[wpiutil] Add ParallelTcpConnector (#3655)

This is a libuv-based parallel, repeating TCP connector for making
multiple parallel DNS resolution and TCP connection attempts to a
network server.

wpiutil/examples/parallelconnect/parallelconnect.cpp

@@ -0,0 +1,50 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <cstdio>
+
+#include "wpi/EventLoopRunner.h"
+#include "wpi/Logger.h"
+#include "wpi/ParallelTcpConnector.h"
+#include "wpi/uv/Error.h"
+#include "wpi/uv/Tcp.h"
+
+namespace uv = wpi::uv;
+
+static void logfunc(unsigned int level, const char* file, unsigned int line,
+                    const char* msg) {
+  std::fprintf(stderr, "(%d) %s:%d: %s\n", level, file, line, msg);
+}
+
+int main() {
+  wpi::Logger logger{logfunc, 0};
+
+  // Kick off the event loop on a separate thread
+  wpi::EventLoopRunner loop;
+  std::shared_ptr<wpi::ParallelTcpConnector> connect;
+  loop.ExecAsync([&](uv::Loop& loop) {
+    connect = wpi::ParallelTcpConnector::Create(
+        loop, uv::Timer::Time{2000}, logger, [&](uv::Tcp& tcp) {
+          std::fputs("Got connection, accepting!\n", stdout);
+          tcp.StartRead();
+          connect->Succeeded(tcp);
+          tcp.end.connect([&] {
+            std::fputs("TCP connection ended, disconnecting!\n", stdout);
+            tcp.Close();
+            connect->Disconnected();
+          });
+          tcp.error.connect([&](uv::Error) {
+            std::fputs("TCP error, disconnecting!\n", stdout);
+            connect->Disconnected();
+          });
+        });
+    connect->SetServers({{{"roborio-294-frc.local", 8080},
+                          {"roborio-294-frc.frc-field.local", 8080},
+                          {"10.2.94.2", 8080},
+                          {"127.0.0.1", 8080}}});
+  });
+
+  // wait for a keypress to terminate
+  std::getchar();
+}