Paper/patches/unapplied/server/1023-Attempt-to-recalculate-regionfile-header-if-it-is-co.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Sun, 2 Feb 2020 02:25:10 -0800
Subject: [PATCH] Attempt to recalculate regionfile header if it is corrupt

Instead of trying to relocate the chunk, which is seems to never
be the correct choice, so we end up duplicating or swapping chunks,
we instead drop the current regionfile header and recalculate -
hoping that at least then we don't swap chunks, and maybe recover
them all.

diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
index 539b36bde9cba3a44184eba36df9aa4c345a5b84..d53c4f3d47a8728d56fbd9b5e12be51885560d52 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
@@ -70,6 +70,18 @@ import net.minecraft.world.ticks.ProtoChunkTicks;
 import org.slf4j.Logger;
 
 public class ChunkSerializer {
+    // Paper start - Attempt to recalculate regionfile header if it is corrupt
+    // TODO: Check on update
+    public static long getLastWorldSaveTime(CompoundTag chunkData) {
+        final int dataVersion = ChunkStorage.getVersion(chunkData);
+        if (dataVersion < 2842) { // Level tag is removed after this version
+            final CompoundTag levelData = chunkData.getCompound("Level");
+            return levelData.getLong("LastUpdate");
+        } else {
+            return chunkData.getLong("LastUpdate");
+        }
+    }
+    // Paper end - Attempt to recalculate regionfile header if it is corrupt
 
     public static final Codec<PalettedContainer<BlockState>> BLOCK_STATE_CODEC = PalettedContainer.codecRW(Block.BLOCK_STATE_REGISTRY, BlockState.CODEC, PalettedContainer.Strategy.SECTION_STATES, Blocks.AIR.defaultBlockState(), null); // Paper - Anti-Xray - Add preset block states
     private static final Logger LOGGER = LogUtils.getLogger();
@@ -455,7 +467,7 @@ public class ChunkSerializer {
         nbttagcompound.putInt("xPos", chunkcoordintpair.x);
         nbttagcompound.putInt("yPos", chunk.getMinSection());
         nbttagcompound.putInt("zPos", chunkcoordintpair.z);
-        nbttagcompound.putLong("LastUpdate", asyncsavedata != null ? asyncsavedata.worldTime : world.getGameTime()); // Paper - async chunk unloading
+        nbttagcompound.putLong("LastUpdate", asyncsavedata != null ? asyncsavedata.worldTime : world.getGameTime()); // Paper - async chunk unloading // Paper - diff on change
         nbttagcompound.putLong("InhabitedTime", chunk.getInhabitedTime());
         nbttagcompound.putString("Status", BuiltInRegistries.CHUNK_STATUS.getKey(chunk.getStatus()).toString());
         BlendingData blendingdata = chunk.getBlendingData();
diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkStorage.java b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkStorage.java
index 645a1773237f2002c233ec1f3ff6f0ca46ca4024..d16d7c2fed89fb1347df7ddd95856e7f08c22e8a 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkStorage.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkStorage.java
@@ -38,7 +38,7 @@ public class ChunkStorage implements AutoCloseable {
 
     public ChunkStorage(Path directory, DataFixer dataFixer, boolean dsync) {
         this.fixerUpper = dataFixer;
-        this.regionFileCache = new RegionFileStorage(directory, dsync); // Paper - rewrite chunk system; async chunk IO
+        this.regionFileCache = new RegionFileStorage(directory, dsync, true); // Paper - rewrite chunk system; async chunk IO & Attempt to recalculate regionfile header if it is corrupt
     }
 
     public boolean isOldChunkAround(ChunkPos chunkPos, int checkRadius) {
diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/RegionBitmap.java b/src/main/java/net/minecraft/world/level/chunk/storage/RegionBitmap.java
index a23dc2f8f4475de1ee35bf18a7a8a53233ccac12..226af44fd469053451a0403a95ffb446face9530 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/RegionBitmap.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/RegionBitmap.java
@@ -9,6 +9,27 @@ import java.util.BitSet;
 public class RegionBitmap {
     private final BitSet used = new BitSet();
 
+    // Paper start - Attempt to recalculate regionfile header if it is corrupt
+    public final void copyFrom(RegionBitmap other) {
+        BitSet thisBitset = this.used;
+        BitSet otherBitset = other.used;
+
+        for (int i = 0; i < Math.max(thisBitset.size(), otherBitset.size()); ++i) {
+            thisBitset.set(i, otherBitset.get(i));
+        }
+    }
+
+    public final boolean tryAllocate(int from, int length) {
+        BitSet bitset = this.used;
+        int firstSet = bitset.nextSetBit(from);
+        if (firstSet > 0 && firstSet < (from + length)) {
+            return false;
+        }
+        bitset.set(from, from + length);
+        return true;
+    }
+    // Paper end - Attempt to recalculate regionfile header if it is corrupt
+
     public void force(int start, int size) {
         this.used.set(start, start + size);
     }
diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFile.java b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFile.java
index 92ba75254f6ffca40abd5485dbb4789de59edebd..6cf83502a954cce9c562ec036bfeddb477d38b73 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFile.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFile.java
@@ -50,6 +50,355 @@ public class RegionFile implements AutoCloseable {
     public final java.util.concurrent.locks.ReentrantLock fileLock = new java.util.concurrent.locks.ReentrantLock(); // Paper
     public final Path regionFile; // Paper
 
