node-api: faster threadsafe_function

Invoke threadsafe_function during the same tick and avoid marshalling
costs between threads and/or churning event loop if either:

1. There's a queued call already
2. `Push()` is called while the main thread was running
   threadsafe_function

Backport-PR-URL: nodejs#38506

src/node_api.cc

@@ -12,6 +12,7 @@
 #include "tracing/traced_value.h"
 #include "util-inl.h"
 
+#include <atomic>
 #include <memory>
 
 struct node_napi_env__ : public napi_env__ {
@@ -131,6 +132,7 @@ class ThreadSafeFunction : public node::AsyncResource {
                                    *v8::String::Utf8Value(env_->isolate, name)),
       thread_count(thread_count_),
       is_closing(false),
+      dispatch_state(kDispatchIdle),
       context(context_),
       max_queue_size(max_queue_size_),
       env(env_),
@@ -170,10 +172,8 @@ class ThreadSafeFunction : public node::AsyncResource {
         return napi_closing;
       }
     } else {
-      if (uv_async_send(&async) != 0) {
-        return napi_generic_failure;
-      }
       queue.push(data);
+      Send();
       return napi_ok;
     }
   }
@@ -205,9 +205,7 @@ class ThreadSafeFunction : public node::AsyncResource {
         if (is_closing && max_queue_size > 0) {
           cond->Signal(lock);
         }
-        if (uv_async_send(&async) != 0) {
-          return napi_generic_failure;
-        }
+        Send();
       }
     }
 
@@ -232,7 +230,6 @@ class ThreadSafeFunction : public node::AsyncResource {
         cond = std::make_unique<node::ConditionVariable>();
       }
       if (max_queue_size == 0 || cond) {
-        CHECK_EQ(0, uv_idle_init(loop, &idle));
         return napi_ok;
       }
 
@@ -257,21 +254,46 @@ class ThreadSafeFunction : public node::AsyncResource {
 
   napi_status Unref() {
     uv_unref(reinterpret_cast<uv_handle_t*>(&async));
-    uv_unref(reinterpret_cast<uv_handle_t*>(&idle));
 
     return napi_ok;
   }
 
   napi_status Ref() {
     uv_ref(reinterpret_cast<uv_handle_t*>(&async));
-    uv_ref(reinterpret_cast<uv_handle_t*>(&idle));
 
     return napi_ok;
   }
 
-  void DispatchOne() {
+  inline void* Context() {
+    return context;
+  }
+
+ protected:
+  void Dispatch() {
+    bool has_more = true;
+
+    // Limit maximum synchronous iteration count to prevent event loop
+    // starvation. See `src/node_messaging.cc` for an inspiration.
+    unsigned int iterations_left = kMaxIterationCount;
+    while (has_more && --iterations_left != 0) {
+      dispatch_state = kDispatchRunning;
+      has_more = DispatchOne();
+
+      // Send() was called while we were executing the JS function
+      if (dispatch_state.exchange(kDispatchIdle) != kDispatchRunning) {
+        has_more = true;
+      }
+    }
+
+    if (has_more) {
+      Send();
+    }
+  }
+
+  bool DispatchOne() {
     void* data = nullptr;
     bool popped_value = false;
+    bool has_more = false;
 
     {
       node::Mutex::ScopedLock lock(this->mutex);
@@ -296,9 +318,9 @@ class ThreadSafeFunction : public node::AsyncResource {
               cond->Signal(lock);
             }
             CloseHandlesAndMaybeDelete();
-          } else {
-            CHECK_EQ(0, uv_idle_stop(&idle));
           }
+        } else {
+          has_more = true;
         }
       }
     }
@@ -316,6 +338,8 @@ class ThreadSafeFunction : public node::AsyncResource {
         call_js_cb(env, js_callback, context, data);
       });
     }
+
+    return has_more;
   }
 
   void Finalize() {
@@ -329,10 +353,6 @@ class ThreadSafeFunction : public node::AsyncResource {
     EmptyQueueAndDelete();
   }
 
