PG unrestricted geometry: trust SRID from geometry_columns

Fixes qgis#38567 where the SRID was not detected if the table was empty.

Also: do not override the SRID passed explicitly in the data source
URI.

Note: the logic in the provider is becoming a bit convoluted, there
are too many corner cases, at least the test coverage is pretty
good.

src/providers/postgres/qgspostgresprovider.cpp

@@ -3874,6 +3874,12 @@ bool QgsPostgresProvider::getGeometryDetails()
     {
       detectedType = result.PQgetvalue( 0, 0 );
 
+      // Do not override the SRID if set in the data source URI
+      if ( detectedSrid.isEmpty() )
+      {
+        detectedSrid = result.PQgetvalue( 0, 1 );
+      }
+
       QString dim = result.PQgetvalue( 0, 2 );
       if ( dim == QLatin1String( "3" ) && !detectedType.endsWith( 'M' ) )
         detectedType += QLatin1String( "Z" );
@@ -4047,7 +4053,6 @@ bool QgsPostgresProvider::getGeometryDetails()
 
   if ( mDetectedGeomType == QgsWkbTypes::Unknown )
   {
-    mDetectedSrid.clear();
 
     QgsPostgresLayerProperty layerProperty;
     if ( !mIsQuery )
@@ -4077,8 +4082,8 @@ bool QgsPostgresProvider::getGeometryDetails()
 
     if ( layerProperty.size() == 0 )
     {
-      // no data - so take what's requested
-      if ( mRequestedGeomType == QgsWkbTypes::Unknown || mRequestedSrid.isEmpty() )
+      // no data - so take what's requested/detected
+      if ( mRequestedGeomType == QgsWkbTypes::Unknown || mDetectedSrid.isEmpty() )
       {
         QgsMessageLog::logMessage( tr( "Geometry type and srid for empty column %1 of %2 undefined." ).arg( mGeometryColumn, mQuery ) );
       }

tests/src/python/test_provider_postgres.py

@@ -2972,6 +2972,18 @@ def testUnrestrictedGeometryType(self):
         with self.assertRaises(StopIteration):
             next(polygons.getFeatures())
 
+        # Test regression GH #38567 (no SRID requested in the data source URI)
+        # Cleanup if needed
+        conn.executeSql('DELETE FROM "qgis_test"."test_unrestricted_geometry" WHERE \'t\'')
+
+        points = QgsVectorLayer(self.dbconn + ' sslmode=disable key=\'gid\' type=POINT table="qgis_test"."test_unrestricted_geometry" (geom) sql=', 'test_points', 'postgres')
+        lines = QgsVectorLayer(self.dbconn + ' sslmode=disable key=\'gid\' type=LINESTRING table="qgis_test"."test_unrestricted_geometry" (geom) sql=', 'test_lines', 'postgres')
+        polygons = QgsVectorLayer(self.dbconn + ' sslmode=disable key=\'gid\' type=POLYGON table="qgis_test"."test_unrestricted_geometry" (geom) sql=', 'test_polygons', 'postgres')
+
+        self.assertTrue(points.isValid())
+        self.assertTrue(lines.isValid())
+        self.assertTrue(polygons.isValid())
+
 
 if __name__ == '__main__':
     unittest.main()