+    // Paper start - Attempt to recalculate regionfile header if it is corrupt
+    private static long roundToSectors(long bytes) {
+        long sectors = bytes >>> 12; // 4096 = 2^12
+        long remainingBytes = bytes & 4095;
+        long sign = -remainingBytes; // sign is 1 if nonzero
+        return sectors + (sign >>> 63);
+    }
+
+    private static final CompoundTag OVERSIZED_COMPOUND = new CompoundTag();
+
+    private CompoundTag attemptRead(long sector, int chunkDataLength, long fileLength) throws IOException {
+        try {
+            if (chunkDataLength < 0) {
+                return null;
+            }
+
+            long offset = sector * 4096L + 4L; // offset for chunk data
+
+            if ((offset + chunkDataLength) > fileLength) {
+                return null;
+            }
+
+            ByteBuffer chunkData = ByteBuffer.allocate(chunkDataLength);
+            if (chunkDataLength != this.file.read(chunkData, offset)) {
+                return null;
+            }
+
+            ((java.nio.Buffer)chunkData).flip();
+
+            byte compressionType = chunkData.get();
+            if (compressionType < 0) { // compressionType & 128 != 0
+                // oversized chunk
+                return OVERSIZED_COMPOUND;
+            }
+
+            RegionFileVersion compression = RegionFileVersion.fromId(compressionType);
+            if (compression == null) {
+                return null;
+            }
+
+            InputStream input = compression.wrap(new ByteArrayInputStream(chunkData.array(), chunkData.position(), chunkDataLength - chunkData.position()));
+
+            return NbtIo.read(new DataInputStream(input));
+        } catch (Exception ex) {
+            return null;
+        }
+    }
+
+    private int getLength(long sector) throws IOException {
+        ByteBuffer length = ByteBuffer.allocate(4);
+        if (4 != this.file.read(length, sector * 4096L)) {
+            return -1;
+        }
+
+        return length.getInt(0);
+    }
+
+    private void backupRegionFile() {
+        Path backup = this.regionFile.getParent().resolve(this.regionFile.getFileName() + "." + new java.util.Random().nextLong() + ".backup");
+        this.backupRegionFile(backup);
+    }
+
+    private void backupRegionFile(Path to) {
+        try {
+            this.file.force(true);
+            LOGGER.warn("Backing up regionfile \"" + this.regionFile.toAbsolutePath() + "\" to " + to.toAbsolutePath());
+            java.nio.file.Files.copy(this.regionFile, to, java.nio.file.StandardCopyOption.COPY_ATTRIBUTES);
+            LOGGER.warn("Backed up the regionfile to " + to.toAbsolutePath());
+        } catch (IOException ex) {
+            LOGGER.error("Failed to backup to " + to.toAbsolutePath(), ex);
+        }
+    }
+
+    private static boolean inSameRegionfile(ChunkPos first, ChunkPos second) {
+        return (first.x & ~31) == (second.x & ~31) && (first.z & ~31) == (second.z & ~31);
+    }
+
+    // note: only call for CHUNK regionfiles
+    boolean recalculateHeader() throws IOException {
+        if (!this.canRecalcHeader) {
+            return false;
+        }
+        ChunkPos ourLowerLeftPosition = RegionFileStorage.getRegionFileCoordinates(this.regionFile);
+        if (ourLowerLeftPosition == null) {
+            LOGGER.error("Unable to get chunk location of regionfile " + this.regionFile.toAbsolutePath() + ", cannot recover header");
+            return false;
+        }
+        synchronized (this) {
+            LOGGER.warn("Corrupt regionfile header detected! Attempting to re-calculate header offsets for regionfile " + this.regionFile.toAbsolutePath(), new Throwable());
+
+            // try to backup file so maybe it could be sent to us for further investigation
+
+            this.backupRegionFile();
+            CompoundTag[] compounds = new CompoundTag[32 * 32]; // only in the regionfile (i.e exclude mojang/aikar oversized data)
+            int[] rawLengths = new int[32 * 32]; // length of chunk data including 4 byte length field, bytes
+            int[] sectorOffsets = new int[32 * 32]; // in sectors
+            boolean[] hasAikarOversized = new boolean[32 * 32];
+
+            long fileLength = this.file.size();
+            long totalSectors = roundToSectors(fileLength);
+
+            // search the regionfile from start to finish for the most up-to-date chunk data
+
+            for (long i = 2, maxSector = Math.min((long)(Integer.MAX_VALUE >>> 8), totalSectors); i < maxSector; ++i) { // first two sectors are header, skip
+                int chunkDataLength = this.getLength(i);
+                CompoundTag compound = this.attemptRead(i, chunkDataLength, fileLength);
+                if (compound == null || compound == OVERSIZED_COMPOUND) {
+                    continue;
+                }
+
+                ChunkPos chunkPos = ChunkSerializer.getChunkCoordinate(compound);
+                if (!inSameRegionfile(ourLowerLeftPosition, chunkPos)) {
+                    LOGGER.error("Ignoring absolute chunk " + chunkPos + " in regionfile as it is not contained in the bounds of the regionfile '" + this.regionFile.toAbsolutePath() + "'. It should be in regionfile (" + (chunkPos.x >> 5) + "," + (chunkPos.z >> 5) + ")");
+                    continue;
+                }
+                int location = (chunkPos.x & 31) | ((chunkPos.z & 31) << 5);
+
+                CompoundTag otherCompound = compounds[location];
+
+                if (otherCompound != null && ChunkSerializer.getLastWorldSaveTime(otherCompound) > ChunkSerializer.getLastWorldSaveTime(compound)) {
+                    continue; // don't overwrite newer data.
+                }
+
+                // aikar oversized?
+                Path aikarOversizedFile = this.getOversizedFile(chunkPos.x, chunkPos.z);
+                boolean isAikarOversized = false;
+                if (Files.exists(aikarOversizedFile)) {
+                    try {
+                        CompoundTag aikarOversizedCompound = this.getOversizedData(chunkPos.x, chunkPos.z);
+                        if (ChunkSerializer.getLastWorldSaveTime(compound) == ChunkSerializer.getLastWorldSaveTime(aikarOversizedCompound)) {
+                            // best we got for an id. hope it's good enough
+                            isAikarOversized = true;
+                        }
+                    } catch (Exception ex) {
+                        LOGGER.error("Failed to read aikar oversized data for absolute chunk (" + chunkPos.x + "," + chunkPos.z + ") in regionfile " + this.regionFile.toAbsolutePath() + ", oversized data for this chunk will be lost", ex);
+                        // fall through, if we can't read aikar oversized we can't risk corrupting chunk data
+                    }
+                }
+
+                hasAikarOversized[location] = isAikarOversized;
+                compounds[location] = compound;
+                rawLengths[location] = chunkDataLength + 4;
+                sectorOffsets[location] = (int)i;
+
+                int chunkSectorLength = (int)roundToSectors(rawLengths[location]);
+                i += chunkSectorLength;
+                --i; // gets incremented next iteration
+            }
+
+            // forge style oversized data is already handled by the local search, and aikar data we just hope
+            // we get it right as aikar data has no identifiers we could use to try and find its corresponding
+            // local data compound
+
+            java.nio.file.Path containingFolder = this.externalFileDir;
+            Path[] regionFiles = Files.list(containingFolder).toArray(Path[]::new);
+            boolean[] oversized = new boolean[32 * 32];
+            RegionFileVersion[] oversizedCompressionTypes = new RegionFileVersion[32 * 32];
+
+            if (regionFiles != null) {
+                int lowerXBound = ourLowerLeftPosition.x; // inclusive
+                int lowerZBound = ourLowerLeftPosition.z; // inclusive
+                int upperXBound = lowerXBound + 32 - 1; // inclusive
+                int upperZBound = lowerZBound + 32 - 1; // inclusive
+
+                // read mojang oversized data
+                for (Path regionFile : regionFiles) {
+                    ChunkPos oversizedCoords = getOversizedChunkPair(regionFile);
+                    if (oversizedCoords == null) {
+                        continue;
+                    }
+
+                    if ((oversizedCoords.x < lowerXBound || oversizedCoords.x > upperXBound) || (oversizedCoords.z < lowerZBound || oversizedCoords.z > upperZBound)) {
+                        continue; // not in our regionfile
+                    }
+
+                    // ensure oversized data is valid & is newer than data in the regionfile
+
+                    int location = (oversizedCoords.x & 31) | ((oversizedCoords.z & 31) << 5);
