Don't try to unlock the GncDbiSqlConnection when it was connected read-only.

If it's locked it's not *our* lock.

libgnucash/backend/dbi/gnc-dbisqlconnection.cpp

@@ -89,9 +89,11 @@ GncDbiSqlConnection::GncDbiSqlConnection (DbType type, QofBackend* qbe,
             make_dbi_provider<DbType::DBI_MYSQL>() :
             make_dbi_provider<DbType::DBI_PGSQL>()},
     m_conn_ok{true}, m_last_error{ERR_BACKEND_NO_ERR}, m_error_repeat{0},
-    m_retry{false}, m_sql_savepoint{0}
+    m_retry{false}, m_sql_savepoint{0}, m_readonly{false}
 {
-    if (mode != SESSION_READ_ONLY && !lock_database(mode == SESSION_BREAK_LOCK))
+    if (mode == SESSION_READ_ONLY)
+        m_readonly = true;
+    else if (!lock_database(mode == SESSION_BREAK_LOCK))
         throw std::runtime_error("Failed to lock database!");
     if (!check_and_rollback_failed_save())
     {
@@ -174,6 +176,7 @@ void
 GncDbiSqlConnection::unlock_database ()
 {
     if (m_conn == nullptr) return;
+    if (m_readonly) return;
     g_return_if_fail (dbi_conn_error (m_conn, nullptr) == 0);
 
     auto tables = m_provider->get_table_list (m_conn, lock_table);

libgnucash/backend/dbi/gnc-dbisqlconnection.hpp

@@ -108,6 +108,7 @@ private:
      */
     bool m_retry;
     unsigned int m_sql_savepoint;
+    bool m_readonly; 
     bool lock_database(bool break_lock);
     void unlock_database();
     bool rename_table(const std::string& old_name, const std::string& new_name);

libgnucash/backend/dbi/test/test-backend-dbi-basic.cpp

@@ -541,7 +541,7 @@ test_dbi_version_control (Fixture* fixture, gconstpointer pData)
     qof_session_end (sess);
     qof_session_destroy (sess);
     sess = qof_session_new (qof_book_new());
-    qof_session_begin (sess, url, SESSION_NORMAL_OPEN);
+    qof_session_begin (sess, url, SESSION_READ_ONLY);
     qof_session_load (sess, NULL);
     err = qof_session_pop_error (sess);
     g_assert_cmpint (err, == , ERR_SQL_DB_TOO_OLD);
@@ -554,7 +554,7 @@ test_dbi_version_control (Fixture* fixture, gconstpointer pData)
     qof_session_end (sess);
     qof_session_destroy (sess);
     sess = qof_session_new (qof_book_new());
-    qof_session_begin (sess, url, SESSION_NORMAL_OPEN);
+    qof_session_begin (sess, url, SESSION_READ_ONLY);
     qof_session_load (sess, NULL);
     qof_session_ensure_all_data_loaded (sess);
     err = qof_session_pop_error (sess);