Bug 797983 - v4.2 report numbers change over gnucash restarts;...

Price Database dropping user:price-editor entries.

A wider problem: QofInstance was unmarking itself dirty as long as the
backend raised an error and unconditionally marking itself non-infant.

This matters because the SQL backend depends on infant status to decide
whether to do an insert or update query; either will fail if the infant
status is wrong.

The price editor dialog clones a price having only its commodity set and
GncSqlPriceBackend declines to save if the currency isn't set. Since the
instance marked itself non-infant even though it wasn't saved subsequent
commits tried to use an update query and since the price wasn't in the
table that inevitably failed.

Requiring that QofBackend::commit should doing the marking-clean
required implementing it in the backends where it wasn't already.

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

@@ -314,6 +314,13 @@ GncXmlBackend::sync(QofBook* book)
     remove_old_files();
 }
 
+void
+GncXmlBackend::commit(QofInstance* instance)
+{
+    if (qof_instance_is_dirty(instance))
+        qof_instance_mark_clean(instance);
+}
+
 bool
 GncXmlBackend::save_may_clobber_data()
 {

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

@@ -43,6 +43,7 @@ public:
     void export_coa(QofBook*) override;
     void sync(QofBook* book) override;
     void safe_sync(QofBook* book) override { sync(book); } // XML sync is inherently safe.
+    void commit(QofInstance* instance) override;
     const char * get_filename() { return m_fullpath.c_str(); }
     QofBook* get_book() { return m_book; }