doc: document ThreadSafeFunction (#494)

* doc: document ThreadSafeFunction

PR-URL: #494
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/threadsafe_function.md

@@ -0,0 +1,303 @@
+# 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 `Napi::Env`, `Napi::Value`, or `Napi::Reference` 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.
+
+`Napi::ThreadSafeFunction` objects are destroyed when every thread which uses
+the object has called `Release()` or has received a return status of
+`napi_closing` in response to a call to `BlockingCall()` or `NonBlockingCall()`.
+The queue is emptied before the `Napi::ThreadSafeFunction` is destroyed. It is
+important that `Release()` be the last API call made in conjunction with a given
+`Napi::ThreadSafeFunction`, because after the call completes, there is no
+guarantee that the `Napi::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 `napi_closing` in response to a call to
+`BlockingCall()` or `NonBlockingCall()`. Data associated with the
+`Napi::ThreadSafeFunction` can be freed in its `Finalizer` callback which was
+passed to `ThreadSafeFunction::New()`.
+
+Once the number of threads making use of a `Napi::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);
+```
+
+- `tsfn`: The `napi_threadsafe_function` which is a handle for an existing
+  thread-safe 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,
+    ContextType* context,
+    Finalizer finalizeCallback,
+    FinalizerDataType* data);
+```
+
+- `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] context`: Data to attach to the resulting `ThreadSafeFunction`.
+- `[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.
+  Can be retreived via `GetContext()`.
+- `[optional] data`: Data to be passed to `finalizeCallback`.
+
+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
+napi_status Napi::ThreadSafeFunction::Acquire()
+```
+
+Returns one of:
+- `napi_ok`: The thread has successfully acquired the thread-safe function
+for its use. 
+- `napi_closing`: The thread-safe function has been marked as closing via a
+previous call to `Abort()`.
+
+### 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
+napi_status Napi::ThreadSafeFunction::Release()
+```
+
+Returns one of:
+- `napi_ok`: The thread-safe function has been successfully released.
+- `napi_invalid_arg`: The thread-safe function's thread-count is zero.
+- `napi_generic_failure`: A generic error occurred when attemping to release
+the thread-safe function.
+
+### 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 thread-safe 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 one of:
+- `napi_ok`: The thread-safe function has been successfully aborted.
+- `napi_invalid_arg`: The thread-safe function's thread-count is zero.
+- `napi_generic_failure`: A generic error occurred when attemping to abort
+the thread-safe function.
+
+### BlockingCall / NonBlockingCall
+
+Calls the Javascript function in 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 value of `clock()` five times at 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 ]
+```