Paper/nms-patches/CommandGamemode.patch

--- a/net/minecraft/server/CommandGamemode.java
+++ b/net/minecraft/server/CommandGamemode.java
@@ -27,6 +27,12 @@
             EntityPlayer entityplayer = astring.length >= 2 ? a(minecraftserver, icommandlistener, astring[1]) : a(icommandlistener);
 
             entityplayer.a(worldsettings_enumgamemode);
+            // CraftBukkit start - handle event cancelling the change
+            if (entityplayer.playerInteractManager.getGameMode() != worldsettings_enumgamemode) {
+                icommandlistener.sendMessage(new ChatComponentText("Failed to set the gamemode of '" + entityplayer.getName() + "'"));
+                return;
+            }
+            // CraftBukkit end
             ChatMessage chatmessage = new ChatMessage("gameMode." + worldsettings_enumgamemode.b(), new Object[0]);
 
             if (icommandlistener.getWorld().getGameRules().getBoolean("sendCommandFeedback")) {
@@ -49,10 +55,17 @@
     }
 
     public List<String> tabComplete(MinecraftServer minecraftserver, ICommandListener icommandlistener, String[] astring, BlockPosition blockposition) {
-        return astring.length == 1 ? a(astring, new String[] { "survival", "creative", "adventure", "spectator"}) : (astring.length == 2 ? a(astring, minecraftserver.getPlayers()) : Collections.emptyList());
+        return astring.length == 1 ? a(astring, new String[] { "survival", "creative", "adventure", "spectator"}) : (astring.length == 2 ? a(astring, minecraftserver.getPlayers()) : Collections.<String>emptyList()); // CraftBukkit - decompile error
     }
 
     public boolean isListStart(String[] astring, int i) {
         return i == 1;
     }
+
+    // CraftBukkit start - fix decompiler error
+    @Override
+    public int compareTo(ICommand o) {
+        return a((ICommand) o);
+    }
+    // CraftBukkit end
 }