wpiutil: Add C++ libuv wrappers (#1166)

- Provide an EventLoopRunner to run uv::Loop on a separate thread.

- Add raw_ostream wrapper for uv::Buffer.

wpiutil/src/main/native/include/wpi/uv/Work.h

@@ -0,0 +1,97 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_WPI_UV_WORK_H_
+#define WPIUTIL_WPI_UV_WORK_H_
+
+#include <uv.h>
+
+#include <functional>
+#include <memory>
+
+#include "wpi/Signal.h"
+#include "wpi/uv/Request.h"
+
+namespace wpi {
+namespace uv {
+
+class Loop;
+
+/**
+ * Work request.
+ * For use with `QueueWork()` function family.
+ */
+class WorkReq : public RequestImpl<WorkReq, uv_work_t> {
+ public:
+  WorkReq();
+
+  Loop& GetLoop() const { return *static_cast<Loop*>(GetRaw()->loop->data); }
+
+  /**
+   * Function(s) that will be run on the thread pool.
+   */
+  sig::Signal<> work;
+
+  /**
+   * Function(s) that will be run on the loop thread after the work on the
+   * thread pool has been completed by the work callback.
+   */
+  sig::Signal<> afterWork;
+};
+
+/**
+ * Initializes a work request which will run on the thread pool.
+ *
+ * @param loop Event loop
+ * @param req request
+ */
+void QueueWork(Loop& loop, const std::shared_ptr<WorkReq>& req);
+
+/**
+ * Initializes a work request which will run on the thread pool.
+ *
+ * @param loop Event loop
+ * @param req request
+ */
+inline void QueueWork(const std::shared_ptr<Loop>& loop,
+                      const std::shared_ptr<WorkReq>& req) {
+  QueueWork(*loop, req);
+}
+
+/**
+ * Initializes a work request which will run on the thread pool.  The work
+ * callback will be run on a thread from the thread pool, and the afterWork
+ * callback will be called on the loop thread after the work completes.  Any
+ * errors are forwarded to the loop.
+ *
+ * @param loop Event loop
+ * @param work Work callback (called from separate thread)
+ * @param afterWork After work callback (called on loop thread)
+ */
+void QueueWork(Loop& loop, std::function<void()> work,
+               std::function<void()> afterWork);
+
+/**
+ * Initializes a work request which will run on the thread pool.  The work
+ * callback will be run on a thread from the thread pool, and the afterWork
+ * callback will be called on the loop thread after the work completes.  Any
+ * errors are forwarded to the loop.
+ *
+ * @param loop Event loop
+ * @param work Work callback (called from separate thread)
+ * @param afterWork After work callback (called on loop thread)
+ */
+inline void QueueWork(const std::shared_ptr<Loop>& loop,
+                      std::function<void()> work,
+                      std::function<void()> afterWork) {
+  QueueWork(*loop, work, afterWork);
+}
+
+}  // namespace uv
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_UV_WORK_H_