filer: bound format chunk sizes when no maxMB is configured

Extent chunks are buffered in memory, so an absent limit must not mean
unlimited. Also close the repack chunk reader to release its private
reader cache, and drop the arithmetic capacity hint on the extended-map
allocation.

weed/server/filer_server_format.go

@@ -28,6 +28,9 @@ import (
 const (
 	maxFormatSidecarBytes = 16 << 20
 	formatSniffBytes      = 512
+	// defaultFormatChunkSizeMB caps extent chunks when no maxMB is configured.
+	// Extent chunks are buffered in memory, so the limit must never be absent.
+	defaultFormatChunkSizeMB = 4
 )
 
 // formatChunkSizeLimit mirrors the autoChunk maxMB resolution.
@@ -37,6 +40,9 @@ func (fs *FilerServer) formatChunkSizeLimit(r *http.Request) int64 {
 	if maxMB <= 0 && fs.option.MaxMB > 0 {
 		maxMB = int32(fs.option.MaxMB)
 	}
+	if maxMB <= 0 {
+		maxMB = defaultFormatChunkSizeMB
+	}
 	return int64(maxMB) * 1024 * 1024
 }
 
@@ -261,6 +267,8 @@ func (fs *FilerServer) formatRepack(ctx context.Context, w http.ResponseWriter,
 	chunkViews := filer.ViewFromChunks(ctx, lookup, oldChunks, 0, size)
 	readerCache := filer.NewReaderCache(8, chunk_cache.NewChunkCacheInMemory(16), lookup, nil)
 	readerAt := filer.NewChunkReaderAtFromClient(ctx, readerCache, chunkViews, size, filer.DefaultPrefetchCount)
+	// Close releases the private reader cache and its in-flight prefetches.
+	defer readerAt.Close()
 
 	hint := format.Hint{Name: entry.Name(), ContentType: entry.Attr.Mime, Size: size}
 	sniffSize := int64(formatSniffBytes)
@@ -321,7 +329,7 @@ func (fs *FilerServer) formatRepack(ctx context.Context, w http.ResponseWriter,
 
 	newEntry := *entry
 	newEntry.Chunks = newChunks
-	newEntry.Extended = make(map[string][]byte, len(entry.Extended)+1)
+	newEntry.Extended = make(map[string][]byte)
 	for k, v := range entry.Extended {
 		newEntry.Extended[k] = v
 	}