Add test for nested connection pool acquire freeze

tests/src/python/test_provider_postgres.py

@@ -44,7 +44,9 @@
     QgsCoordinateReferenceSystem,
     QgsProject,
     QgsWkbTypes,
-    QgsGeometry
+    QgsGeometry,
+    QgsExpression,
+    QgsExpressionContext
 )
 from qgis.gui import QgsGui, QgsAttributeForm
 from qgis.PyQt.QtCore import QDate, QTime, QDateTime, QVariant, QDir, QObject
@@ -1141,6 +1143,27 @@ def testCurveToMultipolygon(self):
         self.assertEqual(g.childCount(), 1)
         self.assertTrue(g.childGeometry(0).vertexCount() > 3)
 
+    def testConcurrency(self):
+        """
+        The connection pool has a maximum of 4 connections defined (+2 spare connections)
+        Make sure that if we exhaust those 4 connections and force another connection
+        it is actually using the spare connections and does not freeze.
+        This situation normally happens when (at least) 4 rendering threads are active
+        in parallel and one requires an expression to be evaluated.
+        """
+        vl = QgsVectorLayer('{conn} srid=4326 table="qgis_test".{table} (geom) sql='.format(conn=self.dbconn, table='someData'), "testgeom", "postgres")
+        self.assertTrue(vl.isValid())
+        it = vl.getFeatures()
+        it2 = vl.getFeatures()
+        it3 = vl.getFeatures()
+        it4 = vl.getFeatures()
+        QgsProject.instance().addMapLayer(vl)
+        feat = next(it)
+        context = QgsExpressionContext()
+        context.setFeature(feat)
+        exp = QgsExpression('get_feature(\'{layer}\', \'pk\', 5)'.format(layer=vl.id()))
+        exp.evaluate(context)
+
 
 class TestPyQgsPostgresProviderCompoundKey(unittest.TestCase, ProviderTestCase):