Bug 797906 - GnuCash data file is locked after a "save-as"...

to mysql database.

It's more general, any save-as to a different backend type, i.e.
xml->sql or sql->xml, left the previous book locked.

Ensure that the session is ended and the book unlocked in the
backend's destructor. This also fixes a memory leak in the SQL
backend because the GncSqlConnection wasn't being deleted.

libgnucash/backend/sql/gnc-sql-backend.cpp

@@ -88,6 +88,11 @@ GncSqlBackend::GncSqlBackend(GncSqlConnection *conn, QofBook* book) :
         connect (conn);
 }
 
+GncSqlBackend::~GncSqlBackend()
+{
+    connect(nullptr);
+}
+
 void
 GncSqlBackend::connect(GncSqlConnection *conn) noexcept
 {

libgnucash/backend/sql/gnc-sql-backend.hpp

@@ -66,7 +66,7 @@ class GncSqlBackend : public QofBackend
 {
 public:
     GncSqlBackend(GncSqlConnection *conn, QofBook* book);
-    virtual ~GncSqlBackend() = default;
+    virtual ~GncSqlBackend();
     /**
      * Load the contents of an SQL database into a book.
      *

libgnucash/backend/sql/test/utest-gnc-backend-sql.cpp

@@ -275,7 +275,7 @@ test_gnc_sql_commit_edit (void)
 {
     QofInstance* inst;
     guint dirty_called = 0;
-    GncMockSqlConnection conn;
+    auto conn{new GncMockSqlConnection};
     const char* msg1 =
         "[GncSqlBackend::commit()] Unknown object type 'null'\n";
     GLogLevelFlags loglevel = static_cast<decltype (loglevel)>
@@ -293,8 +293,7 @@ test_gnc_sql_commit_edit (void)
 
     qof_object_initialize ();
     auto book = qof_book_new();
-    auto sql_be = new GncMockSqlBackend
-        (&conn, book);
+    auto sql_be = new GncMockSqlBackend{conn, book};
     inst  = static_cast<decltype (inst)> (g_object_new (QOF_TYPE_INSTANCE, NULL));
     qof_instance_init_data (inst, QOF_ID_NULL, book);
     qof_book_set_dirty_cb (book, test_dirty_cb, &dirty_called);

libgnucash/backend/xml/gnc-xml-backend.cpp

@@ -49,6 +49,11 @@ extern "C"
 #define FILE_URI_PREFIX "file://"
 static QofLogModule log_module = GNC_MOD_BACKEND;
 
+GncXmlBackend::~GncXmlBackend()
+{
+    session_end();
+};
+
 bool
 GncXmlBackend::check_path (const char* fullpath, bool create)
 {

libgnucash/backend/xml/gnc-xml-backend.hpp

@@ -34,7 +34,7 @@ public:
     GncXmlBackend operator=(const GncXmlBackend&) = delete;
     GncXmlBackend(const GncXmlBackend&&) = delete;
     GncXmlBackend operator=(const GncXmlBackend&&) = delete;
-    ~GncXmlBackend() = default;
+    ~GncXmlBackend();
     void session_begin(QofSession* session, const char* new_uri,
                        SessionOpenMode mode) override;
     void session_end() override;