mount: refuse to merge a whole-file rewrite over a peer's chunk change

maybeMergeChunks re-uploads the handle's whole view as unstamped chunks.
Merged after a peer's change, they tie with a peer's own rewrite or bring
back a range the peer truncated, so that flush fails as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

weed/mount/content_flush_condition_test.go

@@ -308,6 +308,15 @@ func TestMergeContentEntry(t *testing.T) {
 		t.Fatal("merged into a replaced file")
 	}
 
+	rewritten := proto.Clone(base).(*filer_pb.Entry)
+	rewritten.Chunks = []*filer_pb.FileChunk{{Fid: &filer_pb.FileId{VolumeId: 1, FileKey: 8, Cookie: 1}, Size: 2}}
+	if _, ok := mergeContentEntry(stored, base, rewritten, rewritten); ok {
+		t.Fatal("merged a stale whole-file rewrite over a peer append")
+	}
+	if _, ok := mergeContentEntry(proto.Clone(base).(*filer_pb.Entry), base, rewritten, rewritten); !ok {
+		t.Fatal("refused a rewrite with no peer chunk change")
+	}
+
 	empty := flushConditionEntry()
 	inline := proto.Clone(empty).(*filer_pb.Entry)
 	inline.Content = []byte("peer")

weed/mount/weedfs_file_sync.go

@@ -373,9 +373,10 @@ const maxContentFlushMerges = 3
 // content it changed, taking those values from sent (local in stored's id
 // space). Everything else keeps the stored value. A local truncation keeps
 // its size and clips stored chunks to it; otherwise the larger size wins.
-// A different file at the path, inline content changed on both sides, or
-// inline content next to chunks (reads stop at the inline bytes) cannot be
-// merged.
+// A different file at the path, inline content changed on both sides,
+// inline content next to chunks (reads stop at the inline bytes), or a local
+// whole-file rewrite (maybeMergeChunks, unstamped chunks) over a peer's chunk
+// change cannot be merged: the rewrite carries the handle's stale view.
 func mergeContentEntry(stored, base, local, sent *filer_pb.Entry) (*filer_pb.Entry, bool) {
 	if stored == nil || base == nil || stored.IsDirectory || !bytes.Equal(stored.HardLinkId, base.HardLinkId) {
 		return nil, false
@@ -400,14 +401,20 @@ func mergeContentEntry(stored, base, local, sent *filer_pb.Entry) (*filer_pb.Ent
 	for _, c := range local.Chunks {
 		inLocal[contentChunkKey(c)] = true
 	}
+	peerChanged := len(stored.Chunks) != len(base.Chunks)
 	merged.Chunks = merged.Chunks[:0]
 	for _, c := range stored.Chunks {
-		if key := contentChunkKey(c); !inBase[key] || inLocal[key] {
+		key := contentChunkKey(c)
+		peerChanged = peerChanged || !inBase[key]
+		if !inBase[key] || inLocal[key] {
 			merged.Chunks = append(merged.Chunks, proto.Clone(c).(*filer_pb.FileChunk))
 		}
 	}
 	for _, c := range local.Chunks {
 		if !inBase[contentChunkKey(c)] {
+			if peerChanged && c.ModifiedTsNs == 0 && !c.IsChunkManifest {
+				return nil, false
+			}
 			merged.Chunks = append(merged.Chunks, proto.Clone(c).(*filer_pb.FileChunk))
 		}
 	}