Fix lowres changes getting lost when a tile is changed while it is saved, Closes #883

Co-authored-by: Hexay <97907668+Hexay@users.noreply.github.com>

Author: TBlueF

core/src/main/java/de/bluecolored/bluemap/core/map/lowres/LowresLayer.java

@@ -36,6 +36,7 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
@@ -99,10 +100,22 @@ public void removeTileUpdateListener(BiConsumer<Vector2i, Integer> listener) {
     }
 
     public synchronized void save() {
-        pendingChanges.entrySet().removeIf(entry -> saveTile(entry.getKey(), entry.getValue()));
-        if (pendingChanges.size() >= DISCARD_THRESHOLD) {
-            Logger.global.logDebug("Discarding changes of " + pendingChanges.size() + " lowres-tiles that failed to save!");
-            pendingChanges.clear();
+        Map<Vector2i, LowresTile> failed = new HashMap<>();
+
+        // save tiles and remember which tiles are not saved
+        pendingChanges.forEach((pos, tile) -> {
+            if (!saveTile(pos, tile)) failed.put(pos, tile);
+        });
+
+        // if there is more than DISCARD_THRESHOLD failed saves, drop them
+        if (failed.size() >= DISCARD_THRESHOLD) {
+            Logger.global.logDebug("Discarding changes of " + failed.size() + " lowres-tiles that failed to save!");
+            failed.forEach(pendingChanges::remove);
+        }
+
+        // atomically test every tile if it is modified and remove it if it is not
+        for (Vector2i pos : pendingChanges.keySet()) {
+            pendingChanges.computeIfPresent(pos, (_, tile) -> tile.isModified() ? tile : null);
         }
     }
 
@@ -124,6 +137,7 @@ private LowresTile createTile(Vector2i tilePos) {
     }
 
     private boolean saveTile(Vector2i tilePos, LowresTile tile) {
+        if (!tile.isModified()) return true;
 
         // check if storage is closed
         if (storage.isClosed()){
@@ -134,7 +148,8 @@ private boolean saveTile(Vector2i tilePos, LowresTile tile) {
         // save the tile
         try {
             storage.write(tilePos.getX(), tilePos.getY(), tile::save);
-        } catch (IOException e) {
+        } catch (Exception e) {
+            tile.markModified();
             Logger.global.logError("Failed to save tile " + tilePos + " (lod: " + lod + ")", e);
             return false;
         }
@@ -193,35 +208,31 @@ private boolean saveTile(Vector2i tilePos, LowresTile tile) {
         return true;
     }
 
-    private LowresTile accessTile(int x, int z) {
-        Vector2i tilePos = VECTOR_2_I_CACHE.get(x, z);
-        LowresTile tile = tileCache.get(tilePos);
-
-        if (pendingChanges.size() >= MAX_PENDING) save();
-        pendingChanges.put(tilePos, tile);
-
-        return tile;
-    }
-
     void set(int cellX, int cellZ, int pixelX, int pixelZ, Color color, int height, int blockLight) {
-        accessTile(cellX, cellZ)
-                .set(pixelX, pixelZ, color, height, blockLight);
+        setPixel(cellX, cellZ, pixelX, pixelZ, color, height, blockLight);
 
         // for seamless edges
         if (pixelX == 0) {
-            accessTile(cellX - 1, cellZ)
-                    .set(tileGrid.getGridSize().getX(), pixelZ, color, height, blockLight);
+            setPixel(cellX - 1, cellZ, tileGrid.getGridSize().getX(), pixelZ, color, height, blockLight);
         }
 
         if (pixelZ == 0) {
-            accessTile(cellX, cellZ - 1)
-                    .set(pixelX, tileGrid.getGridSize().getY(), color, height, blockLight);
+            setPixel(cellX, cellZ - 1, pixelX, tileGrid.getGridSize().getY(), color, height, blockLight);
         }
 
         if (pixelX == 0 && pixelZ == 0) {
-            accessTile(cellX - 1, cellZ - 1)
-                    .set(tileGrid.getGridSize().getX(), tileGrid.getGridSize().getY(), color, height, blockLight);
+            setPixel(cellX - 1, cellZ - 1, tileGrid.getGridSize().getX(), tileGrid.getGridSize().getY(), color, height, blockLight);
         }
     }
 
+    private void setPixel(int cellX, int cellZ, int pixelX, int pixelZ, Color color, int height, int blockLight) {
+        Vector2i tilePos = VECTOR_2_I_CACHE.get(cellX, cellZ);
+        LowresTile tile = tileCache.get(tilePos);
+
+        tile.set(pixelX, pixelZ, color, height, blockLight);
+
+        pendingChanges.put(tilePos, tile);
+        if (pendingChanges.size() >= MAX_PENDING) save();
+    }
+
 }

core/src/main/java/de/bluecolored/bluemap/core/map/lowres/LowresTile.java

@@ -26,6 +26,7 @@
 
 import com.flowpowered.math.vector.Vector2i;
 import de.bluecolored.bluemap.core.util.math.Color;
+import lombok.Getter;
 
 import javax.imageio.ImageIO;
 import java.awt.image.BufferedImage;
@@ -36,21 +37,24 @@
 
 public class LowresTile {
 
-    public static final int HEIGHT_UNDEFINED = Integer.MIN_VALUE;
-
     private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
 
     private final BufferedImage texture;
     private final Vector2i size;
 
+    @Getter
+    private volatile boolean modified;
+
     public LowresTile(Vector2i tileSize) {
         this.size = tileSize.add(1, 1); // add 1 for seamless edges
         this.texture = new BufferedImage(this.size.getX(), this.size.getY() * 2, BufferedImage.TYPE_INT_ARGB);
+        this.modified = false; // technically true, but no need to save empty tiles
     }
 
     public LowresTile(Vector2i tileSize, InputStream in) throws IOException {
         this.size = tileSize.add(1, 1); // add 1 for seamless edges
         this.texture = ImageIO.read(in);
+        this.modified = false;
 
         if (this.texture == null) {
             throw new IOException("No registered ImageReader is able to read the image-stream");
@@ -64,6 +68,7 @@ public LowresTile(Vector2i tileSize, InputStream in) throws IOException {
     public void set(int x, int z, Color color, int height, int blockLight) {
         lock.readLock().lock();
         try {
+            if (!modified) modified = true;
             texture.setRGB(x, z, color.straight().getInt());
             texture.setRGB(x, size.getY() + z,
                     (height & 0x0000FFFF) |
@@ -75,6 +80,10 @@ public void set(int x, int z, Color color, int height, int blockLight) {
         }
     }
 
+    public void markModified() {
+        modified = true;
+    }
+
     public Color getColor(int x, int z, Color target) {
         return target.set(texture.getRGB(x, z));
     }
@@ -94,6 +103,7 @@ public void save(OutputStream out) throws IOException {
         lock.writeLock().lock();
         try {
             ImageIO.write(texture, "png", out);
+            modified = false;
         } finally {
             lock.writeLock().unlock();
         }