introduce tokenizer modules

This adds the boilerplate for selecting configurable tokenizers.
A tokenizer can be chosen at import time and will then install
itself such that it is fixed for the given database import even
when the software itself is updated.

The legacy tokenizer implements Nominatim's traditional algorithms.

nominatim/tokenizer/factory.py

@@ -0,0 +1,84 @@
+"""
+Functions for creating a tokenizer or initialising the right one for an
+existing database.
+
+A tokenizer is something that is bound to the lifetime of a database. It
+can be choosen and configured before the intial import but then needs to
+be used consistently when querying and updating the database.
+
+This module provides the functions to create and configure a new tokenizer
+as well as instanciating the appropriate tokenizer for updating an existing
+database.
+
+A tokenizer usually also includes PHP code for querying. The appropriate PHP
+normalizer module is installed, when the tokenizer is created.
+"""
+import logging
+import importlib
+
+from ..errors import UsageError
+from ..db import properties
+from ..db.connection import connect
+
+LOG = logging.getLogger()
+
+def _import_tokenizer(name):
+    """ Load the tokenizer.py module from project directory.
+    """
+    try:
+        return importlib.import_module('nominatim.tokenizer.' + name + '_tokenizer')
+    except ModuleNotFoundError as exp:
+        LOG.fatal("No tokenizer named '%s' available. "
+                  "Check the setting of NOMINATIM_TOKENIZER.", name)
+        raise UsageError('Tokenizer not found') from exp
+
+
+def create_tokenizer(config):
+    """ Create a new tokenizer as defined by the given configuration.
+
+        The tokenizer data and code is copied into the 'tokenizer' directory
+        of the project directory and the tokenizer loaded from its new location.
+    """
+    # Create the directory for the tokenizer data
+    basedir = config.project_dir / 'tokenizer'
+    if not basedir.exists():
+        basedir.mkdir()
+    elif not basedir.is_dir():
+        LOG.fatal("Tokenizer directory '%s' cannot be created.", basedir)
+        raise UsageError("Tokenizer setup failed.")
+
+    tokenizer_module = _import_tokenizer(config.TOKENIZER)
+
+    tokenizer = tokenizer_module.create(config.get_libpq_dsn(), basedir)
+    tokenizer.init_new_db(config)
+
+    with connect(config.get_libpq_dsn()) as conn:
+        properties.set_property(conn, 'tokenizer', config.TOKENIZER)
+
+    return tokenizer
+
+
+def get_tokenizer_for_db(config):
+    """ Instantiate a tokenizer for an existing database.
+
+        The function looks up the appropriate tokenizer in the database
+        and initialises it.
+    """
+    basedir = config.project_dir / 'tokenizer'
+    if not basedir.is_dir():
+        LOG.fatal("Cannot find tokenizer data in '%s'.", basedir)
+        raise UsageError('Cannot initialize tokenizer.')
+
+    with connect(config.get_libpq_dsn()) as conn:
+        name = properties.get_property(conn, 'tokenizer')
+
+    if name is None:
+        LOG.fatal("Tokenizer was not set up properly. Database property missing.")
+        raise UsageError('Cannot initialize tokenizer.')
+
+    tokenizer_module = _import_tokenizer(name)
+
+    tokenizer = tokenizer_module.create(config.get_libpq_dsn(), basedir)
+    tokenizer.init_from_project()
+
+    return tokenizer