Expose QgsProject::setInstance to API

Fixes #38755

Needs forward porting to all active branches

python/core/auto_generated/qgsproject.sip.in

@@ -69,6 +69,20 @@ open within the main QGIS application.
 Returns the QgsProject singleton instance
 %End
 
+    static void setInstance( QgsProject *project );
+%Docstring
+Set the current project instance to ``project``
+
+.. note::
+
+   this is used mainly by the server, which caches the projects and (potentially) needs to switch the current instance on every request
+
+.. seealso:: :py:func:`instance`
+
+.. versionadded:: 3.10
+%End
+
+
     explicit QgsProject( QObject *parent /TransferThis/ = 0 );
 %Docstring
 Create a new QgsProject.

src/core/qgsproject.h

@@ -165,6 +165,16 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
     //! Returns the QgsProject singleton instance
     static QgsProject *instance();
 
+    /**
+     * Set the current project instance to \a project
+     *
+     * \note this is used mainly by the server, which caches the projects and (potentially) needs to switch the current instance on every request
+     * \see instance()
+     * \since QGIS 3.10.11
+     */
+    static void setInstance( QgsProject *project ) ;
+
+
     /**
      * Create a new QgsProject.
      *
@@ -1837,15 +1847,6 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
 
     static QgsProject *sProject;
 
-    /**
-     * Set the current project instance to \a project
-     *
-     * \note this is used mainly by the server, which caches the projects and (potentially) needs to switch the current instance on every request
-     * \see instance()
-     * \note not available in Python bindings
-     * \since QGIS 3.2
-     */
-    static void setInstance( QgsProject *project ) SIP_SKIP;
 
     /**
      * Read map layers from project file.

tests/src/python/test_qgsproject.py

@@ -1360,6 +1360,14 @@ class TestQgsProject(unittest.TestCase):
         p.setUseProjectScales(False)
         self.assertEqual(len(spy), 4)
 
+    def testSetInstance(self):
+        """Test singleton API"""
+
+        p = QgsProject()
+        self.assertNotEqual(p, QgsProject.instance())
+        QgsProject.setInstance(p)
+        self.assertEqual(p, QgsProject.instance())
+
 
 if __name__ == '__main__':
     unittest.main()