+
+                    byte[] chunkData;
+                    try {
+                        chunkData = Files.readAllBytes(regionFile);
+                    } catch (Exception ex) {
+                        LOGGER.error("Failed to read oversized chunk data in file " + regionFile.toAbsolutePath() + ", data will be lost", ex);
+                        continue;
+                    }
+
+                    CompoundTag compound = null;
+
+                    // We do not know the compression type, as it's stored in the regionfile. So we need to try all of them
+                    RegionFileVersion compression = null;
+                    for (RegionFileVersion compressionType : RegionFileVersion.VERSIONS.values()) {
+                        try {
+                            DataInputStream in = new DataInputStream(compressionType.wrap(new ByteArrayInputStream(chunkData))); // typical java
+                            compound = NbtIo.read((java.io.DataInput)in);
+                            compression = compressionType;
+                            break; // reaches here iff readNBT does not throw
+                        } catch (Exception ex) {
+                            continue;
+                        }
+                    }
+
+                    if (compound == null) {
+                        LOGGER.error("Failed to read oversized chunk data in file " + regionFile.toAbsolutePath() + ", it's corrupt. Its data will be lost");
+                        continue;
+                    }
+
+                    if (!ChunkSerializer.getChunkCoordinate(compound).equals(oversizedCoords)) {
+                        LOGGER.error("Can't use oversized chunk stored in " + regionFile.toAbsolutePath() + ", got absolute chunkpos: " + ChunkSerializer.getChunkCoordinate(compound) + ", expected " + oversizedCoords);
+                        continue;
+                    }
+
+                    if (compounds[location] == null || ChunkSerializer.getLastWorldSaveTime(compound) > ChunkSerializer.getLastWorldSaveTime(compounds[location])) {
+                        oversized[location] = true;
+                        oversizedCompressionTypes[location] = compression;
+                    }
+                }
+            }
+
+            // now we need to calculate a new offset header
+
+            int[] calculatedOffsets = new int[32 * 32];
+            RegionBitmap newSectorAllocations = new RegionBitmap();
+            newSectorAllocations.force(0, 2); // make space for header
+
+            // allocate sectors for normal chunks
+
+            for (int chunkX = 0; chunkX < 32; ++chunkX) {
+                for (int chunkZ = 0; chunkZ < 32; ++chunkZ) {
+                    int location = chunkX | (chunkZ << 5);
+
+                    if (oversized[location]) {
+                        continue;
+                    }
+
+                    int rawLength = rawLengths[location]; // bytes
+                    int sectorOffset = sectorOffsets[location]; // sectors
+                    int sectorLength = (int)roundToSectors(rawLength);
+
+                    if (newSectorAllocations.tryAllocate(sectorOffset, sectorLength)) {
+                        calculatedOffsets[location] = sectorOffset << 8 | (sectorLength > 255 ? 255 : sectorLength); // support forge style oversized
+                    } else {
+                        LOGGER.error("Failed to allocate space for local chunk (overlapping data??) at (" + chunkX + "," + chunkZ + ") in regionfile " + this.regionFile.toAbsolutePath() + ", chunk will be regenerated");
+                    }
+                }
+            }
+
+            // allocate sectors for oversized chunks
+
+            for (int chunkX = 0; chunkX < 32; ++chunkX) {
+                for (int chunkZ = 0; chunkZ < 32; ++chunkZ) {
+                    int location = chunkX | (chunkZ << 5);
+
+                    if (!oversized[location]) {
+                        continue;
+                    }
+
+                    int sectorOffset = newSectorAllocations.allocate(1);
+                    int sectorLength = 1;
+
+                    try {
+                        this.file.write(this.createExternalStub(oversizedCompressionTypes[location]), sectorOffset * 4096);
+                        // only allocate in the new offsets if the write succeeds
+                        calculatedOffsets[location] = sectorOffset << 8 | (sectorLength > 255 ? 255 : sectorLength); // support forge style oversized
+                    } catch (IOException ex) {
+                        newSectorAllocations.free(sectorOffset, sectorLength);
+                        LOGGER.error("Failed to write new oversized chunk data holder, local chunk at (" + chunkX + "," + chunkZ + ") in regionfile " + this.regionFile.toAbsolutePath() + " will be regenerated");
+                    }
+                }
+            }
+
+            // rewrite aikar oversized data
+
+            this.oversizedCount = 0;
+            for (int chunkX = 0; chunkX < 32; ++chunkX) {
+                for (int chunkZ = 0; chunkZ < 32; ++chunkZ) {
+                    int location = chunkX | (chunkZ << 5);
+                    int isAikarOversized = hasAikarOversized[location] ? 1 : 0;
+
+                    this.oversizedCount += isAikarOversized;
+                    this.oversized[location] = (byte)isAikarOversized;
+                }
+            }
+
+            if (this.oversizedCount > 0) {
+                try {
+                    this.writeOversizedMeta();
+                } catch (Exception ex) {
+                    LOGGER.error("Failed to write aikar oversized chunk meta, all aikar style oversized chunk data will be lost for regionfile " + this.regionFile.toAbsolutePath(), ex);
+                    Files.deleteIfExists(this.getOversizedMetaFile());
+                }
+            } else {
+                Files.deleteIfExists(this.getOversizedMetaFile());
+            }
+
+            this.usedSectors.copyFrom(newSectorAllocations);
+
+            // before we overwrite the old sectors, print a summary of the chunks that got changed.
+
+            LOGGER.info("Starting summary of changes for regionfile " + this.regionFile.toAbsolutePath());
+
+            for (int chunkX = 0; chunkX < 32; ++chunkX) {
+                for (int chunkZ = 0; chunkZ < 32; ++chunkZ) {
+                    int location = chunkX | (chunkZ << 5);
+
+                    int oldOffset = this.offsets.get(location);
+                    int newOffset = calculatedOffsets[location];
+
+                    if (oldOffset == newOffset) {
+                        continue;
+                    }
+
+                    this.offsets.put(location, newOffset); // overwrite incorrect offset
+
+                    if (oldOffset == 0) {
+                        // found lost data
+                        LOGGER.info("Found missing data for local chunk (" + chunkX + "," + chunkZ + ") in regionfile " + this.regionFile.toAbsolutePath());
+                    } else if (newOffset == 0) {
+                        LOGGER.warn("Data for local chunk (" + chunkX + "," + chunkZ + ") could not be recovered in regionfile " + this.regionFile.toAbsolutePath() + ", it will be regenerated");
+                    } else {
+                        LOGGER.info("Local chunk (" + chunkX + "," + chunkZ + ") changed to point to newer data or correct chunk in regionfile " + this.regionFile.toAbsolutePath());
+                    }
+                }
+            }
+
+            LOGGER.info("End of change summary for regionfile " + this.regionFile.toAbsolutePath());
+
+            // simply destroy the timestamp header, it's not used
+
+            for (int i = 0; i < 32 * 32; ++i) {
+                this.timestamps.put(i, calculatedOffsets[i] != 0 ? (int)System.currentTimeMillis() : 0); // write a valid timestamp for valid chunks, I do not want to find out whatever dumb program actually checks this
+            }
+
+            // write new header
+            try {
+                this.flush();
+                this.file.force(true); // try to ensure it goes through...
+                LOGGER.info("Successfully wrote new header to disk for regionfile " + this.regionFile.toAbsolutePath());
+            } catch (IOException ex) {
+                LOGGER.error("Failed to write new header to disk for regionfile " + this.regionFile.toAbsolutePath(), ex);
+            }
+        }
+
+        return true;
+    }
+
+    final boolean canRecalcHeader; // final forces compile fail on new constructor
+    // Paper end - Attempt to recalculate regionfile header if it is corrupt
+
     // Paper start - Cache chunk status
     private final net.minecraft.world.level.chunk.ChunkStatus[] statuses = new net.minecraft.world.level.chunk.ChunkStatus[32 * 32];
 
