mount: tell a shortened base chunk from a whole-file rewrite

A truncate shortens a base chunk in place, so it gets a new reference
key, and an old chunk can carry no timestamp. The rewrite refusal now
applies only to unstamped chunks whose file id is new to the base.

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

weed/mount/content_flush_condition_test.go

@@ -344,6 +344,15 @@ func TestMergeContentEntry(t *testing.T) {
 	if _, ok := mergeContentEntry(proto.Clone(base).(*filer_pb.Entry), base, rewritten, rewritten, -1); !ok {
 		t.Fatal("refused a rewrite with no peer chunk change")
 	}
+	unstamped := flushConditionEntry(4)
+	unstamped.Chunks[0].Size, unstamped.Chunks[0].ModifiedTsNs = 3, 0
+	shortened := proto.Clone(unstamped).(*filer_pb.Entry)
+	shortened.Chunks[0].Size, shortened.Attributes.FileSize = 2, 2
+	peerAppend := proto.Clone(unstamped).(*filer_pb.Entry)
+	peerAppend.Chunks = append(peerAppend.Chunks, &filer_pb.FileChunk{Fid: &filer_pb.FileId{VolumeId: 1, FileKey: 9, Cookie: 1}, Offset: 3, Size: 1, ModifiedTsNs: 9})
+	if _, ok := mergeContentEntry(peerAppend, unstamped, shortened, shortened, 2); !ok {
+		t.Fatal("refused a truncate that shortened an unstamped base chunk")
+	}
 
 	empty := flushConditionEntry()
 	inline := proto.Clone(empty).(*filer_pb.Entry)

weed/mount/weedfs_file_sync.go

@@ -395,8 +395,10 @@ func mergeContentEntry(stored, base, local, sent *filer_pb.Entry, truncatedTo in
 	}
 
 	inBase := make(map[string]bool, len(base.Chunks))
+	baseFileIds := make(map[string]bool, len(base.Chunks))
 	for _, c := range base.Chunks {
 		inBase[contentChunkKey(c)] = true
+		baseFileIds[contentFileIdKey(c)] = true
 	}
 	inLocal := make(map[string]bool, len(local.Chunks))
 	for _, c := range local.Chunks {
@@ -426,7 +428,7 @@ func mergeContentEntry(stored, base, local, sent *filer_pb.Entry, truncatedTo in
 	}
 	for _, c := range local.Chunks {
 		if !inBase[contentChunkKey(c)] {
-			if peerChanged && c.ModifiedTsNs == 0 && !c.IsChunkManifest {
+			if peerChanged && c.ModifiedTsNs == 0 && !c.IsChunkManifest && !baseFileIds[contentFileIdKey(c)] {
 				return nil, false
 			}
 			merged.Chunks = append(merged.Chunks, proto.Clone(c).(*filer_pb.FileChunk))
@@ -496,15 +498,19 @@ func sameContentFile(stored, base *filer_pb.Entry) bool {
 // place, so the same file id at another size is a different reference. Both
 // file id encodings give one key, and the chunk is left as is.
 func contentChunkKey(c *filer_pb.FileChunk) string {
+	return fmt.Sprintf("%s@%d+%d", contentFileIdKey(c), c.Offset, c.Size)
+}
+
+func contentFileIdKey(c *filer_pb.FileChunk) string {
 	fid := c.Fid
 	if fid == nil {
 		if parsed, err := filer_pb.ToFileIdObject(c.FileId); err == nil {
 			fid = parsed
 		} else {
-			return fmt.Sprintf("%s@%d+%d", c.FileId, c.Offset, c.Size)
+			return c.FileId
 		}
 	}
-	return fmt.Sprintf("%d,%x,%x@%d+%d", fid.VolumeId, fid.FileKey, fid.Cookie, c.Offset, c.Size)
+	return fmt.Sprintf("%d,%x,%x", fid.VolumeId, fid.FileKey, fid.Cookie)
 }
 
 // ackedBase is the base an acknowledged create leaves: the entry the filer