fix: recognize timestamp-only retention-removal tombstone in resend compare

retentionRemovedAtSource only recognized representation (1) of a removed
retention: the object lock key present with an empty value. But a removal
that arrived by replication persists representation (2): restoreRetention
(and the receiver's replica update path) writes only the retention ordering
timestamp when the mode is empty, leaving the mode and retain-until-date keys
absent. For that shape the helper returned false, so replicationActionForTarget
skipped the GetObjectRetention confirmation and let getReplicationAction's
replicateNone stand, silently dropping a needed removal when the destination
HEAD hides retention behind a permission-filtered credential.

Recognize representation (2) as well: a present retention ordering timestamp
with the mode value absent or empty is a removal. A present timestamp paired
with a non-empty mode is a retention that was set, not removed, and still
returns false.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L7qJqWwy8oFA6aCXWRzXQe
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/bucket-replication.go

@@ -1067,15 +1067,29 @@ type objectRetentionGetter interface {
 	GetObjectRetention(ctx context.Context, bucketName, objectName, versionID string) (*minio.RetentionMode, *time.Time, error)
 }
 
-// retentionRemovedAtSource reports whether oi carries the shape a removed retention leaves behind,
+// retentionRemovedAtSource reports whether oi carries the shape a removed retention leaves behind.
-// an object lock key present with an empty value (cmd/object-handlers.go:3211-3215).
+// Two representations persist. A retention removed directly on this cluster keeps the object lock
+// keys present with empty values (PutObjectRetentionHandler, cmd/object-handlers.go:3309-3316). A
+// removal that arrived by replication keeps only the retention ordering timestamp, with the mode
+// and retain-until-date keys absent, because restoreRetention and the replica update path write
+// the timestamp alone when the mode is empty (cmd/bucket-object-lock.go:388-399,
+// cmd/object-handlers.go:1782-1797). A present ordering timestamp paired with a non-empty mode is
+// a retention that was set, not removed, and must not be mistaken for one.
 func retentionRemovedAtSource(oi ObjectInfo) bool {
 	lkMap := caseInsensitiveMap(oi.UserDefined)
+	// Representation (1): an object lock key is present with an empty value.
 	for _, k := range []string{xhttp.AmzObjectLockMode, xhttp.AmzObjectLockRetainUntilDate} {
 		if v, ok := lkMap.Lookup(k); ok && v == "" {
 			return true
 		}
 	}
+	// Representation (2): a recorded retention ordering timestamp with the mode value absent or
+	// empty is a removal restoreRetention persisted without the empty public keys.
+	if _, ok := oi.UserDefined[ReservedMetadataPrefixLower+ObjectLockRetentionTimestamp]; ok {
+		if v, ok := lkMap.Lookup(xhttp.AmzObjectLockMode); !ok || v == "" {
+			return true
+		}
+	}
 	return false
 }
 

cmd/bucket-replication_test.go

@@ -844,6 +844,8 @@ func (f *fakeRetentionGetter) GetObjectRetention(_ context.Context, _, _, _ stri
 func TestRetentionRemovedAtSource(t *testing.T) {
 	modeKey := strings.ToLower(xhttp.AmzObjectLockMode)
 	dateKey := strings.ToLower(xhttp.AmzObjectLockRetainUntilDate)
+	tsKey := ReservedMetadataPrefixLower + ObjectLockRetentionTimestamp
+	stamp := "2026-09-06T01:00:00Z"
 	tests := []struct {
 		name string
 		meta map[string]string
@@ -856,6 +858,11 @@ func TestRetentionRemovedAtSource(t *testing.T) {
 		{"real retention", map[string]string{modeKey: "GOVERNANCE", dateKey: "2026-10-05T10:00:00.000Z"}, false},
 		{"canonical case", map[string]string{xhttp.AmzObjectLockMode: ""}, true},
 		{"empty user metadata", map[string]string{"x-amz-meta-foo": ""}, false},
+		// Representation (2): a replicated removal persists the ordering timestamp alone,
+		// with the mode and retain-until-date keys absent (restoreRetention).
+		{"timestamp only, mode absent", map[string]string{tsKey: stamp}, true},
+		{"timestamp with empty mode", map[string]string{tsKey: stamp, modeKey: ""}, true},
+		{"timestamp with real retention is a set, not a removal", map[string]string{tsKey: stamp, modeKey: "GOVERNANCE", dateKey: "2026-10-05T10:00:00.000Z"}, false},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
@@ -1030,3 +1037,68 @@ func TestReplicationActionForTargetNullVersionResync(t *testing.T) {
 		})
 	}
 }
+
+// TestReplicationActionForTargetTimestampOnlyRemoval covers representation (2) of a removed
+// retention. A removal that arrived by replication persists only the retention ordering timestamp,
+// with the mode and retain-until-date keys absent, because restoreRetention writes the timestamp
+// alone when the mode is empty (cmd/bucket-object-lock.go). The comparison in getReplicationAction
+// reads such a source as in sync with a matching destination, so only the GetObjectRetention
+// confirmation separates a destination that dropped the retention from one hiding it behind a
+// permission-filtered HEAD. The source is built through the real restoreRetention path so the
+// fixture is the metadata a replicated removal actually leaves on disk, not a hand-rolled map.
+func TestReplicationActionForTargetTimestampOnlyRemoval(t *testing.T) {
+	governance := minio.Governance
+	stamp := time.Date(2026, 9, 6, 1, 0, 0, 0, time.UTC).Format(time.RFC3339Nano)
+
+	tests := []struct {
+		name      string
+		mode      *minio.RetentionMode
+		err       error
+		want      replicationAction
+		wantCalls int
+	}{
+		{
+			// The reference case: a destination that denies the retention read is
+			// indistinguishable from one still holding it, so the removal is resent.
+			name:      "retention hidden from HEAD by permissions",
+			err:       minio.ErrorResponse{Code: "AccessDenied"},
+			want:      replicateMetadata,
+			wantCalls: 1,
+		},
+		{
+			name:      "destination still holds the retention",
+			mode:      &governance,
+			want:      replicateMetadata,
+			wantCalls: 1,
+		},
+		{
+			name:      "removal confirmed by destination",
+			err:       minio.ErrorResponse{Code: "NoSuchObjectLockConfiguration"},
+			want:      replicateNone,
+			wantCalls: 1,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			src, tgtInfo := newMatchingReplicationPair()
+			// Persist the timestamp-only tombstone the same way an applied replica removal does.
+			objectLockState{retentionTimestamp: stamp}.restoreRetention(src.UserDefined)
+			if !retentionRemovedAtSource(src) {
+				t.Fatalf("restoreRetention fixture not recognized as a removal: %v", src.UserDefined)
+			}
+			if _, ok := src.UserDefined[strings.ToLower(xhttp.AmzObjectLockMode)]; ok {
+				t.Fatalf("restoreRetention fixture wrote a mode key, fixture is not timestamp-only: %v", src.UserDefined)
+			}
+
+			tgt := &fakeRetentionGetter{mode: test.mode, err: test.err}
+			got := replicationActionForTarget(t.Context(), src, tgtInfo, replication.HealReplicationType, tgt, "bucket", "object")
+			if got != test.want {
+				t.Fatalf("replicationActionForTarget() = %q, want %q", got, test.want)
+			}
+			if tgt.calls != test.wantCalls {
+				t.Fatalf("GetObjectRetention called %d times, want %d", tgt.calls, test.wantCalls)
+			}
+		})
+	}
+}