add database migration

Clear voters bloomfilter and initialize with an (almost) empty one.

isso/db/__init__.py

@@ -1,6 +1,9 @@
 # -*- encoding: utf-8 -*-
 
 import sqlite3
+import logging
+
+logger = logging.getLogger("isso")
 
 class IssoDBException(Exception):
     pass
@@ -11,12 +14,24 @@ from isso.db.threads import Threads
 
 class SQLite3:
 
+    MAX_VERSION = 1
+
     def __init__(self, path, conf):
 
         self.path = path
         self.conf = conf
         self.mode = 1
 
+        rv = self.execute([
+            "SELECT name FROM sqlite_master"
+            "   WHERE type='table' AND name IN ('threads', 'comments')"]
+        ).fetchall()
+
+        if rv:
+            self.migrate(to=SQLite3.MAX_VERSION)
+        else:
+            self.execute("PRAGMA user_version = %i" % SQLite3.MAX_VERSION)
+
         self.threads = Threads(self)
         self.comments = Comments(self)
 
@@ -34,3 +49,24 @@ class SQLite3:
 
         with sqlite3.connect(self.path) as con:
             return con.execute(sql, args)
+
+    @property
+    def version(self):
+        return self.execute("PRAGMA user_version").fetchone()[0]
+
+    def migrate(self, to):
+
+        if self.version >= to:
+            return
+
+        logger.info("migrate database from version %i to %i", self.version, to)
+
+        if self.version == 0:
+
+            from isso.utils import Bloomfilter
+            bf = buffer(Bloomfilter(iterable=["127.0.0.0"]).array)
+
+            with sqlite3.connect(self.path) as con:
+                con.execute('UPDATE comments SET voters=?', (bf, ))
+                con.execute('PRAGMA user_version = 1')
+                logger.info("%i rows changed", con.total_changes)