@@ -77,8 +426,19 @@ public class RegionFile implements AutoCloseable {
     public RegionFile(Path file, Path directory, boolean dsync) throws IOException {
         this(file, directory, RegionFileVersion.getCompressionFormat(), dsync); // Paper - Configurable region compression format
     }
+    // Paper start - add can recalc flag
+    public RegionFile(Path file, Path directory, boolean dsync, boolean canRecalcHeader) throws IOException {
+        this(file, directory, RegionFileVersion.getCompressionFormat(), dsync, canRecalcHeader);
+    }
+    // Paper end - add can recalc flag
 
     public RegionFile(Path file, Path directory, RegionFileVersion outputChunkStreamVersion, boolean dsync) throws IOException {
+        // Paper start - add can recalc flag
+        this(file, directory, outputChunkStreamVersion, dsync, false);
+    }
+    public RegionFile(Path file, Path directory, RegionFileVersion outputChunkStreamVersion, boolean dsync, boolean canRecalcHeader) throws IOException {
+        this.canRecalcHeader = canRecalcHeader;
+        // Paper end - add can recalc flag
         this.header = ByteBuffer.allocateDirect(8192);
         this.regionFile = file; // Paper
         initOversizedState(); // Paper
@@ -107,14 +467,16 @@ public class RegionFile implements AutoCloseable {
                     RegionFile.LOGGER.warn("Region file {} has truncated header: {}", file, i);
                 }
 
