PaperMC/Spigot-Server-Patches/0177-Properly-handle-async-calls-to-restart-the-server.patch

From 5e520316a953640c0e50801cd486c34a367dbd0b Mon Sep 17 00:00:00 2001
From: Zach Brown <zach.brown@destroystokyo.com>
Date: Fri, 12 May 2017 23:34:11 -0500
Subject: [PATCH] Properly handle async calls to restart the server

The watchdog thread calls the server restart function asynchronously. Prior to
this change, it attempted to do several non-safe operations from the watchdog
thread, rather than the main. Specifically, because of a separate upstream change,
it causes player entities to be ticked asynchronously, among other things.

This is dangerous.

This patch moves the old handling into a synchronous variant, for calls from the
restart command, and adds separate handling for async calls, such as those from
the watchdog thread.

When calling from the watchdog thread, we cannot assume the main thread is in a
tickable state; it may be completely deadlocked. In order to handle this, we mark
the server as stopping, in order to account for situations where the server should
complete a tick reasonbly soon, i.e. 99% of cases.

Should the server not enter a state where it is stopping within 10 seconds, We
will assume that the server has in fact deadlocked and will proceed to force
kill the server.

This modification does not force restart the server should we actually enter a
deadlocked state where the server is stopping, whereas this will in most cases
exit within a reasonable amount of time, to put a fixed limit on a process that
will have plugins and worlds saving to the disk has a high potential to result
in corruption/dataloss.

diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
index ee13260af7..5d8dbdf8a3 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -89,6 +89,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
     public final Map<DimensionManager, WorldServer> worldServer = Maps.newIdentityHashMap();
     private PlayerList s;
     private boolean isRunning = true;
+    private boolean isRestarting = false; // Paper - flag to signify we're attempting to restart
     private boolean isStopped;
     private int ticks;
     protected final Proxy c;
@@ -655,7 +656,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         if (this.s != null) {
             MinecraftServer.LOGGER.info("Saving players");
             this.s.savePlayers();
-            this.s.u();
+            this.s.u(isRestarting); // Paper
             try { Thread.sleep(100); } catch (InterruptedException ex) {} // CraftBukkit - SPIGOT-625 - give server at least a chance to send packets
         }
 
@@ -705,9 +706,16 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         return this.isRunning;
     }
 
+    // Paper start - allow passing of the intent to restart
     public void safeShutdown() {
+        safeShutdown(false);
+    }
+
+    public void safeShutdown(boolean isRestarting) {
         this.isRunning = false;
+        this.isRestarting = isRestarting;
     }
+    // Paper end
 
     private boolean aT() {
         return System.nanoTime() - lastTick + catchupTime < TICK_TIME; // Paper - improved "are we lagging" check to match our own
diff --git a/src/main/java/net/minecraft/server/PlayerList.java b/src/main/java/net/minecraft/server/PlayerList.java
index cc0ed8546f..a9906f03c9 100644
--- a/src/main/java/net/minecraft/server/PlayerList.java
+++ b/src/main/java/net/minecraft/server/PlayerList.java
@@ -1332,10 +1332,15 @@ public abstract class PlayerList {
         entityplayer.playerInteractManager.b(generatoraccess.getWorldData().getGameType());
     }
 
+    // Paper start - Extract method to allow for restarting flag
     public void u() {
+        u(false);
+    }
+
+    public void u(boolean isRestarting) {
         // CraftBukkit start - disconnect safely
         for (EntityPlayer player : this.players) {
-            player.playerConnection.disconnect(this.server.server.getShutdownMessage()); // CraftBukkit - add custom shutdown message
+            player.playerConnection.disconnect(!isRestarting ? this.server.server.getShutdownMessage() : org.spigotmc.SpigotConfig.restartMessage); // CraftBukkit - add custom shutdown message // Paper - add isRestarting flag
         }
         // CraftBukkit end
         // Paper start - Remove collideRule team if it exists
@@ -1346,6 +1351,7 @@ public abstract class PlayerList {
         }
         // Paper end
     }
+    // Paper end
 
     // CraftBukkit start
     public void sendMessage(IChatBaseComponent[] iChatBaseComponents) {
diff --git a/src/main/java/org/spigotmc/RestartCommand.java b/src/main/java/org/spigotmc/RestartCommand.java
index 947c43a5d0..f15fd9f370 100644
--- a/src/main/java/org/spigotmc/RestartCommand.java
+++ b/src/main/java/org/spigotmc/RestartCommand.java
@@ -46,88 +46,123 @@ public class RestartCommand extends Command
         org.spigotmc.AsyncCatcher.shuttingDown = true; // Paper
         try
         {
-            if ( script.isFile() )
+            // Paper - extract method and cleanup
+            boolean isRestarting = addShutdownHook(script);
+            if (isRestarting) {
+                System.out.println("Attempting to restart with " + SpigotConfig.restartScript);
+            } else {
+                System.out.println( "Startup script '" + SpigotConfig.restartScript + "' does not exist! Stopping server." );
+            }
+
+            // Stop the watchdog
+            WatchdogThread.doStop();
+
+            shutdownServer(isRestarting);
+        } catch ( Exception ex )
+        {
+            ex.printStackTrace();
+        }
+    }
+
+    // Paper start - sync copied from above with minor changes, async added
+    private static void shutdownServer(boolean isRestarting)
+    {
+        if (MinecraftServer.getServer().isMainThread())
+        {
+            // Kick all players
+            for ( EntityPlayer p : com.google.common.collect.ImmutableList.copyOf( MinecraftServer.getServer().getPlayerList().players ) )
+            {
+                p.playerConnection.disconnect(SpigotConfig.restartMessage);
+            }
+            // Give the socket a chance to send the packets
+            try
+            {
+                Thread.sleep( 100 );
+            } catch ( InterruptedException ex )
             {
-                System.out.println( "Attempting to restart with " + SpigotConfig.restartScript );
+            }
 
