Keep stale virtual member copy when the remote lookup fails

internal/virtual/engine_stale_test.go

@@ -328,3 +328,18 @@ func TestMergePrunesExpiredCopiesOfOtherMembers(t *testing.T) {
 		t.Fatal("any merge should prune expired copies of removed members")
 	}
 }
+
+func TestServeStaleLookupErrorKeepsCopy(t *testing.T) {
+	e, _ := staleEngine(true, func() *RPMMember { return rpmPkgMember("a", "1") })
+	v1 := fetchRepomd(t, e)
+	e.getRemote = func(context.Context, string) (*models.Remote, error) { return nil, errors.New("db down") }
+	if got := fetchRepomd(t, e); got != v1 {
+		t.Fatal("a failed remote lookup should keep serving the previous copy")
+	}
+	waitRefresh(e, rpmStaleKey)
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	if e.stale[rpmStaleKey] == nil || e.stale[rpmStaleKey].m == nil {
+		t.Fatal("a failed remote lookup must not drop the copy")
+	}
+}