perf: improve prefetch logic with pipeline optimizations

internal/stream/pipe.go

@@ -138,7 +138,15 @@ func (p *StreamPipe) Close() error {
 	return nil
 }
 
+// blockResult holds the outcome of a single block download.
+type blockResult struct {
+	data []byte
+	err  error
+}
+
 // prefetch runs in a goroutine, fetching blocks concurrently and sending to blockQueue.
+// each block is pushed to the reader as soon as it's ready (in order), rather than
+// waiting for the entire batch to complete — this minimizes time-to-first-byte.
 func (p *StreamPipe) prefetch() {
 	defer close(p.blockQueue)
 
@@ -159,34 +167,24 @@ func (p *StreamPipe) prefetch() {
 		default:
 		}
 
-		// fetch a batch of blocks concurrently
+		// fetch a batch of blocks concurrently, each with its own result channel
 		batchSize := min(config.ValueOf.StreamConcurrency, totalBlocks-currentBlock)
-		blocks := make([][]byte, batchSize)
-
-		var wg sync.WaitGroup
-		var fetchErr error
-		var errMu sync.Mutex
+		results := make([]chan blockResult, batchSize)
 
 		for i := range batchSize {
-			wg.Add(1)
+			results[i] = make(chan blockResult, 1)
 			go func(idx int) {
-				defer wg.Done()
-
 				blockNum := currentBlock + idx
 				blockOffset := offset + int64(idx)*p.blockSize
 
 				data, err := p.downloadBlockWithRetry(blockOffset)
-				dataLen := int64(len(data))
-
 				if err != nil {
-					errMu.Lock()
-					if fetchErr == nil {
-						fetchErr = err
-					}
-					errMu.Unlock()
+					results[idx] <- blockResult{err: err}
 					return
 				}
 
+				dataLen := int64(len(data))
+
 				// trim first/last block to exact range
 				if totalBlocks == 1 {
 					if dataLen < rightTrim {
@@ -207,30 +205,26 @@ func (p *StreamPipe) prefetch() {
 					}
 				}
 
-				blocks[idx] = data
+				results[idx] <- blockResult{data: data}
 			}(i)
 		}
 
-		wg.Wait()
-
-		// handle errors
-		// ignore context cancellation cuz it's expected on disconnect
-		if fetchErr != nil {
-			if p.ctx.Err() == nil {
-				p.log.Error("block download failed", zap.Error(fetchErr))
-			}
-			return
-		}
-
-		// send blocks to queue in order
-		for _, block := range blocks {
-			if block == nil {
-				// a fetch failure that wasn't captured, should not happen but just in case.
-				p.log.Error("unexpected nil block in batch, aborting prefetch")
-				return
-			}
+		// drain results in order — block 0 is sent to reader as soon as it's ready,
+		// without waiting for blocks 1, 2, 3 to finish
+		for i := range batchSize {
 			select {
-			case p.blockQueue <- block:
+			case res := <-results[i]:
+				if res.err != nil {
+					if p.ctx.Err() == nil {
+						p.log.Error("block download failed", zap.Error(res.err))
+					}
+					return
+				}
+				select {
+				case p.blockQueue <- res.data:
+				case <-p.ctx.Done():
+					return
+				}
 			case <-p.ctx.Done():
 				return
 			}