For GDAL sake extract a dbname from the connection

Works around GDAL limitation reported in:
https://trac.osgeo.org/gdal/ticket/6910

Ref #16625
Closes #16626

Author: strk

python/plugins/db_manager/db_plugins/postgis/plugin.py

@@ -303,8 +303,6 @@ def info(self):
     def gdalUri(self, uri=None):
         if not uri:
             uri = self.database().uri()
-        # NOTE: service-only URIs won't work with GDAL up to 2.2.x
-        # See: https://issues.qgis.org/issues/16626
         service = (u'service=%s' % uri.service()) if uri.service() else ''
         schema = (u'schema=%s' % self.schemaName()) if self.schemaName() else ''
         dbname = (u'dbname=%s' % uri.database()) if uri.database() else ''
@@ -313,6 +311,15 @@ def gdalUri(self, uri=None):
         passw = (u'password=%s' % uri.password()) if uri.password() else ''
         port = (u'port=%s' % uri.port()) if uri.port() else ''
 
+        if not dbname:
+            # GDAL postgisraster driver *requires* ad dbname
+            # See: https://trac.osgeo.org/gdal/ticket/6910
+            # TODO: cache this ?
+            connector = self.database().connector
+            r = connector._execute(None, "SELECT current_database()")
+            dbname = (u'dbname=%s' % connector._fetchone(r)[0])
+            connector._close_cursor(r)
+
         # Find first raster field
         col = ''
         for fld in self.fields():