-                long j = Files.size(file);
+                final long j = Files.size(file); final long regionFileSize = j; // Paper - recalculate header on header corruption
 
-                for (int k = 0; k < 1024; ++k) {
-                    int l = this.offsets.get(k);
+                boolean needsHeaderRecalc = false; // Paper - recalculate header on header corruption
+                boolean hasBackedUp = false; // Paper - recalculate header on header corruption
+                for (int k = 0; k < 1024; ++k) { final int headerLocation = k; // Paper - we expect this to be the header location
+                    final int l = this.offsets.get(k);
 
                     if (l != 0) {
-                        int i1 = RegionFile.getSectorNumber(l);
-                        int j1 = RegionFile.getNumSectors(l);
+                        final int i1 = RegionFile.getSectorNumber(l); final int offset = i1; // Paper - we expect this to be offset in file in sectors
+                        int j1 = RegionFile.getNumSectors(l); final int sectorLength; // Paper - diff on change, we expect this to be sector length of region - watch out for reassignments
                         // Spigot start
                         if (j1 == 255) {
                             // We're maxed out, so we need to read the proper length from the section
@@ -123,32 +485,102 @@ public class RegionFile implements AutoCloseable {
                             j1 = (realLen.getInt(0) + 4) / 4096 + 1;
                         }
                         // Spigot end
+                        sectorLength = j1; // Paper - diff on change, we expect this to be sector length of region
 
