Set statement_timeout for DB connections

The patch sets the statement_timeout parameter for the ocitysmap
sessions to 18mn. This should be enough. I checked that an exception
is raised when the timeout appears (=> ocitysmap crashes
immediatly). The other sessions (eg. osm2pgsql) should not be
affected. Issue "show statement_timeout" on a new DB connection to
make sure (should reply '0').

This is a "quick and dirty fix": the timeout should be configurable
from a config file or from the cmdline. For now it's hard-coded in
street_index.py.

Patch by David Decotigny <d2@maposmatic.org>

ocitysmap/street_index.py

@@ -32,6 +32,7 @@ import map_canvas, grid, utils
 from draw_utils import enclose_in_frame
 
 LOG = logging.getLogger('ocitysmap')
+STATEMENT_TIMEOUT_MINUTES = 18
 
 class BaseOCitySMapError(Exception):
     """Base class for exceptions thrown by OCitySMap."""
@@ -294,6 +295,16 @@ class OCitySMap:
                           datasource['password'], datasource['host'],
                           datasource['dbname'])
 
+        # Set session timeout parameter (18mn)
+        cursor = db.cursor()
+        cursor.execute("show statement_timeout;")
+        LOG.debug("Initial statement timeout: %s" % cursor.fetchall()[0][0])
+        cursor.execute("set session statement_timeout=%d;"
+                       % (STATEMENT_TIMEOUT_MINUTES*60*1000))
+        cursor.execute("show statement_timeout;")
+        LOG.info("Configured statement timeout: %s" % cursor.fetchall()[0][0])
+        del cursor
+
         if self.city_name:
             self.boundingbox = self.find_bounding_box_by_name(db, self.city_name)
         elif self.osmid: