doc: document ThreadSafeFunction

doc/threadsafe_function.md

@@ -0,0 +1,219 @@
+# ThreadSafeFunction
+
+JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then node-addon-api functions that require a `Env`, `Value`, or `Ref` must not be called from those threads.
+
+When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.
+
+These APIs provide the type `Napi::ThreadSafeFunction` as well as APIs to create, destroy, and call objects of this type. `Napi::ThreadSafeFunction::New()` creates a persistent reference that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function.
+
+`ThreadSafeFunction` objects are destroyed when every thread which uses the object has called `Release()` or has received a return status of `CLOSING` in response to a call to `BlockingCall()` or `NonBlockingCall()`. The queue is emptied before the `ThreadSafeFunction` is destroyed. It is important that `Release()` be the last API call made in conjunction with a given `ThreadSafeFunction`, because after the call completes, there is no guarantee that the `ThreadSafeFunction` is still allocated. For the same reason it is also important that no more use be made of a thread-safe function after receiving a return value of `CLOSING` in response to a call to `BlockingCall()` or `NonBlockingCall()`. Data associated with the `ThreadSafeFunction` can be freed in its `Finalizer` callback which was passed to `ThreadSafeFunction::New()`.
+
+Once the number of threads making use of a `ThreadSafeFunction` reaches zero, no further threads can start making use of it by calling `Acquire()`. In fact, all subsequent API calls associated with it, except `Release()`, will return an error value of napi_closing.
+
+## Methods
+
+### Constructor
+
+Creates a new empty instance of `Napi::ThreadSafeFunction`.
+
+```cpp
+Napi::Function::ThreadSafeFunction();
+```
+
+### Constructor
+
+Creates a new instance of the `Napi::ThreadSafeFunction` object.
+
+```cpp
+Napi::ThreadSafeFunction::ThreadSafeFunction(napi_threadsafe_function tsfn);
+```
+
+- `[in] value`: The `napi_threadsafe_function` which is a handle for an existing napi threadsafe function.
+
+Returns a non-empty `Napi::ThreadSafeFunction` instance.
+
+### New
+
+Creates a new instance of the `Napi::ThreadSafeFunction` object. The `New` function has several overloads for the various optional parameters: skip the optional parameter for that specific overload.
+
+```cpp
+New(napi_env env,
+    const Function& callback,
+    const Object& resource,
+    ResourceString resourceName,
+    size_t maxQueueSize,
+    size_t initialThreadCount,
+    DataType* data,
+    Finalizer finalizeCallback,
+    Context* context);
+```
+
+- `env`: The `napi_env` environment in which to construct the `Napi::ThreadSafeFunction` object.
+- `callback`: The `Function` to call from another thread.
+- `[optional] resource`: An object associated with the async work that will be passed to possible async_hooks init hooks.
+- `resourceName`: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
+- `maxQueueSize`: Maximum size of the queue. `0` for no limit.
+- `initialThreadCount`: The initial number of threads, including the main thread, which will be making use of this function.
+- `[optional] data`: Data to be passed to `finalizeCallback`.
+- `[optional] finalizeCallback`: Function to call when the `ThreadSafeFunction` is being destroyed.  This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction (if given), and provides an opportunity for cleaning up after the threads e.g. by calling `uv_thread_join()`. It is important that, aside from the main loop thread, there be no threads left using the thread-safe function after the finalize callback completes. Must implement `void operator()(Env env, DataType* data, Context* hint)`, skipping `data` or `hint` if they are not provided.
+- `[optional] context`: Data to attach to the resulting `ThreadSafeFunction`. Can be retreived via `GetContext()`.
+
+Returns a non-empty `Napi::ThreadSafeFunction` instance.
+
+### Acquire
+
+Add a thread to this thread-safe function object, indicating that a new thread will start making use of the thread-safe function. 
+
+```cpp
+bool Napi::ThreadSafeFunction::Acquire()
+```
+
+Returns `true` if the thread can successfully be added to the thread count.
+
+### Release
+
+Indicate that an existing thread will stop making use of the thread-safe function. A thread should call this API when it stops making use of this thread-safe function. Using any thread-safe APIs after having called this API has undefined results in the current thread, as it may have been destroyed.
+
+```cpp
+bool Napi::ThreadSafeFunction::Release()
+```
+
+Returns `true` if the thread-safe function has successfully released.
+
+### Abort
+
+"Abort" the thread-safe function. This will cause all subsequent APIs associated with the thread-safe function except `Release()` to return `napi_closing` even before its reference count reaches zero. In particular, `BlockingCall` and `NonBlockingCall()` will return `napi_closing`, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of `napi_closing` from a threadsafe function call a thread must make no further use of the thread-safe function because it is no longer guaranteed to be allocated.