                         if (i1 < 2) {
                             RegionFile.LOGGER.warn("Region file {} has invalid sector at index: {}; sector {} overlaps with header", new Object[]{file, k, i1});
-                            this.offsets.put(k, 0);
+                            //this.offsets.put(k, 0); // Paper - we catch this, but need it in the header for the summary change
                         } else if (j1 == 0) {
                             RegionFile.LOGGER.warn("Region file {} has an invalid sector at index: {}; size has to be > 0", file, k);
-                            this.offsets.put(k, 0);
+                            //this.offsets.put(k, 0); // Paper - we catch this, but need it in the header for the summary change
                         } else if ((long) i1 * 4096L > j) {
                             RegionFile.LOGGER.warn("Region file {} has an invalid sector at index: {}; sector {} is out of bounds", new Object[]{file, k, i1});
-                            this.offsets.put(k, 0);
+                            //this.offsets.put(k, 0); // Paper - we catch this, but need it in the header for the summary change
                         } else {
-                            this.usedSectors.force(i1, j1);
+                            //this.usedSectors.force(i1, j1); // Paper - move this down so we can check if it fails to allocate
+                        }
+                        // Paper start - recalculate header on header corruption
+                        if (offset < 2 || sectorLength <= 0 || ((long)offset * 4096L) > regionFileSize) {
+                            if (canRecalcHeader) {
+                                LOGGER.error("Detected invalid header for regionfile " + this.regionFile.toAbsolutePath() + "! Recalculating header...");
+                                needsHeaderRecalc = true;
+                                break;
+                            } else {
+                                // location = chunkX | (chunkZ << 5);
+                                LOGGER.error("Detected invalid header for regionfile " + this.regionFile.toAbsolutePath() +
+                                        "! Cannot recalculate, removing local chunk (" + (headerLocation & 31) + "," + (headerLocation >>> 5) + ") from header");
+                                if (!hasBackedUp) {
+                                    hasBackedUp = true;
+                                    this.backupRegionFile();
+                                }
+                                this.timestamps.put(headerLocation, 0); // be consistent, delete the timestamp too
+                                this.offsets.put(headerLocation, 0); // delete the entry from header
+                                continue;
+                            }
+                        }
+                        boolean failedToAllocate = !this.usedSectors.tryAllocate(offset, sectorLength);
+                        if (failedToAllocate) {
+                            LOGGER.error("Overlapping allocation by local chunk (" + (headerLocation & 31) + "," + (headerLocation >>> 5) + ") in regionfile " + this.regionFile.toAbsolutePath());
                         }
+                        if (failedToAllocate & !canRecalcHeader) {
+                            // location = chunkX | (chunkZ << 5);
+                            LOGGER.error("Detected invalid header for regionfile " + this.regionFile.toAbsolutePath() +
+                                    "! Cannot recalculate, removing local chunk (" + (headerLocation & 31) + "," + (headerLocation >>> 5) + ") from header");
+                            if (!hasBackedUp) {
+                                hasBackedUp = true;
+                                this.backupRegionFile();
+                            }
+                            this.timestamps.put(headerLocation, 0); // be consistent, delete the timestamp too
+                            this.offsets.put(headerLocation, 0); // delete the entry from header
+                            continue;
+                        }
+                        needsHeaderRecalc |= failedToAllocate;
+                        // Paper end - recalculate header on header corruption
                     }
                 }
+                // Paper start - recalculate header on header corruption
+                // we move the recalc here so comparison to old header is correct when logging to console
+                if (needsHeaderRecalc) { // true if header gave us overlapping allocations or had other issues
+                    LOGGER.error("Recalculating regionfile " + this.regionFile.toAbsolutePath() + ", header gave erroneous offsets & locations");
+                    this.recalculateHeader();
+                }
+                // Paper end
             }
 
         }
     }
 
     private Path getExternalChunkPath(ChunkPos chunkPos) {
-        String s = "c." + chunkPos.x + "." + chunkPos.z + ".mcc";
+        String s = "c." + chunkPos.x + "." + chunkPos.z + ".mcc"; // Paper - diff on change
 
