n-api: add API for asynchronous functions

Bundle a uv_async_t and a napi_ref to make it possible to call into JS
from another thread. The API accepts a void data and context pointer,
an optional native-to-JS function argument marshaller, and a
JS-to-native return value marshaller.

Fixes: #13512

doc/api/n-api.md

@@ -47,6 +47,7 @@ The documentation for N-API is structured as follows:
 * [Custom Asynchronous Operations][]
 * [Promises][]
 * [Script Execution][]
+* [Asynchronous Thread-safe Function Calls][]
 
 The N-API is a C API that ensures ABI stability across Node.js versions
 and different compiler levels. However, we also understand that a C++
@@ -203,6 +204,36 @@ typedef void (*napi_async_complete_callback)(napi_env env,
                                              void* data);
 ```
 
+#### napi_threadsafe_function_marshal
+Function pointer used with asynchronous thread-safe function calls. The callback
+will be called on the main thread. Its purpose is to compute the JavaScript
+function context and its arguments from the native data associated with the
+thread-safe function and store them in `recv` and `argv`, respectively.
+Callback functions must satisfy the following signature:
+
+```C
+typedef napi_status(*napi_threadsafe_function_marshal)(napi_env env,
+                                                       void* data,
+                                                       napi_value* recv,
+                                                       size_t argc,
+                                                       napi_value* argv);
+```
+
+#### napi_threadsafe_function_process_result
+Function pointer used with asynchronous thread-safe function calls. The callback
+will be called on the main thread after the JavaScript function has returned.
+If an exception was thrown during the execution of the JavaScript function, it
+will be made available in the `error` parameter. The `result` parameter will
+have the function's return value. Both parameters may be `NULL`, but one of them
+will always be set.
+
+```C
+typedef void(*napi_threadsafe_function_process_result)(napi_env env,
+                                                       void* data,
+                                                       napi_value error,
+                                                       napi_value result);
+```
+
 ## Error Handling
 N-API uses both return values and JavaScript exceptions for error handling.
 The following sections explain the approach for each case.
@@ -3705,6 +3736,105 @@ NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
 - `[in] env`: The environment that the API is invoked under.
 - `[out] loop`: The current libuv loop instance.
 
+<!-- it's very convenient to have all the anchors indexed -->
+<!--lint disable no-unused-definitions remark-lint-->
+## Asynchronous Thread-safe Function Calls
+JavaScript functions can normally only be called from a native addon's main
+thread. If an addon creates additional threads then N-API functions that require
+a `napi_env`, `napi_value`, or `napi_ref` must not be called from those threads.
+
+This API provides the type `napi_threadsafe_function` as well as APIs to create,
+destroy, and call objects of this type. `napi_threadsafe_function` creates a
+permanent reference to a `napi_value` that holds a JavaScript function, and
+uses `uv_async_t` from libuv to coordinate calls to the JavaScript function from
+all threads.
+
+The user provides callbacks `marshal_cb` and `process_result_cb` to handle the
+conversion of the native data to JavaScript function argfuments, and to process
+the JavaScript function return value or a possible error condition,
+respectively.
+
+`napi_threadsafe_function` objects are destroyed by passing them to
+`napi_delete_threadsafe_function()`. Make sure that all threads that have
+references to the `napi_threadsafe_function` object are stopped before deleting
+the object.
+
+Since `uv_async_t` is used in the implementation, the caveat whereby multiple
+invocations on the secondary thread may result in only one invocation of the
+JavaScript function also applies to `napi_threadsafe_function`.
+
+### napi_create_threadsafe_function
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status
+napi_create_threadsafe_function(napi_env env,
+                                napi_value func,
+                                void* data,
+                                size_t argc,
+                                napi_threadsafe_function_marshal marshal_cb,
+                                napi_threadsafe_function_process_result
+                                    process_result_cb,
+                                napi_threadsafe_function* result);
+```
+
+- `[in] env`: The environment that the API is invoked under.
+- `[in] func`: The JavaScript function to call from another thread.
