Use TRANSFER_BACK annotation instead of FACTORY for QgsProviderRegist…

…ry::createProvider

Paraphrasing Phil Thomson's advice on https://www.riverbankcomputing.com/pipermail/pyqt/2017-July/039450.html:

"
/Factory/ is used when the instance returned is guaranteed to be new to Python.
In this case it isn't because it has already been seen when being returned by by the python function
creating the provider subclass.

(However for a different sub-class implemented in C++ then it would be the first time it was seen
by Python so the /Factory/ on create() would be correct.)

You might try using /TransferBack/ on createProvider() instead - that might be the best compromise.

python/core/auto_generated/qgsproviderregistry.sip.in

@@ -77,10 +77,11 @@ Returns the library directory where plugins are found.
 Sets library directory where to search for plugins
 %End
 
+
     QgsDataProvider *createProvider( const QString &providerKey,
                                      const QString &dataSource,
                                      const QgsDataProvider::ProviderOptions &options = QgsDataProvider::ProviderOptions(),
-                                     QgsDataProvider::ReadFlags flags = QgsDataProvider::ReadFlags() ) /Factory/;
+                                     QgsDataProvider::ReadFlags flags = QgsDataProvider::ReadFlags() ) /TransferBack/;
 %Docstring
 Creates a new instance of a provider.
 

src/core/qgsproviderregistry.h

@@ -111,6 +111,23 @@ class CORE_EXPORT QgsProviderRegistry
     //! Sets library directory where to search for plugins
     void setLibraryDirectory( const QDir &path );
 
+    /*
+     * IMPORTANT: While it seems like /Factory/ would be the correct annotation here, that's not
+     * the case.
+     * Paraphrasing Phil Thomson's advice on https://www.riverbankcomputing.com/pipermail/pyqt/2017-July/039450.html:
+     *
+     * "
+     * /Factory/ is used when the instance returned is guaranteed to be new to Python.
+     * In this case it isn't because it has already been seen when being returned by by the python function
+     * creating the provider subclass.
+     *
+     * (However for a different sub-class implemented in C++ then it would be the first time it was seen
+     * by Python so the /Factory/ on create() would be correct.)
+     *
+     * You might try using /TransferBack/ on createProvider() instead - that might be the best compromise.
+     * "
+     */
+
     /**
      * Creates a new instance of a provider.
      * \param providerKey identifier of the provider
@@ -124,7 +141,7 @@ class CORE_EXPORT QgsProviderRegistry
     QgsDataProvider *createProvider( const QString &providerKey,
                                      const QString &dataSource,
                                      const QgsDataProvider::ProviderOptions &options = QgsDataProvider::ProviderOptions(),
-                                     QgsDataProvider::ReadFlags flags = QgsDataProvider::ReadFlags() ) SIP_FACTORY;
+                                     QgsDataProvider::ReadFlags flags = QgsDataProvider::ReadFlags() ) SIP_TRANSFERBACK;
 
     /**
      * Returns the provider capabilities