Fixed #19152 -- Allowed PostGIS tests to be run without template

From version 2, a PostGIS template is no longer required to create
a GIS-enabled database.

django/contrib/gis/db/backends/postgis/creation.py

@@ -1,12 +1,23 @@
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
 from django.db.backends.postgresql_psycopg2.creation import DatabaseCreation
+from django.utils.functional import cached_property
+
 
 class PostGISCreation(DatabaseCreation):
     geom_index_type = 'GIST'
     geom_index_ops = 'GIST_GEOMETRY_OPS'
     geom_index_ops_nd = 'GIST_GEOMETRY_OPS_ND'
 
+    @cached_property
+    def template_postgis(self):
+        template_postgis = getattr(settings, 'POSTGIS_TEMPLATE', 'template_postgis')
+        cursor = self.connection.cursor()
+        cursor.execute('SELECT 1 FROM pg_database WHERE datname = %s LIMIT 1;', (template_postgis,))
+        if cursor.fetchone():
+            return template_postgis
+        return None
+
     def sql_indexes_for_field(self, model, f, style):
         "Return any spatial index creation SQL for the field."
         from django.contrib.gis.db.models.fields import GeometryField
@@ -67,5 +78,19 @@ class PostGISCreation(DatabaseCreation):
         return output
 
     def sql_table_creation_suffix(self):
-        postgis_template = getattr(settings, 'POSTGIS_TEMPLATE', 'template_postgis')
-        return ' TEMPLATE %s' % self.connection.ops.quote_name(postgis_template)
+        if self.template_postgis is not None:
+            return ' TEMPLATE %s' % (
+                self.connection.ops.quote_name(self.template_postgis),)
+        return ''
+
+    def _create_test_db(self, verbosity, autoclobber):
+        test_database_name = super(PostGISCreation, self)._create_test_db(verbosity, autoclobber)
+        if self.template_postgis is None:
+            # Connect to the test database in order to create the postgis extension
+            self.connection.close()
+            self.connection.settings_dict["NAME"] = test_database_name
+            cursor = self.connection.cursor()
+            cursor.execute("CREATE EXTENSION postgis")
+            cursor.commit()
+
+        return test_database_name