         return this.externalFileDir.resolve(s);
     }
 
+    // Paper start
+    private static ChunkPos getOversizedChunkPair(Path file) {
+        String fileName = file.getFileName().toString();
+
+        if (!fileName.startsWith("c.") || !fileName.endsWith(".mcc")) {
+            return null;
+        }
+
+        String[] split = fileName.split("\\.");
+
+        if (split.length != 4) {
+            return null;
+        }
+
+        try {
+            int x = Integer.parseInt(split[1]);
+            int z = Integer.parseInt(split[2]);
+
+            return new ChunkPos(x, z);
+        } catch (NumberFormatException ex) {
+            return null;
+        }
+    }
+    // Paper end
+
     @Nullable
     public synchronized DataInputStream getChunkDataInputStream(ChunkPos pos) throws IOException {
         int i = this.getOffset(pos);
@@ -172,6 +604,11 @@ public class RegionFile implements AutoCloseable {
             ((java.nio.Buffer) bytebuffer).flip(); // CraftBukkit - decompile error
             if (bytebuffer.remaining() < 5) {
                 RegionFile.LOGGER.error("Chunk {} header is truncated: expected {} but read {}", new Object[]{pos, l, bytebuffer.remaining()});
+                // Paper start - recalculate header on regionfile corruption
+                if (this.canRecalcHeader && this.recalculateHeader()) {
+                    return this.getChunkDataInputStream(pos);
+                }
+                // Paper end - recalculate header on regionfile corruption
                 return null;
             } else {
                 int i1 = bytebuffer.getInt();
@@ -179,6 +616,11 @@ public class RegionFile implements AutoCloseable {
 
                 if (i1 == 0) {
                     RegionFile.LOGGER.warn("Chunk {} is allocated, but stream is missing", pos);
+                    // Paper start - recalculate header on regionfile corruption
+                    if (this.canRecalcHeader && this.recalculateHeader()) {
+                        return this.getChunkDataInputStream(pos);
+                    }
+                    // Paper end - recalculate header on regionfile corruption
                     return null;
                 } else {
                     int j1 = i1 - 1;
@@ -186,17 +628,44 @@ public class RegionFile implements AutoCloseable {
                     if (RegionFile.isExternalStreamChunk(b0)) {
                         if (j1 != 0) {
                             RegionFile.LOGGER.warn("Chunk has both internal and external streams");
+                            // Paper start - recalculate header on regionfile corruption
+                            if (this.canRecalcHeader && this.recalculateHeader()) {
+                                return this.getChunkDataInputStream(pos);
+                            }
+                            // Paper end - recalculate header on regionfile corruption
                         }
 
-                        return this.createExternalChunkInputStream(pos, RegionFile.getExternalChunkVersion(b0));
+                        // Paper start - recalculate header on regionfile corruption
+                        final DataInputStream ret = this.createExternalChunkInputStream(pos, RegionFile.getExternalChunkVersion(b0));
+                        if (ret == null && this.canRecalcHeader && this.recalculateHeader()) {
+                            return this.getChunkDataInputStream(pos);
+                        }
+                        return ret;
+                        // Paper end - recalculate header on regionfile corruption
                     } else if (j1 > bytebuffer.remaining()) {
                         RegionFile.LOGGER.error("Chunk {} stream is truncated: expected {} but read {}", new Object[]{pos, j1, bytebuffer.remaining()});
+                        // Paper start - recalculate header on regionfile corruption
+                        if (this.canRecalcHeader && this.recalculateHeader()) {
+                            return this.getChunkDataInputStream(pos);
+                        }
+                        // Paper end - recalculate header on regionfile corruption
                         return null;
                     } else if (j1 < 0) {
                         RegionFile.LOGGER.error("Declared size {} of chunk {} is negative", i1, pos);
+                        // Paper start - recalculate header on regionfile corruption
+                        if (this.canRecalcHeader && this.recalculateHeader()) {
+                            return this.getChunkDataInputStream(pos);
+                        }
+                        // Paper end - recalculate header on regionfile corruption
                         return null;
                     } else {
-                        return this.createChunkInputStream(pos, b0, RegionFile.createStream(bytebuffer, j1));
+                        // Paper start - recalculate header on regionfile corruption
+                        final DataInputStream ret = this.createChunkInputStream(pos, b0, RegionFile.createStream(bytebuffer, j1));
+                        if (ret == null && this.canRecalcHeader && this.recalculateHeader()) {
+                            return this.getChunkDataInputStream(pos);
+                        }
+                        return ret;
+                        // Paper end - recalculate header on regionfile corruption
                     }
                 }
             }
@@ -371,10 +840,15 @@ public class RegionFile implements AutoCloseable {
     }
 
     private ByteBuffer createExternalStub() {
+        // Paper start - add version param
+        return this.createExternalStub(this.version);
+    }
+    private ByteBuffer createExternalStub(RegionFileVersion version) {
+        // Paper end - add version param
         ByteBuffer bytebuffer = ByteBuffer.allocate(5);
 
         bytebuffer.putInt(1);
-        bytebuffer.put((byte) (this.version.getId() | 128));
+        bytebuffer.put((byte) (version.getId() | 128)); // Paper - replace with version param
         ((java.nio.Buffer) bytebuffer).flip(); // CraftBukkit - decompile error
         return bytebuffer;
     }
diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileStorage.java b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileStorage.java
index 42d37bee3a459adcd46408596ccf93abbcff51fe..fe312b1aef579cb4bf81bdd967cf72ff880d7505 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileStorage.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileStorage.java
@@ -24,6 +24,7 @@ public class RegionFileStorage implements AutoCloseable {
     public final Long2ObjectLinkedOpenHashMap<RegionFile> regionCache = new Long2ObjectLinkedOpenHashMap();
     private final Path folder;
     private final boolean sync;
+    private final boolean isChunkData; // Paper
 
