address review comments (round 2)

- Return io.ErrUnexpectedEOF when HTTP response is truncated
  This prevents silent data corruption from incomplete reads
- Simplify errgroup error handling by using g.Wait() error directly
  Remove redundant task.err field and manual error aggregation loop
- Define minReadConcurrency constant instead of magic number 4
  Improves code readability and maintainability

Note: Context propagation to startCaching() is intentionally NOT changed.
The downloaded chunk is a shared resource that may be used by multiple
readers. Using context.Background() ensures the download completes even
if one reader cancels, preventing data loss for other waiting readers.

weed/filer/reader_at.go

@@ -21,6 +21,11 @@ import (
 // the prefetch count is derived from the -concurrentReaders option.
 const DefaultPrefetchCount = 4
 
+// minReadConcurrency is the minimum number of parallel chunk fetches.
+// This ensures at least some parallelism even when prefetchCount is low,
+// improving throughput for reads spanning multiple chunks.
+const minReadConcurrency = 4
+
 type ChunkReadAt struct {
 	masterClient  *wdclient.MasterClient
 	chunkViews    *IntervalList[*ChunkView]
@@ -185,7 +190,6 @@ type chunkReadTask struct {
 	chunkOffset  uint64 // offset within the chunk to read from
 	bytesRead    int
 	modifiedTsNs int64
-	err          error
 }
 
 func (c *ChunkReadAt) doReadAt(ctx context.Context, p []byte, offset int64) (n int, ts int64, err error) {
@@ -252,10 +256,10 @@ func (c *ChunkReadAt) doReadAt(ctx context.Context, p []byte, offset int64) (n i
 		// This significantly improves throughput when chunks are on different volume servers
 		g, gCtx := errgroup.WithContext(ctx)
 
-		// Limit concurrency to prefetchCount to avoid overwhelming the system
+		// Limit concurrency to avoid overwhelming the system
 		concurrency := c.prefetchCount
-		if concurrency < 4 {
-			concurrency = 4
+		if concurrency < minReadConcurrency {
+			concurrency = minReadConcurrency
 		}
 		if concurrency > len(tasks) {
 			concurrency = len(tasks)
@@ -269,24 +273,19 @@ func (c *ChunkReadAt) doReadAt(ctx context.Context, p []byte, offset int64) (n i
 				copied, readErr := c.readChunkSliceAtForParallel(gCtx, p[task.bufferStart:task.bufferEnd], task.chunk, task.chunkOffset)
 				task.bytesRead = copied
 				task.modifiedTsNs = task.chunk.ModifiedTsNs
-				task.err = readErr
-				if readErr != nil {
-					return readErr
-				}
-				return nil
+				return readErr
 			})
 		}
 
 		// Wait for all chunk reads to complete
-		_ = g.Wait()
+		if waitErr := g.Wait(); waitErr != nil {
+			err = waitErr
+		}
 
 		// Aggregate results (order is preserved since we read directly into buffer positions)
 		for _, task := range tasks {
 			n += task.bytesRead
 			ts = max(ts, task.modifiedTsNs)
-			if task.err != nil && err == nil {
-				err = task.err
-			}
 		}
 
 		if err != nil {

weed/util/http/http_global_client_util.go

@@ -645,6 +645,11 @@ func readUrlDirectToBuffer(ctx context.Context, fileUrl, jwt string, buffer []by
 		totalRead += m
 		if readErr != nil {
 			if readErr == io.EOF {
+				// Return io.ErrUnexpectedEOF if we haven't filled the buffer
+				// This prevents silent data corruption from truncated responses
+				if totalRead < len(buffer) {
+					return totalRead, true, io.ErrUnexpectedEOF
+				}
 				return totalRead, false, nil
 			}
 			return totalRead, true, readErr