Initial reload command support.

What is missing:
- Changing/removing servers "in-flight"
- Query stuff
- A way to preserve existing user connections while closing the old port

api/src/main/java/com/velocitypowered/api/event/proxy/ProxyReloadEvent.java

@@ -0,0 +1,12 @@
+package com.velocitypowered.api.event.proxy;
+
+/**
+ * This event is fired when the proxy is reloaded by the user using {@code /velocity reload}.
+ */
+public class ProxyReloadEvent {
+
+  @Override
+  public String toString() {
+    return "ProxyInitializeEvent";
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -7,6 +7,7 @@ import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.velocitypowered.api.event.EventManager;
 import com.velocitypowered.api.event.proxy.ProxyInitializeEvent;
+import com.velocitypowered.api.event.proxy.ProxyReloadEvent;
 import com.velocitypowered.api.event.proxy.ProxyShutdownEvent;
 import com.velocitypowered.api.plugin.PluginContainer;
 import com.velocitypowered.api.plugin.PluginManager;
@@ -40,6 +41,7 @@ import com.velocitypowered.proxy.util.VelocityChannelRegistrar;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiter;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiters;
 import io.netty.bootstrap.Bootstrap;
+import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -232,6 +234,37 @@ public class VelocityServer implements ProxyServer {
     return shutdown;
   }
 
+  public boolean reloadConfiguration() throws IOException {
+    Path configPath = Paths.get("velocity.toml");
+    VelocityConfiguration newConfiguration = VelocityConfiguration.read(configPath);
+
+    if (!newConfiguration.validate()) {
+      return false;
+    }
+
+    // If we have a new bind address, bind to it
+    if (!configuration.getBind().equals(newConfiguration.getBind())) {
+      this.cm.bind(newConfiguration.getBind());
+    }
+
+    // Re-register servers
+    for (Map.Entry<String, String> entry : newConfiguration.getServers().entrySet()) {
+      ServerInfo newInfo =
+          new ServerInfo(entry.getKey(), AddressUtil.parseAddress(entry.getValue()));
+      Optional<RegisteredServer> rs = servers.getServer(entry.getKey());
+      if (!rs.isPresent()) {
+        servers.register(newInfo);
+      } else if (!rs.get().getServerInfo().equals(newInfo)) {
+        throw new IllegalStateException("Unable to replace servers in flight!");
+      }
+    }
+
+    ipAttemptLimiter = Ratelimiters.createWithMilliseconds(newConfiguration.getLoginRatelimit());
+    this.configuration = newConfiguration;
+    eventManager.fireAndForget(new ProxyReloadEvent());
+    return true;
+  }
+
   public void shutdown(boolean explicitExit) {
     if (eventManager == null || pluginManager == null || cm == null || scheduler == null) {
       throw new AssertionError();

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommand.java

@@ -10,6 +10,8 @@ import com.velocitypowered.api.plugin.PluginContainer;
 import com.velocitypowered.api.plugin.PluginDescription;
 import com.velocitypowered.api.proxy.ProxyServer;
 import com.velocitypowered.api.util.ProxyVersion;
+import com.velocitypowered.proxy.VelocityServer;
+import java.io.IOException;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Locale;
@@ -22,6 +24,8 @@ import net.kyori.text.event.HoverEvent;
 import net.kyori.text.event.HoverEvent.Action;
 import net.kyori.text.format.TextColor;
 import net.kyori.text.format.TextDecoration;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.checkerframework.checker.nullness.qual.NonNull;
 
 public class VelocityCommand implements Command {
@@ -32,10 +36,11 @@ public class VelocityCommand implements Command {
    * Initializes the command object for /velocity.
    * @param server the Velocity server
    */
-  public VelocityCommand(ProxyServer server) {
+  public VelocityCommand(VelocityServer server) {
     this.subcommands = ImmutableMap.<String, Command>builder()
         .put("version", new Info(server))
         .put("plugins", new Plugins(server))
+        .put("reload", new Reload(server))
         .build();
   }
 
@@ -103,6 +108,39 @@ public class VelocityCommand implements Command {
     return command.hasPermission(source, actualArgs);
   }
 
+  private static class Reload implements Command {
+
+    private static final Logger logger = LogManager.getLogger(Reload.class);
+    private final VelocityServer server;
+
+    private Reload(VelocityServer server) {
+      this.server = server;
+    }
+
+    @Override
+    public void execute(CommandSource source, String @NonNull [] args) {
+      try {
+        if (server.reloadConfiguration()) {
+          source.sendMessage(TextComponent.of("Configuration reloaded.", TextColor.GREEN));
+        } else {
+          source.sendMessage(TextComponent.of(
+              "Unable to reload your configuration. Check the console for more details.",
+              TextColor.RED));
+        }
+      } catch (Exception e) {
+        logger.error("Unable to reload configuration", e);
+        source.sendMessage(TextComponent.of(
+            "Unable to reload your configuration. Check the console for more details.",
+            TextColor.RED));
+      }
+    }
+
+    @Override
+    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+      return source.getPermissionValue("velocity.command.reload") == Tristate.TRUE;
+    }
+  }
+
   private static class Info implements Command {
 
     private final ProxyServer server;