From c1479920cb39b85e9dc4993c9c425dae8a22acb1 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 11 Aug 2018 00:49:20 -0400
Subject: [PATCH] Detect and repair corrupt Region Files

If the file has partial data written but not the full 8192 bytes,
then the server will be unable to load that region file...

I don't know why mojang only checks for 4096, when anything less than 8192 is a crash.

But to be safe, it will attempt to back up the file.

diff --git a/src/main/java/net/minecraft/server/RegionFile.java b/src/main/java/net/minecraft/server/RegionFile.java
index 5d2853b9ce..c35974aa7c 100644
--- a/src/main/java/net/minecraft/server/RegionFile.java
+++ b/src/main/java/net/minecraft/server/RegionFile.java
@@ -40,7 +40,20 @@ public class RegionFile {
             }
 
             this.c = new RandomAccessFile(file1, "rw");
-            if (this.c.length() < 4096L) {
+            // Paper start - detect and fix incomplete headers
+            long length = this.c.length();
+            if (length < 8192 && length > 0) {
+                File corrupt = new File(file1.getParentFile(), file1.getName() + ".bak");
+                org.apache.logging.log4j.Logger logger = org.apache.logging.log4j.LogManager.getLogger();
+                logger.error("Region file " + file1 + " was incomplete. Backing up to " + corrupt + " and repairing");
+                try {
+                    java.nio.file.Files.copy(file1.toPath(), corrupt.toPath());
+                } catch (IOException e) {
+                    logger.error("Error backing up corrupt file", e);
+                }
+            }
+            if (length < 8192L) {
+                // Paper end
                 this.c.write(a);
                 this.c.write(a);
                 this.g += 8192;
-- 
2.19.0