Fix async arguments

docs.md

@@ -1016,15 +1016,19 @@ end)
 async:send()
 ```
 
-### `uv.new_async(callback)`
+### `uv.new_async(callback, [size])`
 
 **Parameters:**
 - `callback`: `callable`
   - `...`: `threadargs` passed to/from `uv.async_send(async, ...)`
+- `size`: `integer` or `nil` (default: `0`)
 
 Creates and initializes a new `uv_async_t`. Returns the Lua userdata wrapping
 it.
 
+If size is omitted (or 0), each call to send discards any pending one; otherwise, a call to send could fail with `ENOSPC` when there are already `size` pending send.
+
+
 **Returns:** `uv_async_t userdata` or `fail`
 
 **Note**: Unlike other handle initialization functions, this immediately starts
@@ -1050,6 +1054,7 @@ every call to it will yield an execution of the callback. For example: if
 `uv.async_send()` is called 5 times in a row before the callback is called, the
 callback will only be called once. If `uv.async_send()` is called again after
 the callback was called, it will be called again.
+When specifying a `size` greater than 0, this function will fail when there are pending calls.
 
 ## `uv_poll_t` — Poll handle
 

src/async.c

@@ -16,25 +16,95 @@
  */
 #include "private.h"
 
+typedef struct luv_async_send_s {
+  luv_thread_arg_t targ;
+  struct luv_async_send_s* next;
+} luv_async_send_t;
+
+typedef struct {
+  luv_thread_arg_t targ;
+  uv_mutex_t mutex;
+  int max; // FIFO queue in case of max > 0
+  int count;
+  luv_async_send_t* first;
+  luv_async_send_t* last;
+} luv_async_arg_t;
+
+#define luv_get_async_arg_from_handle(H) ((luv_async_arg_t *) ((luv_handle_t*) (H)->data)->extra)
+
 static uv_async_t* luv_check_async(lua_State* L, int index) {
   uv_async_t* handle = (uv_async_t*)luv_checkudata(L, index, "uv_async");
   luaL_argcheck(L, handle->type == UV_ASYNC && handle->data, index, "Expected uv_async_t");
   return handle;
 }
 
+#define luv_is_async_queue(AA) ((AA)->max > 0)
+
+static luv_async_send_t* luv_async_pop(luv_async_arg_t* asarg) {
+  luv_async_send_t* sendarg = asarg->first;
+  if (sendarg != NULL) {
+    asarg->count--;
+    asarg->first = sendarg->next;
+    if (asarg->first == NULL) {
+      asarg->last = NULL;
+    }
+  }
+  return sendarg;
+}
+
+static luv_async_send_t* luv_async_push(luv_async_arg_t* asarg) {
+  luv_async_send_t* sendarg = (luv_async_send_t*)malloc(sizeof(luv_async_send_t));
+  if (sendarg != NULL) {
+    memset(sendarg, 0, sizeof(luv_async_send_t));
+    asarg->count++;
+    if (asarg->last != NULL) {
+      asarg->last->next = sendarg;
+    }
+    asarg->last = sendarg;
+    if (asarg->first == NULL) {
+      asarg->first = sendarg;
+    }
+  }
+  return sendarg;
+}
+
 static void luv_async_cb(uv_async_t* handle) {
   luv_handle_t* data = (luv_handle_t*)handle->data;
   lua_State* L = data->ctx->L;
-  int n = luv_thread_arg_push(L, (luv_thread_arg_t*)data->extra, LUVF_THREAD_SIDE_MAIN);
-  luv_call_callback(L, data, LUV_ASYNC, n);
-  luv_thread_arg_clear(L, (luv_thread_arg_t*)data->extra, LUVF_THREAD_SIDE_MAIN);
+  luv_async_arg_t* asarg = luv_get_async_arg_from_handle(handle);
+  uv_mutex_t *argmutex = &asarg->mutex;
+  luv_thread_arg_t targcpy; // work on a copy of the arguments
+  int n;
+  int q = luv_is_async_queue(asarg);
+  do {
+    uv_mutex_lock(argmutex);
+    if (q) {
+      luv_async_send_t* sendarg = luv_async_pop(asarg);
+      if (sendarg == NULL) {
+        uv_mutex_unlock(argmutex);
+        return;
+      }
+      targcpy = sendarg->targ;
+      free(sendarg);
+    } else {
+      targcpy = asarg->targ;
+      asarg->targ.argc = 0; // empty the shared original, nothing to clear
+    }
+    uv_mutex_unlock(argmutex);