+- `[in] data`: Optional data to attach to the resulting `napi_threadsafe_function`.
+- `[in] context`: Optional context associated with `data`.
+- `[in] argc`: Number of arguments the JavaScript function will have.
+- `[in] marshal_cb`: Optional callback to convert `data` and `context` to
+JavaScript function arguments. The callback will always be called on the main
+thread.
+- `[in] process_result_cb`: Optional callback to handle the return value and/or
+exception resulting from the invocation of the JavaScript function. The callback
+will always be called on the main thread.
+- `[out] result`: The asynchronous thread-safe JavaScript function.
+
+### napi_call_threadsafe_function
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status
+napi_call_threadsafe_function(napi_threadsafe_function func);
+```
+
+- `[in] func`: The asynchronous thread-safe JavaScript function to invoke. This
+API may be called from any thread.
+
+### napi_get_threadsafe_function_data
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status
+napi_get_threadsafe_function_data(napi_threadsafe_function func,
+                                  void** data);
+```
+
+- `[in] func`: The asynchronous thread-safe JavaScript function whose associated
+data to retrieve.
+- `[out] data`: Optional pointer to receive the data associated with the
+thread-safe JavaScript function.
+- `[out]: context`: Optional pointer to receive the context associated with the
+thread-safe JavaScript function.
+
+### napi_delete_threadsafe_function
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status
+napi_delete_threadsafe_function(napi_env env,
+                                napi_threadsafe_function func);
+```
+
+- `[in] env`: The environment that the API is invoked under.
+- `[in] func`: The asynchronous thread-safe JavaScript function to delete.
+
+[Asynchronous Thread-safe Function Calls]: #n_api_asynchronous_thread-safe_function_calls
 [Promises]: #n_api_promises
 [Simple Asynchronous Operations]: #n_api_simple_asynchronous_operations
 [Custom Asynchronous Operations]: #n_api_custom_asynchronous_operations

src/node_api.cc

@@ -18,7 +18,7 @@
 #include "node_api.h"
 #include "node_internals.h"
 
-#define NAPI_VERSION  2
+#define NAPI_VERSION  3
 
 static
 napi_status napi_set_last_error(napi_env env, napi_status error_code,
@@ -3514,3 +3514,189 @@ napi_status napi_run_script(napi_env env,
   *result = v8impl::JsValueFromV8LocalValue(script_result.ToLocalChecked());
   return GET_RETURN_STATUS(env);
 }
+
+struct napi_threadsafe_function__ {
+  uv_async_t async;
+  napi_ref ref;
+  napi_env env;
+  size_t argc;
+  void* data;
+  void* context;
+  napi_threadsafe_function_marshal marshal_cb;
+  napi_threadsafe_function_process_result process_result_cb;
+};
+
+static napi_value napi_threadsafe_function_error(napi_env env,
+                                                 const char* message) {
+  napi_value result, js_message;
+  if (napi_create_string_utf8(env, message, NAPI_AUTO_LENGTH, &js_message) ==
+      napi_ok) {
+    if (napi_create_error(env, nullptr, js_message, &result) == napi_ok) {
+      return result;
+    }
+  }
+
+  napi_fatal_error("N-API thread-safe function", NAPI_AUTO_LENGTH,
+      (std::string("Failed to create JS error: ") +
+      std::string(message)).c_str(), NAPI_AUTO_LENGTH);
+  return nullptr;
+}
+
+static void napi_threadsafe_function_cb(uv_async_t* uv_async) {
+  napi_threadsafe_function async =
+      node::ContainerOf(&napi_threadsafe_function__::async, uv_async);
+  v8::HandleScope handle_scope(async->env->isolate);
+
+  napi_value js_cb;
+  napi_value recv;
+  napi_value js_result = nullptr;
+  napi_value exception = nullptr;
+  std::vector<napi_value> argv(async->argc);
+
+  napi_status status = napi_get_reference_value(async->env, async->ref, &js_cb);
+  if (status != napi_ok) {
+    exception = napi_threadsafe_function_error(async->env,
+        "Failed to retrieve JS callback");
+    goto done;
+  }
+
+  status = async->marshal_cb(async->env, async->data, &recv, async->argc,
+      argv.data());
+  if (status != napi_ok) {
+    exception = napi_threadsafe_function_error(async->env,
+        "Failed to marshal JS callback arguments");
+    goto done;
+  }
+
+  status = napi_make_callback(async->env, nullptr, recv, js_cb, async->argc,
+      argv.data(), &js_result);
+  if (status != napi_ok) {
+    if (status == napi_pending_exception) {
+      status = napi_get_and_clear_last_exception(async->env, &exception);
+      if (status != napi_ok) {
+        exception = napi_threadsafe_function_error(async->env,
+            "Failed to retrieve JS callback exception");
+        goto done;
+      }
+    } else {
+      exception = napi_threadsafe_function_error(async->env,
+          "Failed to call JS callback");
+      goto done;
+    }
+  }
+
+done:
