Better handling of lack of powa extension on remote servers.

As reported by Thoma Reiss, if the extension isn't available on the remote
server, users get an error message that's not helpful at all.  To fix, first
do the sanity check for the extension in the catalog to possibly avoid to hit
an error when trying to load powa, and also add a specific error message if the
extension cannot be loaded.

Patch by Thomas Reiss and Julien Rouhaud.

CHANGELOG

@@ -1,6 +1,8 @@
 0.0.3
   Bugfix:
     - Support standard_conforming_strings = off
+    - Better error message for remote servers lacking powa extension (Thomas
+      Reiss and Julien Rouhaud)
 
 0.0.2
   Bugfix:

powa_collector/powa_worker.py

@@ -80,11 +80,25 @@ def __check_powa(self):
                 self.logger.error("PoWA extension not found")
                 self.__disconnect_all()
                 self.__stopping.set()
+                return
             elif (res[0] < 4):
                 self.logger.error("Incompatible PoWA version, found %s,"
                                   " requires at least 4.0.0" % res[1])
                 self.__disconnect_all()
                 self.__stopping.set()
+                return
+
+            try:
+                # make sure the GUC are present in case powa isn't in
+                # shared_preload_librairies
+                cur = self.__remote_conn.cursor()
+                cur.execute("LOAD 'powa'")
+                cur.close()
+                self.__remote_conn.commit()
+            except psycopg2.Error as e:
+                self.logger.error("Could not load extension powa:\n%s" % e)
+                self.__disconnect_all()
+                self.__stopping.set()
 
     def __reload(self):
         self.logger.info("Reloading configuration")