Handle bad chunks more gracefully

Prior to this change the server would crash when attempting to load a
chunk from a region with bad data.

After this change the server will defer back to vanilla behavior. At
this time, that means attempting to generate a chunk in its place
(and occasionally just not generating anything and leaving small
holes in the world).

Should Mojang choose to alter this behavior in the future, this change
will simply defer to whatever that new behavior is.

Spigot-Server-Patches/0289-Handle-bad-chunks-more-gracefully.patch

@@ -0,0 +1,53 @@
+From 2b86619b9cdca537a011f0b56ce28ecd4925ba8d Mon Sep 17 00:00:00 2001
+From: Zach Brown <zach.brown@destroystokyo.com>
+Date: Tue, 17 Apr 2018 21:26:31 -0400
+Subject: [PATCH] Handle bad chunks more gracefully
+
+Prior to this change the server would crash when attempting to load a
+chunk from a region with bad data.
+
+After this change the server will defer back to vanilla behavior. At
+this time, that means attempting to generate a chunk in its place
+(and occasionally just not generating anything and leaving small
+holes in the world).
+
+Should Mojang choose to alter this behavior in the future, this change
+will simply defer to whatever that new behavior is.
+
+diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+index 1e84afb0a..9055da64a 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -167,6 +167,13 @@ public class ChunkProviderServer implements IChunkProvider {
+             chunk = originalGetChunkAt(i, j);
+         }
+ 
++        // Paper start - If there was an issue loading the chunk from region, stage1 will fail and stage2 will load it sync
++        // all we need to do is fetch an instance
++        if (chunk == null) {
++            chunk = getChunkIfLoaded(i, j);
++        }
++        // Paper end
++
+         // If we didn't load the chunk async and have a callback run it now
+         if (runnable != null) {
+             runnable.run();
+diff --git a/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java
+index ef9529add..cdf3b614c 100644
+--- a/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java
++++ b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java
+@@ -30,6 +30,11 @@ class ChunkIOProvider implements AsynchronousExecutor.CallBackProvider<QueuedChu
+             return null;
+         } catch (IOException ex) {
+             throw new RuntimeException(ex);
++        // Paper - Mirror vanilla by catching everything (else) rather than immediately crashing the server
++        // stage2 will receive a null chunk and then load it synchronously, where vanilla MC will properly log and recover
++        // stage2 will _not_ however return that instance, only load it
++        } catch (Exception ex) {
++            return null;
+         }
+     }
+ 
+-- 
+2.14.3
+