \
             DFHack::Lua::Push(state, arg2); \
             DFHack::Lua::Push(state, arg3); \
@@ -443,7 +458,7 @@ namespace DFHack {namespace Lua {
     static DFHack::Lua::Notification name##_event(df::wrap_function(handler, true)); \
     void name(color_ostream &out, arg_type1 arg1, arg_type2 arg2, arg_type3 arg3, arg_type4 arg4, arg_type5 arg5) { \
         handler(out, arg1, arg2, arg3, arg4, arg5); \
-        if (auto state = name##_event.get_state()) { \
+        if (auto state = name##_event.state_if_count()) { \
             DFHack::Lua::Push(state, arg1); \
             DFHack::Lua::Push(state, arg2); \
             DFHack::Lua::Push(state, arg3); \

library/include/PluginManager.h

@@ -173,31 +173,16 @@ namespace DFHack
         PluginManager * parent;
         plugin_state state;
 
-        struct LuaCommand {
-            Plugin *owner;
-            std::string name;
-            int (*command)(lua_State *state);
-            LuaCommand(Plugin *owner, std::string name) : owner(owner), name(name) {}
-        };
+        struct LuaCommand;
         std::map<std::string, LuaCommand*> lua_commands;
         static int lua_cmd_wrapper(lua_State *state);
 
-        struct LuaFunction {
-            Plugin *owner;
-            std::string name;
-            function_identity_base *identity;
-            LuaFunction(Plugin *owner, std::string name) : owner(owner), name(name) {}
-        };
+        struct LuaFunction;
         std::map<std::string, LuaFunction*> lua_functions;
         static int lua_fun_wrapper(lua_State *state);
         void push_function(lua_State *state, LuaFunction *fn);
 
-        struct LuaEvent {
-            LuaFunction handler;
-            Lua::Notification *event;
-            bool active;
-            LuaEvent(Plugin *owner, std::string name) : handler(owner,name), active(false) {}
-        };
+        struct LuaEvent;
         std::map<std::string, LuaEvent*> lua_events;
 
         void index_lua(DFLibrary *lib);

library/lua/dfhack.lua

@@ -122,8 +122,9 @@ end
 -- Misc functions
 
 function printall(table)
-    if type(table) == 'table' or df.isvalid(table) == 'ref' then
-        for k,v in pairs(table) do
+    local ok,f,t,k = pcall(pairs,table)
+    if ok then
+        for k,v in f,t,k do
             print(string.format("%-23s\t = %s",tostring(k),tostring(v)))
         end
     end
@@ -177,10 +178,6 @@ end
 
 -- String conversions
 
-function dfhack.event:__tostring()
-    return "<event>"
-end
-
 function dfhack.persistent:__tostring()
     return "<persistent "..self.entry_id..":"..self.key.."=\""
            ..self.value.."\":"..table.concat(self.ints,",")..">"