Coalesce overlapping chunk batches

internal/world/cache.go

@@ -242,18 +242,27 @@ func (c *Cache) chunkAtErr(cx, cz int32) (*Chunk, error) {
 	}
 
 	c.mu.Lock()
+	if c.loads[key] != pending {
+		// An overlapping batch completed this target while its own generation was
+		// running. Keep the first published result and do not close done twice.
+		ch, loadErr = pending.ch, pending.err
+		c.mu.Unlock()
+		return ch, loadErr
+	}
 	if loadErr == nil {
 		for batchKey, batchChunk := range generatedBatch {
 			if batchKey == key || batchChunk == nil || batchChunk.X != batchKey[0] || batchChunk.Z != batchKey[1] {
 				continue
 			}
-			if _, loading := c.loads[batchKey]; loading {
-				continue
-			}
 			if _, exists := c.chunks[batchKey]; !exists {
 				c.chunks[batchKey] = batchChunk
 				c.touch(batchKey)
 			}
+			if neighborLoad := c.loads[batchKey]; neighborLoad != nil {
+				neighborLoad.ch = c.chunks[batchKey]
+				delete(c.loads, batchKey)
+				close(neighborLoad.done)
+			}
 		}
 		c.chunks[key] = ch
 		c.touch(key)

internal/world/cache_test.go

@@ -231,17 +231,18 @@ func TestBatchGeneratorPublishesNeighborsAtomically(t *testing.T) {
 	}
 }
 
-func TestConcurrentOverlappingBatchLoadsKeepOwnership(t *testing.T) {
+func TestConcurrentOverlappingBatchLoadsShareFirstResult(t *testing.T) {
 	started := make(chan struct{}, 2)
 	release := make(chan struct{})
 	c := NewCache(-1, GenerateFlat)
 	c.SetBatchGenerator(func(cx, cz int32) (map[[2]int32]*Chunk, error) {
 		started <- struct{}{}
 		<-release
+		biome := uint16(cx + 10)
 		return map[[2]int32]*Chunk{
-			{cx, cz}:     NewChunk(cx, cz, BiomePlains),
+			{cx, cz}:     NewChunk(cx, cz, biome),
-			{cx + 1, cz}: NewChunk(cx+1, cz, BiomePlains),
+			{cx + 1, cz}: NewChunk(cx+1, cz, biome),
-			{cx - 1, cz}: NewChunk(cx-1, cz, BiomePlains),
+			{cx - 1, cz}: NewChunk(cx-1, cz, biome),
 		}, nil
 	})
 
@@ -261,15 +262,24 @@ func TestConcurrentOverlappingBatchLoadsKeepOwnership(t *testing.T) {
 	<-started
 	<-started
 	close(release)
+	loaded := make(map[[2]int32]*Chunk)
 	for range 2 {
 		result := <-results
 		if result.err != nil || result.chunk == nil {
 			t.Fatalf("overlapping batch load = %v, %v", result.chunk, result.err)
 		}
+		loaded[[2]int32{result.chunk.X, result.chunk.Z}] = result.chunk
 	}
-	if c.chunkAt(0, 0) == nil || c.chunkAt(1, 0) == nil {
+	left, right := c.chunkAt(0, 0), c.chunkAt(1, 0)
+	if left == nil || right == nil {
 		t.Fatal("overlapping targets were not retained")
 	}
+	if left != loaded[[2]int32{0, 0}] || right != loaded[[2]int32{1, 0}] {
+		t.Fatal("requesters did not receive the canonical cached batch results")
+	}
+	if left.GetBiome(0, MinY, 0) != right.GetBiome(0, MinY, 0) {
+		t.Fatal("overlapping chunks came from different batch results")
+	}
 }
 
 func TestBatchGeneratorRejectsWrongTargetCoordinates(t *testing.T) {