wpiutil: Add a signal-slot implementation. (#1163)

Imported from https://github.com/palacaze/sigslot

Classes were renamed from lowercase_me to UppercaseMe style, primarily
to avoid conflicting with the C standard library "signal" function.  They
were also moved to the "wpi::sig" namespace.

wpiutil/src/test/native/cpp/sigslot/signal-threaded.cpp

@@ -0,0 +1,93 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+/*
+
+Sigslot, a signal-slot library
+
+https://github.com/palacaze/sigslot
+
+MIT License
+
+Copyright (c) 2017 Pierre-Antoine Lacaze
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+ */
+
+#include "wpi/Signal.h"  // NOLINT(build/include_order)
+
+#include "gtest/gtest.h"  // NOLINT(build/include_order)
+
+#include <array>
+#include <atomic>
+#include <thread>
+
+using namespace wpi::sig;
+
+namespace {
+
+std::atomic<int> sum{0};
+
+void f(int i) { sum += i; }
+
+void emit_many(Signal<int>& sig) {
+  for (int i = 0; i < 10000; ++i) sig(1);
+}
+
+void connect_emit(Signal<int>& sig) {
+  for (int i = 0; i < 100; ++i) {
+    auto s = sig.connect_scoped(f);
+    for (int j = 0; j < 100; ++j) sig(1);
+  }
+}
+
+}  // namespace
+
+namespace wpi {
+
+TEST(Signal, ThreadedMix) {
+  sum = 0;
+
+  Signal<int> sig;
+
+  std::array<std::thread, 10> threads;
+  for (auto& t : threads) t = std::thread(connect_emit, std::ref(sig));
+
+  for (auto& t : threads) t.join();
+}
+
+TEST(Signal, ThreadedEmission) {
+  sum = 0;
+
+  Signal<int> sig;
+  sig.connect(f);
+
+  std::array<std::thread, 10> threads;
+  for (auto& t : threads) t = std::thread(emit_many, std::ref(sig));
+
+  for (auto& t : threads) t.join();
+
+  ASSERT_EQ(sum, 100000);
+}
+
+}  // namespace wpi