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.

nominatim/db/sql_preprocessor.py

@@ -0,0 +1,62 @@
+"""
+Preprocessing of SQL files.
+"""
+import jinja2
+
+
+def _get_partitions(conn):
+    """ Get the set of partitions currently in use.
+    """
+    with conn.cursor() as cur:
+        cur.execute('SELECT DISTINCT partition FROM country_name')
+        partitions = set([0])
+        for row in cur:
+            partitions.add(row[0])
+
+    return partitions
+
+
+def _get_tables(conn):
+    """ Return the set of tables currently in use.
+        Only includes non-partitioned
+    """
+    with conn.cursor() as cur:
+        cur.execute("SELECT tablename FROM pg_tables WHERE schemaname = 'public'")
+
+        return set((row[0] for row in list(cur)))
+
+class SQLPreprocessor: # pylint: disable=too-few-public-methods
+    """ A environment for preprocessing SQL files from the
+        lib-sql directory.
+
+        The preprocessor provides a number of default filters and variables.
+        The variables may be overwritten when rendering an SQL file.
+
+        The preprocessing is currently based on the jinja2 templating library
+        and follows its syntax.
+    """
+
+    def __init__(self, conn, config, sqllib_dir):
+        self.env = jinja2.Environment(autoescape=False,
+                                      loader=jinja2.FileSystemLoader(str(sqllib_dir)))
+
+        db_info = {}
+        db_info['partitions'] = _get_partitions(conn)
+        db_info['tables'] = _get_tables(conn)
+        db_info['reverse_only'] = 'search_name' not in db_info['tables']
+
+        self.env.globals['config'] = config
+        self.env.globals['db'] = db_info
+        self.env.globals['modulepath'] = config.DATABASE_MODULE_PATH or \
+                                         str((config.project_dir / 'module').resolve())
+
+
+    def run_sql_file(self, conn, name, **kwargs):
+        """ Execute the given SQL file on the connection. The keyword arguments
+            may supply additional parameters for preprocessing.
+        """
+        sql = self.env.get_template(name).render(**kwargs)
+
+        with conn.cursor() as cur:
+            cur.execute(sql)
+        conn.commit()