+
+```cpp
+napi_status Napi::ThreadSafeFunction::Abort()
+```
+
+Returns `napi_ok` if the thread-safe function has successfully aborted.
+
+### BlockingCall / NonBlockingCall
+
+Calls the Javascript function in a either a blocking or non-blocking fashion.
+- `BlockingCall()`: the API blocks until space becomes available in the queue. Will never block if the thread-safe function was created with a maximum queue size of `0`.
+- `NonBlockingCall()`: will return `napi_queue_full` if the queue was full, preventing data from being successfully added to the queue
+
+There are several overloaded implementations of `BlockingCall()` and `NonBlockingCall()` for use with optional parameters: skip the optional parameter for that specific overload.
+
+```cpp
+napi_status Napi::ThreadSafeFunction::BlockingCall(DataType* data, Callback callback) const
+
+napi_status Napi::ThreadSafeFunction::NonBlockingCall(DataType* data, Callback callback) const
+```
+
+- `[optional] data`: Data to pass to `callback`.
+- `[optional] callback`: C++ function that is invoked on the main thread. The callback receives the `ThreadSafeFunction`'s JavaScript callback function to call as an `Napi::Function` in its parameters and the `DataType*` data pointer (if provided). Must implement `void operator()(napi_env env, Function jsCallback, DataType* data)`, skipping `data` if not provided. It is not necessary to call into JavaScript via `MakeCallback()` because N-API runs `callback` in a context appropriate for callbacks.
+
+Returns one of:
+- `napi_ok`: The call was successfully added to the queue.
+- `napi_queue_full`: The queue was full when trying to call in a non-blocking method.
+- `napi_closing`: The thread-safe function is aborted and cannot accept more calls.
+- `napi_invalid_arg`: The thread-safe function is closed.
+- `napi_generic_failure`: A generic error occurred when attemping to add to the queue.
+
+## Example
+
+```cpp
+#include <chrono>
+#include <thread>
+#include <napi.h>
+
+using namespace Napi;
+
+std::thread nativeThread;
+ThreadSafeFunction tsfn;
+
+Value Start( const CallbackInfo& info )
+{
+  Napi::Env env = info.Env();
+
+  if ( info.Length() < 2 )
+  {
+    throw TypeError::New( env, "Expected two arguments" );
+  }
+  else if ( !info[0].IsFunction() )
+  {
+    throw TypeError::New( env, "Expected first arg to be function" );
+  }
+  else if ( !info[1].IsNumber() )
+  {
+    throw TypeError::New( env, "Expected second arg to be number" );
+  }
+
+  int count = info[1].As<Number>().Int32Value();
+
+  // Create a ThreadSafeFunction
+  tsfn = ThreadSafeFunction::New(
+      env,
+      info[0].As<Function>(),  // JavaScript function called asynchronously
+      "Resource Name",         // Name
+      0,                       // Unlimited queue
+      1,                       // Only one thread will use this initially
+      []( Napi::Env ) {        // Finalizer used to clean threads up
+        nativeThread.join();
+      } );
+
+  // Create a native thread
+  nativeThread = std::thread( [count] {
+    auto callback = []( Napi::Env env, Function jsCallback, int* value ) {
+      // Transform native data into JS data, passing it to the provided `jsCallback` -- the TSFN's JavaScript function.
+      jsCallback.Call( {Number::New( env, *value )} );
+
+      // We're finished with the data.
+      delete value;
+    };
+
+    for ( int i = 0; i < count; i++ )
+    {
+      // Create new data
+      int* value = new int( clock() );
+
+      // Perform a blocking call
+      napi_status status = tsfn.BlockingCall( value, callback );
+      if ( status != napi_ok )
+      {
+        // Handle error
+        break;
+      }
+
+      std::this_thread::sleep_for( std::chrono::seconds( 1 ) );
+    }
+
+    // Release the thread-safe function
+    tsfn.Release();
+  } );
+
+  return Boolean::New(env, true);
+}
+
+Napi::Object Init( Napi::Env env, Object exports )
+{
+  exports.Set( "start", Function::New( env, Start ) );
+  return exports;
+}
+
+NODE_API_MODULE( clock, Init )
+```
+
+The above code can be used from JavaScript as follows:
+
+```js
+const { start } = require('bindings')('clock');
+
+start(function () {
+    console.log("JavaScript callback called with arguments", Array.from(arguments));
+}, 5);
+```
+
+When executed, the output will show the output of `clock()` five times in one second intervals:
+
+```
+JavaScript callback called with arguments [ 84745 ]
+JavaScript callback called with arguments [ 103211 ]
+JavaScript callback called with arguments [ 104516 ]
+JavaScript callback called with arguments [ 105104 ]
+JavaScript callback called with arguments [ 105691 ]
+```