#2770: Handle posix signals SIGTERM, SIGINT, SIGHUP gracefully

- Move working contents of Bungeecord.stop() to a separate function
named independentThreadStop() intended to be called from a separate thread.
- Added a new generic shutdown hook to call independentThreadStop when
the JVM begins shutting down.

proxy/src/main/java/net/md_5/bungee/BungeeCord.java

@@ -40,6 +40,7 @@ import java.util.TimerTask;
 import java.util.UUID;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.logging.Handler;
 import java.util.logging.Level;
@@ -134,6 +135,11 @@ public class BungeeCord extends ProxyServer
     private final Map<UUID, UserConnection> connectionsByOfflineUUID = new HashMap<>();
     private final Map<UUID, UserConnection> connectionsByUUID = new HashMap<>();
     private final ReadWriteLock connectionLock = new ReentrantReadWriteLock();
+    /**
+     * Lock to protect the shutdown process from being triggered simultaneously
+     * from multiple sources.
+     */
+    private final ReentrantLock shutdownLock = new ReentrantLock();
     /**
      * Plugin manager.
      */
@@ -297,6 +303,15 @@ public class BungeeCord extends ProxyServer
             }
         }, 0, TimeUnit.MINUTES.toMillis( 5 ) );
         metricsThread.scheduleAtFixedRate( new Metrics(), 0, TimeUnit.MINUTES.toMillis( Metrics.PING_INTERVAL ) );
+
+        Runtime.getRuntime().addShutdownHook( new Thread()
+        {
+            @Override
+            public void run()
+            {
+                independentThreadStop( getTranslation( "restart" ), false );
+            }
+        } );
     }
 
     public void startListeners()
@@ -385,21 +400,36 @@ public class BungeeCord extends ProxyServer
     }
 
     @Override
-    public synchronized void stop(final String reason)
+    public void stop(final String reason)
     {
+        new Thread( "Shutdown Thread" )
+        {
+            @Override
+            public void run()
+            {
+                independentThreadStop( reason, true );
+            }
+        }.start();
+    }
+
+    // This must be run on a separate thread to avoid deadlock!
+    @SuppressFBWarnings("DM_EXIT")
+    @SuppressWarnings("TooBroadCatch")
+    private void independentThreadStop(final String reason, boolean callSystemExit)
+    {
+        // Acquire the shutdown lock
+        // This needs to actually block here, otherwise running 'end' and then ctrl+c will cause the thread to terminate prematurely
+        shutdownLock.lock();
+
+        // Acquired the shutdown lock
         if ( !isRunning )
         {
+            // Server is already shutting down - nothing to do
+            shutdownLock.unlock();
             return;
         }
         isRunning = false;
 
-        new Thread( "Shutdown Thread" )
-        {
-            @Override
-            @SuppressFBWarnings("DM_EXIT")
-            @SuppressWarnings("TooBroadCatch")
-            public void run()
-            {
         stopListeners();
         getLogger().info( "Closing pending connections" );
 
@@ -432,7 +462,6 @@ public class BungeeCord extends ProxyServer
         saveThread.cancel();
         metricsThread.cancel();
 
-                // TODO: Fix this shit
         getLogger().info( "Disabling plugins" );
         for ( Plugin plugin : Lists.reverse( new ArrayList<>( pluginManager.getPlugins() ) ) )
         {
@@ -466,9 +495,15 @@ public class BungeeCord extends ProxyServer
         {
             handler.close();
         }
+
+        // Unlock the thread before optionally calling system exit, which might invoke this function again.
+        // If that happens, the system will obtain the lock, and then see that isRunning == false and return without doing anything.
+        shutdownLock.unlock();
+
+        if ( callSystemExit )
+        {
             System.exit( 0 );
         }
-        }.start();
     }
 
     /**