-  inline void* Context() {
-    return context;
-  }
-
   void CloseHandlesAndMaybeDelete(bool set_closing = false) {
     v8::HandleScope scope(env->isolate);
     if (set_closing) {
@@ -352,18 +372,20 @@ class ThreadSafeFunction : public node::AsyncResource {
           ThreadSafeFunction* ts_fn =
               node::ContainerOf(&ThreadSafeFunction::async,
                                 reinterpret_cast<uv_async_t*>(handle));
-          v8::HandleScope scope(ts_fn->env->isolate);
-          ts_fn->env->node_env()->CloseHandle(
-              reinterpret_cast<uv_handle_t*>(&ts_fn->idle),
-              [](uv_handle_t* handle) -> void {
-                ThreadSafeFunction* ts_fn =
-                    node::ContainerOf(&ThreadSafeFunction::idle,
-                                      reinterpret_cast<uv_idle_t*>(handle));
-                ts_fn->Finalize();
-              });
+          ts_fn->Finalize();
         });
   }
 
+  void Send() {
+    // Ask currently running Dispatch() to make one more iteration
+    unsigned char current_state = dispatch_state.fetch_or(kDispatchPending);
+    if ((current_state & kDispatchRunning) == kDispatchRunning) {
+      return;
+    }
+
+    CHECK_EQ(0, uv_async_send(&async));
+  }
+
   // Default way of calling into JavaScript. Used when ThreadSafeFunction is
   //  without a call_js_cb_.
   static void CallJs(napi_env env, napi_value cb, void* context, void* data) {
@@ -387,16 +409,10 @@ class ThreadSafeFunction : public node::AsyncResource {
     }
   }
 
-  static void IdleCb(uv_idle_t* idle) {
-    ThreadSafeFunction* ts_fn =
-        node::ContainerOf(&ThreadSafeFunction::idle, idle);
-    ts_fn->DispatchOne();
-  }
-
   static void AsyncCb(uv_async_t* async) {
     ThreadSafeFunction* ts_fn =
         node::ContainerOf(&ThreadSafeFunction::async, async);
-    CHECK_EQ(0, uv_idle_start(&ts_fn->idle, IdleCb));
+    ts_fn->Dispatch();
   }
 
   static void Cleanup(void* data) {
@@ -405,14 +421,20 @@ class ThreadSafeFunction : public node::AsyncResource {
   }
 
  private:
+  static const unsigned char kDispatchIdle = 0;
+  static const unsigned char kDispatchRunning = 1 << 0;
+  static const unsigned char kDispatchPending = 1 << 1;
+
+  static const unsigned int kMaxIterationCount = 1000;
+
   // These are variables protected by the mutex.
   node::Mutex mutex;
   std::unique_ptr<node::ConditionVariable> cond;
   std::queue<void*> queue;
   uv_async_t async;
-  uv_idle_t idle;
   size_t thread_count;
   bool is_closing;
+  std::atomic_uchar dispatch_state;
 
   // These are variables set once, upon creation, and then never again, which
   // means we don't need the mutex to read them.

test/node-api/test_threadsafe_function/binding.c

@@ -7,7 +7,7 @@
 #include <node_api.h>
 #include "../../js-native-api/common.h"
 
-#define ARRAY_LENGTH 10
+#define ARRAY_LENGTH 10000
 #define MAX_QUEUE_SIZE 2
 
 static uv_thread_t uv_threads[2];
@@ -72,7 +72,7 @@ static void data_source_thread(void* data) {
   for (index = ARRAY_LENGTH - 1; index > -1 && !queue_was_closing; index--) {
     status = napi_call_threadsafe_function(ts_fn, &ints[index],
         ts_fn_info->block_on_full);
-    if (ts_fn_info->max_queue_size == 0) {
+    if (ts_fn_info->max_queue_size == 0 && (index % 1000 == 0)) {
       // Let's make this thread really busy for 200 ms to give the main thread a
       // chance to abort.
       uint64_t start = uv_hrtime();

test/node-api/test_threadsafe_function/test.js

@@ -210,6 +210,15 @@ new Promise(function testWithoutJSMarshaller(resolve) {
 }))
 .then((result) => assert.strictEqual(result.indexOf(0), -1))
 
+// Make sure that threadsafe function isn't stalled when we hit
+// `kMaxIterationCount` in `src/node_api.cc`
+.then(() => testWithJSMarshaller({
+  threadStarter: 'StartThreadNonblocking',
+  maxQueueSize: binding.ARRAY_LENGTH >>> 1,
+  quitAfter: binding.ARRAY_LENGTH
+}))
+.then((result) => assert.deepStrictEqual(result, expectedArray))
+
 // Start a child process to test rapid teardown
 .then(() => testUnref(binding.MAX_QUEUE_SIZE))