feat(async-worker): introduce a KerberosWorker using lambdas

This allows us to signifcantly reduce the amount of boilerplate
required for async workers, by replacing the implementation with
a single lambda. This should also make it much easier to split up
implementation from definition in order to support both unix and
windows implementations.

src/common.h

@@ -5,6 +5,10 @@
 #include "kerberos_gss.h"
 
 // Provide a default custom delter for the `gss_result` type
+inline void ResultDeleter(gss_result* result) {
+  free(result);
+}
+
 struct FreeDeleter {
     void operator()(void* result) {
         free(result);

src/kerberos.cc

@@ -4,6 +4,7 @@
 #include "kerberos.h"
 #include "kerberos_client.h"
 #include "kerberos_server.h"
+#include "kerberos_worker.h"
 
 using v8::FunctionTemplate;
 
@@ -159,44 +160,28 @@ NAN_METHOD(PrincipalDetails) {
     AsyncQueueWorker(new PrincipalDetailsWorker(service, hostname, callback));
 }
 
-class CheckPasswordWorker : public Nan::AsyncWorker {
-   public:
-    CheckPasswordWorker(std::string username,
-                        std::string password,
-                        std::string service,
-                        std::string defaultRealm,
-                        Nan::Callback* callback)
-        : AsyncWorker(callback, "kerberos:CheckPassword"),
-          _username(username),
-          _password(password),
-          _service(service),
-          _defaultRealm(defaultRealm) {}
-
-    virtual void Execute() {
-        std::unique_ptr<gss_result, FreeDeleter> result(authenticate_user_krb5pwd(
-            _username.c_str(), _password.c_str(), _service.c_str(), _defaultRealm.c_str()));
-
-        if (result->code == AUTH_GSS_ERROR) {
-            SetErrorMessage(result->message);
-            return;
-        }
-    }
-
-   private:
-    std::string _username;
-    std::string _password;
-    std::string _service;
-    std::string _defaultRealm;
-};
-
 NAN_METHOD(CheckPassword) {
     std::string username(*Nan::Utf8String(info[0]));
     std::string password(*Nan::Utf8String(info[1]));
     std::string service(*Nan::Utf8String(info[2]));
     std::string defaultRealm(*Nan::Utf8String(info[3]));
     Nan::Callback* callback = new Nan::Callback(Nan::To<v8::Function>(info[4]).ToLocalChecked());
 
-    AsyncQueueWorker(new CheckPasswordWorker(username, password, service, defaultRealm, callback));
+    KerberosWorker::Run(callback, "kerberos:CheckPassword", [=](KerberosWorker::SetOnFinishedHandler onFinished) {
+        std::shared_ptr<gss_result> result(authenticate_user_krb5pwd(
+            username.c_str(), password.c_str(), service.c_str(), defaultRealm.c_str()), ResultDeleter);
+
+        return onFinished([=](KerberosWorker* worker) {
+            Nan::HandleScope scope;
+            if (result->code == AUTH_GSS_ERROR) {
+                v8::Local<v8::Value> argv[] = {Nan::New(result->message).ToLocalChecked(), Nan::Null()};
+                worker->Call(2, argv);
+            } else {
+                v8::Local<v8::Value> argv[] = {Nan::Null(), Nan::Null()};
+                worker->Call(2, argv);
+            }
+        });
+    });
 }
 
 NAN_MODULE_INIT(Init) {

src/kerberos_worker.h

@@ -0,0 +1,48 @@
+#ifndef ASYNC_WORKER_H
+#define ASYNC_WORKER_H
+
+#include <functional>
+#include <nan.h>
+
+class KerberosWorker : public Nan::AsyncWorker {
+ public:
+    typedef std::function<void(KerberosWorker*)>  OnFinishedHandler;
+    typedef std::function<void(OnFinishedHandler)> SetOnFinishedHandler;
+    typedef std::function<void(SetOnFinishedHandler)> ExecuteHandler;
+
+    explicit KerberosWorker(Nan::Callback *callback, const char* resource_name, ExecuteHandler handler)
+        : Nan::AsyncWorker(callback, resource_name), execute_handler(handler) {}
+
+    template <class... T>
+    void Call(T... t) {
+      callback->Call(t..., async_resource);
+    }
+
+    virtual void Execute() {
+        execute_handler([=] (OnFinishedHandler handler) {
+            on_finished_handler = handler;
+        });
+    }
+
+    static void Run(Nan::Callback *callback, const char* resource_name, ExecuteHandler handler) {
+        Nan::TryCatch tryCatch;
+        if (tryCatch.HasCaught()) {
+            tryCatch.ReThrow();
+            return; // don't proceed in case there were any previous errors
+        }
+
+        auto worker = new KerberosWorker(callback, resource_name, handler);
+        Nan::AsyncQueueWorker(worker);
+    }
+
+ protected:
+    void HandleOKCallback() {
+        on_finished_handler(this);
+    }
+
+ private:
+    ExecuteHandler execute_handler;
+    OnFinishedHandler on_finished_handler;
+};
+
+#endif // ASYNC_WORKER_H