+    n = luv_thread_arg_push(L, &targcpy, LUVF_THREAD_SIDE_MAIN);
+    if (n >= 0) {
+      luv_call_callback(L, data, LUV_ASYNC, n);
+    }
+    luv_thread_arg_clear(L, &targcpy, LUVF_THREAD_SIDE_MAIN); // clear the copy
+  } while (q);
 }
 
 static int luv_new_async(lua_State* L) {
   uv_async_t* handle;
   luv_handle_t* data;
   int ret;
   luv_ctx_t* ctx = luv_context(L);
+  int max = luaL_optinteger(L, 2, 0);
   luaL_checktype(L, 1, LUA_TFUNCTION);
   handle = (uv_async_t*)luv_newuserdata(L, uv_handle_size(UV_ASYNC));
   ret = uv_async_init(ctx->loop, handle, luv_async_cb);
@@ -43,21 +113,83 @@ static int luv_new_async(lua_State* L) {
     return luv_error(L, ret);
   }
   data = luv_setup_handle(L, ctx);
-  data->extra = (luv_thread_arg_t*)malloc(sizeof(luv_thread_arg_t));
+  luv_async_arg_t* asarg = (luv_async_arg_t*)malloc(sizeof(luv_async_arg_t));
+  memset(asarg, 0, sizeof(luv_async_arg_t));
+  asarg->max = max;
+  ret = uv_mutex_init(&asarg->mutex);
+  if (ret < 0) { // unlikely
+    abort();
+  }
+  data->extra = asarg;
   data->extra_gc = free;
-  memset(data->extra, 0, sizeof(luv_thread_arg_t));
   handle->data = data;
   luv_check_callback(L, (luv_handle_t*)handle->data, LUV_ASYNC, 1);
   return 1;
 }
 
+// From handle.c
+static int luv_handle_gc(lua_State* L);
+// From thread.c
+static void luv_thread_arg_free(luv_thread_arg_t* args);
+
+static int luv_async_gc(lua_State* L) {
+  uv_async_t* handle = *(uv_async_t**)lua_touserdata(L, 1);
+  luv_async_arg_t* asarg = luv_get_async_arg_from_handle(handle);
+  uv_mutex_t *argmutex = &asarg->mutex;
+  uv_mutex_lock(argmutex);
+  if (luv_is_async_queue(asarg)) {
+    luv_async_send_t* sendarg;
+    while ((sendarg = luv_async_pop(asarg)) != NULL) {
+      luv_thread_arg_free(&sendarg->targ);
+      free(sendarg);
+    }
+  } else {
+    luv_thread_arg_free(&asarg->targ); // in case of a pending send
+  }
+  uv_mutex_unlock(argmutex);
+  uv_mutex_destroy(argmutex);
+  return luv_handle_gc(L);
+}
+
 static int luv_async_send(lua_State* L) {
   int ret;
   uv_async_t* handle = luv_check_async(L, 1);
-  luv_thread_arg_t* arg = (luv_thread_arg_t *)((luv_handle_t*) handle->data)->extra;
-
-  luv_thread_arg_set(L, arg, 2, lua_gettop(L), LUVF_THREAD_MODE_ASYNC|LUVF_THREAD_SIDE_CHILD);
+  luv_async_arg_t* asarg = luv_get_async_arg_from_handle(handle);
+  uv_mutex_t *argmutex = &asarg->mutex;
+  luv_thread_arg_t targcpy;
+  ret = luv_thread_arg_set(L, &targcpy, 2, lua_gettop(L), LUVF_THREAD_MODE_ASYNC|LUVF_THREAD_SIDE_CHILD);
+  if (ret < 0) {
+    luv_thread_arg_free(&targcpy);
+    return luv_thread_arg_error(L);
+  }
+  uv_mutex_lock(argmutex);
+  if (luv_is_async_queue(asarg)) {
+    luv_async_send_t* sendarg = NULL;
+    ret = UV_ENOSPC;
+    if (asarg->count < asarg->max) {
+      sendarg = luv_async_push(asarg);
+      if (sendarg == NULL) {
+        ret = UV_ENOMEM;
+      }
+    }
+    if (sendarg == NULL) {
+      uv_mutex_unlock(argmutex);
+      luv_thread_arg_free(&targcpy);
+      return luv_error(L, ret);
+    }
+    sendarg->targ = targcpy;
+  } else {
+    luv_thread_arg_free(&asarg->targ); // in case of a pending send
+    asarg->targ = targcpy;
+  }
+  uv_mutex_unlock(argmutex);
   ret = uv_async_send(handle);
-  luv_thread_arg_clear(L, arg, LUVF_THREAD_SIDE_CHILD);
   return luv_result(L, ret);
 }
+
+static void luv_async_init(lua_State* L) {
+  luaL_getmetatable(L, "uv_async");
+  lua_pushcfunction(L, luv_async_gc);
+  lua_setfield(L, -2, "__gc");
+  lua_pop(L, 1);
+}

src/luv.c

@@ -908,6 +908,7 @@ LUALIB_API int luaopen_luv (lua_State* L) {
 
   luv_req_init(L);
   luv_handle_init(L);
+  luv_async_init(L);
 #if LUV_UV_VERSION_GEQ(1, 28, 0)
   luv_dir_init(L);
 #endif