From 73949daa987c62678dcda3845362c044b274eb85 Mon Sep 17 00:00:00 2001
From: scooby87 <alexeyartamonov1987@gmail.com>
Date: Mon, 12 Oct 2026 12:41:01 +0300
Subject: [PATCH] filer: keep sequential prefetch within prefetchCount chunks
 (#11674)

ChunkReadAt.ReadAt asks the ReaderCache to prefetch after every read,
and MaybeCache counted only the downloads it newly started toward the
window, so each call reached prefetchCount chunks further than the last.
A reader consuming a chunk in several reads, as the S3 GET path does
with a 256 KiB copy buffer over 4 MiB chunks, ran the read-ahead up to
the ReaderCache downloader limit and kept most of a large object in
memory, enough to get an S3 gateway OOM-killed mid-response.

Count every chunk in the window, downloading or not, so the read-ahead
stays at prefetchCount chunks past the reader.


Assisted-by: LLM

Signed-off-by: Alexey Artamonov <aleksei.artamonov@aenix.io>
---
 weed/filer/reader_at_prefetch_window_test.go | 74 ++++++++++++++++++++
 weed/filer/reader_cache.go                   |  7 +-
 2 files changed, 78 insertions(+), 3 deletions(-)
 create mode 100644 weed/filer/reader_at_prefetch_window_test.go

diff --git a/weed/filer/reader_at_prefetch_window_test.go b/weed/filer/reader_at_prefetch_window_test.go
new file mode 100644
index 000000000..982bc40cb
--- /dev/null
+++ b/weed/filer/reader_at_prefetch_window_test.go
@@ -0,0 +1,74 @@
+package filer
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/util/chunk_cache"
+	util_http "github.com/seaweedfs/seaweedfs/weed/util/http"
+)
+
+// A sequential reader that consumes a chunk in several small ReadAt calls, as
+// the S3 GET path does with its 256 KiB copy buffer over 4 MiB chunks, must
+// keep its read-ahead at prefetchCount chunks. Each ReadAt triggers a
+// prefetch; if chunks already being fetched are not counted toward the
+// window, every call starts prefetchCount more downloads and the read-ahead
+// runs to the ReaderCache downloader limit, buffering most of the object.
+func TestChunkReadAtSequentialPrefetchStaysWithinWindow(t *testing.T) {
+	const chunkSize = 64 << 10
+	const chunkCount = 64
+	const sliceSize = 16 << 10
+
+	var readerChunk atomic.Int64
+	var maxLead atomic.Int64
+	rc := NewReaderCache(256, (*chunk_cache.TieredChunkCache)(nil), func(context.Context, string) ([]string, error) {
+		return []string{"unused"}, nil
+	}, nil)
+	defer rc.destroy()
+	rc.fetchChunkDataFn = func(_ context.Context, buffer []byte, _ []string, _ []byte, _ bool, _ bool, _ int64, fileId string, _ util_http.RefreshUrlsFunc) (int, error) {
+		idx, err := strconv.Atoi(strings.TrimPrefix(fileId, "chunk"))
+		if err != nil {
+			return 0, err
+		}
+		lead := int64(idx) - readerChunk.Load()
+		for {
+			cur := maxLead.Load()
+			if lead <= cur || maxLead.CompareAndSwap(cur, lead) {
+				break
+			}
+		}
+		return len(buffer), nil
+	}
+
+	views := NewIntervalList[*ChunkView]()
+	for i := 0; i < chunkCount; i++ {
+		views.AppendInterval(&Interval[*ChunkView]{
+			StartOffset: int64(i * chunkSize),
+			StopOffset:  int64((i + 1) * chunkSize),
+			Value: &ChunkView{
+				FileId:     fmt.Sprintf("chunk%d", i),
+				ViewSize:   chunkSize,
+				ViewOffset: int64(i * chunkSize),
+				ChunkSize:  chunkSize,
+			},
+		})
+	}
+	reader := NewChunkReaderAtFromClient(context.Background(), rc, views, chunkSize*chunkCount, DefaultPrefetchCount)
+	defer reader.ReleaseStream()
+
+	buf := make([]byte, sliceSize)
+	for off := int64(0); off < chunkSize*chunkCount; off += sliceSize {
+		readerChunk.Store(off / chunkSize)
+		if _, err := reader.ReadAt(buf, off); err != nil && off+sliceSize < chunkSize*chunkCount {
+			t.Fatalf("ReadAt(%d): %v", off, err)
+		}
+	}
+
+	if got := maxLead.Load(); got > DefaultPrefetchCount {
+		t.Fatalf("read-ahead reached %d chunks past the reader, want at most %d", got, DefaultPrefetchCount)
+	}
+}
diff --git a/weed/filer/reader_cache.go b/weed/filer/reader_cache.go
index 1c2ba7252..cd6049bee 100644
--- a/weed/filer/reader_cache.go
+++ b/weed/filer/reader_cache.go
@@ -92,8 +92,10 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView], count int) {
 		return
 	}
 
-	cached := 0
-	for x := chunkViews; x != nil && cached < count; x = x.Next {
+	// The window is the next count chunks, whether or not a download for them
+	// is already running: counting only newly started downloads would push the
+	// window count chunks further on every call.
+	for x, i := chunkViews, 0; x != nil && i < count; x, i = x.Next, i+1 {
 		chunkView := x.Value
 		if _, found := rc.downloaders[chunkView.FileId]; found {
 			continue
@@ -124,7 +126,6 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView], count int) {
 		rc.downloaders[chunkView.FileId] = cacher
 		go cacher.startCaching()
 		started = append(started, cacher)
-		cached++
 	}
 
 	return