n-api: add API for asynchronous functions

Bundle a `uv_async_t`, a `uv_idle_t`, a `uv_mutex_t`, a `uv_cond_t`, and a `v8::Persistent<v8::Function>` to make it possible to call into JS from another thread. The API accepts a void data pointer and a callback which will be invoked on the loop thread and which will receive the `napi_value` representing the JavaScript function to call so as to perform the call into JS. The callback is run inside a `node::CallbackScope`. A `std::queue<void*>` is used to store calls from the secondary threads, and an idle loop is started by the `uv_async_t` callback on the loop thread to drain the queue, calling into JS with each item. Items can be added to the queue blockingly or non-blockingly. The thread-safe function can be referenced or unreferenced, with the same semantics as libuv handles. Re: nodejs/help#1035 Re: #20964 Fixes: #13512 Backport-PR-URL: #25002 PR-URL: #17887 Reviewed-By: Matteo Collina <matteo.collina@gmail.com> Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
nodejs · MylesBorins · Apr 16, 2019 · Dec 14, 2018 · Dec 19, 2018 · Dec 26, 2018
commit 759a0180b5194686692caab955e371c5efc82e4e
diff --git a/doc/api/errors.md b/doc/api/errors.md
@@ -1107,6 +1107,31 @@ multiple of the element size.
 While calling `napi_create_typedarray()`, `(length * size_of_element) +
 byte_offset` was larger than the length of given `buffer`.
 
+<a id="ERR_NAPI_TSFN_CALL_JS"></a>
+### ERR_NAPI_TSFN_CALL_JS
+
+An error occurred while invoking the JavaScript portion of the thread-safe
+function.
+
+<a id="ERR_NAPI_TSFN_GET_UNDEFINED"></a>
+### ERR_NAPI_TSFN_GET_UNDEFINED
+
+An error occurred while attempting to retrieve the JavaScript `undefined`
+value.
+
+<a id="ERR_NAPI_TSFN_START_IDLE_LOOP"></a>
+### ERR_NAPI_TSFN_START_IDLE_LOOP
+
+On the main thread, values are removed from the queue associated with the
+thread-safe function in an idle loop. This error indicates that an error
+has occurred when attemping to start the loop.
+
+<a id="ERR_NAPI_TSFN_STOP_IDLE_LOOP"></a>
+### ERR_NAPI_TSFN_STOP_IDLE_LOOP
+
+Once no more items are left in the queue, the idle loop must be suspended. This
+error indicates that the idle loop has failed to stop.
+
 <a id="ERR_NO_ICU"></a>
 ### ERR_NO_ICU
 

diff --git a/doc/api/n-api.md b/doc/api/n-api.md