-                // Disable Watchdog
-                WatchdogThread.doStop();
+            closeSocket();
 
-                // Kick all players
-                for ( EntityPlayer p : (List< EntityPlayer>) MinecraftServer.getServer().getPlayerList().players )
-                {
-                    p.playerConnection.disconnect(SpigotConfig.restartMessage);
-                }
-                // Give the socket a chance to send the packets
-                try
-                {
-                    Thread.sleep( 100 );
-                } catch ( InterruptedException ex )
-                {
-                }
-                // Close the socket so we can rebind with the new process
-                MinecraftServer.getServer().getServerConnection().b();
+            // Actually shutdown
+            try
+            {
+                MinecraftServer.getServer().stop();
+            } catch ( Throwable t )
+            {
+            }
 
-                // Give time for it to kick in
-                try
-                {
-                    Thread.sleep( 100 );
-                } catch ( InterruptedException ex )
-                {
-                }
+            // Actually stop the JVM
+            System.exit(0);
 
-                // Actually shutdown
-                try
-                {
-                    MinecraftServer.getServer().stop();
-                } catch ( Throwable t )
-                {
-                }
+        } else
+        {
+            // Mark the server to shutdown at the end of the tick
+            MinecraftServer.getServer().safeShutdown(isRestarting);
 
-                // This will be done AFTER the server has completely halted
-                Thread shutdownHook = new Thread()
-                {
-                    @Override
-                    public void run()
-                    {
-                        try
-                        {
-                            String os = System.getProperty( "os.name" ).toLowerCase(java.util.Locale.ENGLISH);
-                            if ( os.contains( "win" ) )
-                            {
-                                Runtime.getRuntime().exec( "cmd /c start " + script.getPath() );
-                            } else
-                            {
-                                Runtime.getRuntime().exec( new String[]
-                                {
-                                    "sh", script.getPath()
-                                } );
-                            }
-                        } catch ( Exception e )
-                        {
-                            e.printStackTrace();
-                        }
-                    }
-                };
 
-                shutdownHook.setDaemon( true );
-                Runtime.getRuntime().addShutdownHook( shutdownHook );
-            } else
+            // wait 10 seconds to see if we're actually going to try shutdown
+            try
+            {
+                Thread.sleep(10000);
+            }
+            catch (InterruptedException ignored)
             {
-                System.out.println( "Startup script '" + SpigotConfig.restartScript + "' does not exist! Stopping server." );
-
-                // Actually shutdown
-                try
-                {
-                    MinecraftServer.getServer().stop();
-                } catch ( Throwable t )
-                {
-                }
             }
+
+            // Check if we've actually hit a state where the server is going to safely shutdown
+            // if we have, let the server stop as usual
+            if (MinecraftServer.getServer().isStopped()) return;
+
+            // If the server hasn't stopped by now, assume worse case and kill
+            closeSocket();
             System.exit( 0 );
-        } catch ( Exception ex )
+        }
+    }
+
+    // Paper - Split from moved code
+    private static void closeSocket() {
+        // Close the socket so we can rebind with the new process
+        MinecraftServer.getServer().getServerConnection().b();
+
+        // Give time for it to kick in
+        try
         {
-            ex.printStackTrace();
+            Thread.sleep( 100 );
+        } catch ( InterruptedException ex )
+        {
+        }
+    }
+    // Paper end
+
+    // Paper - copied from above and modified to return if the hook registered
+    private static boolean addShutdownHook(final File script) {
+
+        if (script.isFile()) {
+            Thread shutdownHook = new Thread() {
+                @Override
+                public void run() {
+                    try {
+                        String os = System.getProperty("os.name").toLowerCase(java.util.Locale.ENGLISH);
+                        if (os.contains("win")) {
+                            Runtime.getRuntime().exec("cmd /c start " + script.getPath());
+                        } else {
+                            Runtime.getRuntime().exec(new String[]
+                                    {
+                                            "sh", script.getPath()
+                                    });
+                        }
+                    } catch (Exception e) {
+                        e.printStackTrace();
+                    }
+                }
+            };
+
+            shutdownHook.setDaemon(true);
+            Runtime.getRuntime().addShutdownHook(shutdownHook);
+            return true;
+        } else {
+            return false;
         }
     }
 }
-- 
2.18.0