+  async->process_result_cb(async->env, async->data, exception, js_result);
+}
+
+static napi_status napi_threadsafe_function_default_marshal(napi_env env,
+                                                            void* data,
+                                                            napi_value* recv,
+                                                            size_t argc,
+                                                            napi_value* argv) {
+  napi_status status;
+  for (size_t index = 0; index < argc; index++) {
+    status = napi_get_undefined(env, &argv[index]);
+    if (status != napi_ok) {
+      return status;
+    }
+  }
+  return napi_get_global(env, recv);
+}
+
+static void napi_threadsafe_function_default_process_result(napi_env env,
+                                                            void* data,
+                                                            napi_value error,
+                                                            napi_value result) {
+  if (error != nullptr) {
+    napi_throw(env, error);
+  }
+}
+
+NAPI_EXTERN napi_status
+napi_create_threadsafe_function(napi_env env,
+                                napi_value func,
+                                void* data,
+                                size_t argc,
+                                napi_threadsafe_function_marshal marshal_cb,
+                                napi_threadsafe_function_process_result
+                                    process_result_cb,
+                                napi_threadsafe_function* result) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, func);
+  CHECK_ARG(env, result);
+
+  napi_valuetype func_type;
+  napi_status status = napi_typeof(env, func, &func_type);
+  if (status != napi_ok) {
+    return status;
+  }
+
+  if (func_type != napi_function) {
+    return napi_set_last_error(env, napi_function_expected);
+  }
+
+  napi_threadsafe_function async = new napi_threadsafe_function__;
+  if (async == nullptr) {
+    return napi_set_last_error(env, napi_generic_failure);
+  }
+
+  status = napi_create_reference(env, func, 1, &async->ref);
+  if (status != napi_ok) {
+    delete async;
+    return status;
+  }
+
+  if (uv_async_init(uv_default_loop(), &async->async,
+      napi_threadsafe_function_cb) != 0) {
+    napi_delete_reference(env, async->ref);
+    delete async;
+    return napi_set_last_error(env, napi_generic_failure);
+  }
+
+  async->argc = argc;
+  async->marshal_cb = marshal_cb == nullptr ?
+      napi_threadsafe_function_default_marshal : marshal_cb;
+  async->process_result_cb =
+      process_result_cb == nullptr ?
+          napi_threadsafe_function_default_process_result : process_result_cb;
+  async->data = data;
+  async->env = env;
+
+  *result = async;
+  return napi_clear_last_error(env);
+}
+
+NAPI_EXTERN napi_status
+napi_get_threadsafe_function_data(napi_threadsafe_function async,
+                                  void** data) {
+  if (data != nullptr) {
+    *data = async->data;
+  }
+  return napi_ok;
+}
+
+NAPI_EXTERN napi_status
+napi_call_threadsafe_function(napi_threadsafe_function async) {
+  return uv_async_send(&async->async) == 0 ?
+      napi_ok : napi_generic_failure;
+}
+
+NAPI_EXTERN napi_status
+napi_delete_threadsafe_function(napi_env env,
+                                napi_threadsafe_function async) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, async);
+
+  napi_status status = napi_delete_reference(env, async->ref);
+  if (status != napi_ok) {
+    return status;
+  }
+
+  uv_close(reinterpret_cast<uv_handle_t*>(&async->async),
+      [] (uv_handle_t* handle) -> void {
+        delete handle;
+      });
+
+  return napi_clear_last_error(env);
+}

src/node_api.h

@@ -587,6 +587,28 @@ NAPI_EXTERN napi_status napi_run_script(napi_env env,
 NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                                struct uv_loop_s** loop);
 
+// Calling into JS from other threads
+NAPI_EXTERN napi_status
+napi_create_threadsafe_function(napi_env env,
+                                napi_value func,
+                                void* data,
+                                size_t argc,
+                                napi_threadsafe_function_marshal marshal_cb,
+                                napi_threadsafe_function_process_result
+                                    process_result_cb,
+                                napi_threadsafe_function* result);
+
+NAPI_EXTERN napi_status
+napi_call_threadsafe_function(napi_threadsafe_function func);
+
+NAPI_EXTERN napi_status
+napi_get_threadsafe_function_data(napi_threadsafe_function func,
+                                  void** data);
+
+NAPI_EXTERN napi_status
+napi_delete_threadsafe_function(napi_env env,
+                                napi_threadsafe_function func);
+
 EXTERN_C_END
 
 #endif  // SRC_NODE_API_H_