Paper/Spigot-Server-Patches/0142-Properly-handle-async-calls-to-restart-the-server.patch

From 0000000000000000000000000000000000000000 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 283c1111d99b6ae09b6db0c0079eeb0f1cbb7b2b..d92ca78e483b3f085e3bad1d1250cac2f9031fa7 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -201,6 +201,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
     public final Map<ResourceKey<World>, WorldServer> worldServer;
     private PlayerList playerList;
     private volatile boolean isRunning;
+    private volatile boolean isRestarting = false; // Paper - flag to signify we're attempting to restart
     private boolean isStopped;
     private int ticks;
     protected final Proxy proxy;
@@ -860,7 +861,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
         if (this.playerList != null) {
             MinecraftServer.LOGGER.info("Saving players");
             this.playerList.savePlayers();
-            this.playerList.shutdown();
+            this.playerList.shutdown(this.isRestarting); // Paper
             try { Thread.sleep(100); } catch (InterruptedException ex) {} // CraftBukkit - SPIGOT-625 - give server at least a chance to send packets
         }
 
@@ -925,8 +926,13 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
         return this.isRunning;
     }
 
+    // Paper start - allow passing of the intent to restart
     public void safeShutdown(boolean flag) {
+        this.safeShutdown(flag, false);
+    }
+    public void safeShutdown(boolean flag, boolean isRestarting) {
         this.isRunning = false;
+        this.isRestarting = isRestarting;
         if (flag) {
             try {
                 this.serverThread.join();
@@ -936,6 +942,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
         }
 
     }
+    // Paper end
 
     // Spigot Start
     private static double calcTps(double avg, double exp, double tps)
diff --git a/src/main/java/net/minecraft/server/players/PlayerList.java b/src/main/java/net/minecraft/server/players/PlayerList.java
index eb5c22d8af3a3cfadd581d641010942caa6bed54..6c80f328016b6cd30c77b5a5b1e2f6be0b3cd2f0 100644
--- a/src/main/java/net/minecraft/server/players/PlayerList.java
+++ b/src/main/java/net/minecraft/server/players/PlayerList.java
@@ -1158,9 +1158,15 @@ public abstract class PlayerList {
         entityplayer.playerInteractManager.b(worldserver.worldDataServer.getGameType()); // CraftBukkit
     }
 
+    // Paper start - Extract method to allow for restarting flag
     public void shutdown() {
+        this.shutdown(false);
+    }
+
+    public void shutdown(boolean isRestarting) {
         // CraftBukkit start - disconnect safely
         for (EntityPlayer player : this.players) {
+            if (isRestarting) player.playerConnection.disconnect(org.spigotmc.SpigotConfig.restartMessage); else // Paper
             player.playerConnection.disconnect(PaperAdventure.asVanilla(this.server.server.shutdownMessage())); // CraftBukkit - add custom shutdown message // Paper - Adventure
         }
         // CraftBukkit end
@@ -1173,6 +1179,7 @@ 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 a45155cdd7d7a302c119f75bfe2b428ae5e8ab47..b45d7e5c108c7a8541fcbc9ad92d1a79a94746a1 100644
--- a/src/main/java/org/spigotmc/RestartCommand.java
+++ b/src/main/java/org/spigotmc/RestartCommand.java
@@ -46,86 +46,134 @@ public class RestartCommand extends Command
         org.spigotmc.AsyncCatcher.shuttingDown = true; // Paper
         try
         {
-            String[] split = restartScript.split( " " );
-            if ( split.length > 0 && new File( split[0] ).isFile() )
+            // Paper - extract method and cleanup
+            boolean isRestarting = addShutdownHook( restartScript );
+            if ( isRestarting )
             {
-                System.out.println( "Attempting to restart with " + restartScript );
+                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();
 
-                // Disable Watchdog
-                WatchdogThread.doStop();
+            shutdownServer( isRestarting );
+            // Paper end
+        } catch ( Exception ex )
+        {
+            ex.printStackTrace();
+        }
+    }
 
-                // 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();
+    // 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 )
+            {
+            }
 
-                // Give time for it to kick in
-                try
-                {
-                    Thread.sleep( 100 );
-                } catch ( InterruptedException ex )
-                {
-                }
+            closeSocket();
 
-                // Actually shutdown
-                try
-                {
-                    MinecraftServer.getServer().close();
-                } catch ( Throwable t )
-                {
-                }
+            // Actually shutdown
+            try
+            {
+                MinecraftServer.getServer().close(); // calls stop()
+            } catch ( Throwable t )
+            {
+            }
+
+            // Actually stop the JVM
+            System.exit( 0 );
 
-                // This will be done AFTER the server has completely halted
-                Thread shutdownHook = new Thread()
+        } else
+        {
+            // Mark the server to shutdown at the end of the tick
+            MinecraftServer.getServer().safeShutdown( false, isRestarting );
+
+            // wait 10 seconds to see if we're actually going to try shutdown
+            try
+            {
+                Thread.sleep( 10000 );
+            }
+            catch (InterruptedException ignored)
+            {
+            }
+
+            // 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 );
+        }
+    }
+    // Paper end
+
+    // 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
+        {
+            Thread.sleep( 100 );
+        } catch ( InterruptedException ex )
+        {
+        }
+    }
+    // Paper end
+
+    // Paper start - copied from above and modified to return if the hook registered
+    private static boolean addShutdownHook(String restartScript)
+    {
+        String[] split = restartScript.split( " " );
+        if ( split.length > 0 && new File( split[0] ).isFile() )
+        {
+            Thread shutdownHook = new Thread()
+            {
+                @Override
+                public void run()
                 {
-                    @Override
-                    public void run()
+                    try
                     {
-                        try
+                        String os = System.getProperty( "os.name" ).toLowerCase(java.util.Locale.ENGLISH);
+                        if ( os.contains( "win" ) )
                         {
-                            String os = System.getProperty( "os.name" ).toLowerCase(java.util.Locale.ENGLISH);
-                            if ( os.contains( "win" ) )
-                            {
-                                Runtime.getRuntime().exec( "cmd /c start " + restartScript );
-                            } else
-                            {
-                                Runtime.getRuntime().exec( "sh " + restartScript );
-                            }
-                        } catch ( Exception e )
+                            Runtime.getRuntime().exec( "cmd /c start " + restartScript );
+                        } else
                         {
-                            e.printStackTrace();
+                            Runtime.getRuntime().exec( "sh " + restartScript );
                         }
+                    } catch ( Exception e )
+                    {
+                        e.printStackTrace();
                     }
-                };
-
-                shutdownHook.setDaemon( true );
-                Runtime.getRuntime().addShutdownHook( shutdownHook );
-            } else
-            {
-                System.out.println( "Startup script '" + SpigotConfig.restartScript + "' does not exist! Stopping server." );
-
-                // Actually shutdown
-                try
-                {
-                    MinecraftServer.getServer().close();
-                } catch ( Throwable t )
-                {
                 }
-            }
-            System.exit( 0 );
-        } catch ( Exception ex )
+            };
+
+            shutdownHook.setDaemon( true );
+            Runtime.getRuntime().addShutdownHook( shutdownHook );
+            return true;
+        } else
         {
-            ex.printStackTrace();
+            return false;
         }
     }
+    // Paper end
+
 }