     // Paper start - cache regionfile does not exist state
     static final int MAX_NON_EXISTING_CACHE = 1024 * 64;
@@ -55,6 +56,12 @@ public class RegionFileStorage implements AutoCloseable {
     // Paper end - cache regionfile does not exist state
 
     protected RegionFileStorage(Path directory, boolean dsync) { // Paper - protected constructor
+        // Paper start - add isChunkData param
+        this(directory, dsync, false);
+    }
+    RegionFileStorage(Path directory, boolean dsync, boolean isChunkData) {
+        this.isChunkData = isChunkData;
+        // Paper end - add isChunkData param
         this.folder = directory;
         this.sync = dsync;
     }
@@ -122,7 +129,7 @@ public class RegionFileStorage implements AutoCloseable {
             // Paper - only create directory if not existing only - moved down
             Path path = this.folder;
             int j = chunkcoordintpair.getRegionX();
-            Path path1 = path.resolve("r." + j + "." + chunkcoordintpair.getRegionZ() + ".mca");
+            Path path1 = path.resolve("r." + j + "." + chunkcoordintpair.getRegionZ() + ".mca"); // Paper - diff on change
             if (existingOnly && !java.nio.file.Files.exists(path1)) { // Paper start - cache regionfile does not exist state
                 this.markNonExisting(regionPos);
                 return null; // CraftBukkit
@@ -131,7 +138,7 @@ public class RegionFileStorage implements AutoCloseable {
             }
             // Paper end - cache regionfile does not exist state
             FileUtil.createDirectoriesSafe(this.folder); // Paper - only create directory if not existing only - moved from above
-            RegionFile regionfile1 = new RegionFile(path1, this.folder, this.sync);
+            RegionFile regionfile1 = new RegionFile(path1, this.folder, this.sync, this.isChunkData); // Paper - allow for chunk regionfiles to regen header
 
             this.regionCache.putAndMoveToFirst(i, regionfile1);
             // Paper start
@@ -188,6 +195,13 @@ public class RegionFileStorage implements AutoCloseable {
         if (regionfile == null) {
             return null;
         }
+        // Paper start - Add regionfile parameter
+        return this.read(pos, regionfile);
+    }
+    public CompoundTag read(ChunkPos pos, RegionFile regionfile) throws IOException {
+        // We add the regionfile parameter to avoid the potential deadlock (on fileLock) if we went back to obtain a regionfile
+        // if we decide to re-read
+        // Paper end
         // CraftBukkit end
         try { // Paper
         DataInputStream datainputstream = regionfile.getChunkDataInputStream(pos);
@@ -204,6 +218,20 @@ public class RegionFileStorage implements AutoCloseable {
             try {
                 if (datainputstream != null) {
                     nbttagcompound = NbtIo.read((DataInput) datainputstream);
+                    // Paper start - recover from corrupt regionfile header
+                    if (this.isChunkData) {
+                        ChunkPos chunkPos = ChunkSerializer.getChunkCoordinate(nbttagcompound);
+                        if (!chunkPos.equals(pos)) {
+                            net.minecraft.server.MinecraftServer.LOGGER.error("Attempting to read chunk data at " + pos + " but got chunk data for " + chunkPos + " instead! Attempting regionfile recalculation for regionfile " + regionfile.regionFile.toAbsolutePath());
+                            if (regionfile.recalculateHeader()) {
+                                regionfile.fileLock.lock(); // otherwise we will unlock twice and only lock once.
+                                return this.read(pos, regionfile);
+                            }
+                            net.minecraft.server.MinecraftServer.LOGGER.error("Can't recalculate regionfile header, regenerating chunk " + pos + " for " + regionfile.regionFile.toAbsolutePath());
+                            return null;
+                        }
+                    }
+                    // Paper end - recover from corrupt regionfile header
                     break label43;
                 }
 
diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileVersion.java b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileVersion.java
index 581912e1315a4a8b46042a27df54826fa63e9c75..7baa12b53588d39fe1e6f130992dca591f246c83 100644
--- a/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileVersion.java
+++ b/src/main/java/net/minecraft/world/level/chunk/storage/RegionFileVersion.java
@@ -14,7 +14,7 @@ import javax.annotation.Nullable;
 import net.minecraft.util.FastBufferedInputStream;
 
 public class RegionFileVersion {
-    private static final Int2ObjectMap<RegionFileVersion> VERSIONS = new Int2ObjectOpenHashMap<>();
+    public static final Int2ObjectMap<RegionFileVersion> VERSIONS = new Int2ObjectOpenHashMap<>(); // Paper - private -> public
     public static final RegionFileVersion VERSION_GZIP = register(
         new RegionFileVersion(
             1, stream -> new FastBufferedInputStream(new GZIPInputStream(stream)), stream -> new BufferedOutputStream(new GZIPOutputStream(stream))