introduce jinja2 for preprocessing SQL

Replaces various hand-crafted replacements of varying format with
a single Jinja2 templating mechanism. Allows full access to
configuration if necessary.

test/python/test_db_sql_preprocessor.py

@@ -0,0 +1,51 @@
+"""
+Tests for SQL preprocessing.
+"""
+from pathlib import Path
+
+import pytest
+
+from nominatim.db.sql_preprocessor import SQLPreprocessor
+
+@pytest.fixture
+def sql_factory(tmp_path):
+    def _mk_sql(sql_body):
+        (tmp_path / 'test.sql').write_text("""
+          CREATE OR REPLACE FUNCTION test() RETURNS TEXT
+          AS $$
+          BEGIN
+            {}
+          END;
+          $$ LANGUAGE plpgsql IMMUTABLE;""".format(sql_body))
+        return 'test.sql'
+
+    return _mk_sql
+
+
+@pytest.fixture
+def sql_preprocessor(temp_db_conn, tmp_path, def_config, monkeypatch, table_factory):
+    monkeypatch.setenv('NOMINATIM_DATABASE_MODULE_PATH', '.')
+    table_factory('country_name', 'partition INT', (0, 1, 2))
+    return SQLPreprocessor(temp_db_conn, def_config, tmp_path)
+
+@pytest.mark.parametrize("expr,ret", [
+    ("'a'", 'a'),
+    ("'{{db.partitions|join}}'", '012'),
+    ("{% if 'country_name' in db.tables %}'yes'{% else %}'no'{% endif %}", "yes"),
+    ("{% if 'xxx' in db.tables %}'yes'{% else %}'no'{% endif %}", "no"),
+    ("'{{config.DATABASE_MODULE_PATH}}'", '.')
+    ])
+def test_load_file_simple(sql_preprocessor, sql_factory, temp_db_conn, temp_db_cursor, expr, ret):
+    sqlfile = sql_factory("RETURN {};".format(expr))
+
+    sql_preprocessor.run_sql_file(temp_db_conn, sqlfile)
+
+    assert temp_db_cursor.scalar('SELECT test()') == ret
+
+
+def test_load_file_with_params(sql_preprocessor, sql_factory, temp_db_conn, temp_db_cursor):
+    sqlfile = sql_factory("RETURN '{{ foo }} {{ bar }}';")
+
+    sql_preprocessor.run_sql_file(temp_db_conn, sqlfile, bar='XX', foo='ZZ')
+
+    assert temp_db_cursor.scalar('SELECT test()') == 'ZZ XX'