mount: refuse a merge that shortens a manifest at the truncate

Clipping a manifest reference leaves its members whole, so peer bytes past
the handle's truncate could reappear once the file grew again.

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

weed/mount/content_flush_condition_test.go

@@ -316,6 +316,13 @@ func TestMergeContentEntry(t *testing.T) {
 		t.Fatalf("truncate then write kept %v, size %d", got, merged.Attributes.FileSize)
 	}
 
+	// Shortening a manifest reference leaves its members whole.
+	peerManifest := proto.Clone(base).(*filer_pb.Entry)
+	peerManifest.Chunks = append(peerManifest.Chunks, &filer_pb.FileChunk{Fid: &filer_pb.FileId{VolumeId: 1, FileKey: 5, Cookie: 1}, Offset: 0, Size: 10, IsChunkManifest: true})
+	if _, ok := mergeContentEntry(peerManifest, base, regrown, regrown, 1, false); ok {
+		t.Fatal("merged a peer manifest across the truncate")
+	}
+
 	// A peer truncate stands when the handle only overwrote inside it.
 	peerTruncated := proto.Clone(base).(*filer_pb.Entry)
 	peerTruncated.Chunks, peerTruncated.Attributes.FileSize = peerTruncated.Chunks[:1], 1

weed/mount/weedfs_file_sync.go

@@ -379,7 +379,8 @@ const maxContentFlushMerges = 3
 // (truncatedTo, or -1); otherwise the stored size stands unless the handle
 // changed its size or wrote past it.
 // 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
+// inline content next to chunks (reads stop at the inline bytes), a manifest
+// across the truncate (its members keep their sizes), or a local
 // whole-file rewrite (rewrote) 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, truncatedTo int64, rewrote bool) (*filer_pb.Entry, bool) {
@@ -417,6 +418,9 @@ func mergeContentEntry(stored, base, local, sent *filer_pb.Entry, truncatedTo in
 		}
 		c = proto.Clone(c).(*filer_pb.FileChunk)
 		if truncatedTo >= 0 && c.Offset+int64(c.Size) > truncatedTo {
+			if c.IsChunkManifest {
+				return nil, false
+			}
 			c.Size = uint64(truncatedTo - c.Offset)
 		}
 		merged.Chunks = append(merged.Chunks, c)