Make the port script drop NUL values in all tables

Postgres doesn't support NULs in strings so it makes the script
throw an exception and stop if any values contain \0. Drop them
with appropriate warning.

scripts/synapse_port_db

@@ -320,7 +320,7 @@ class Porter(object):
                     backward_chunk = min(row[0] for row in brows) - 1
 
                 rows = frows + brows
-                self._convert_rows(table, headers, rows)
+                rows = self._convert_rows(table, headers, rows)
 
                 def insert(txn):
                     self.postgres_store.insert_many_txn(
@@ -556,17 +556,29 @@ class Porter(object):
             i for i, h in enumerate(headers) if h in bool_col_names
         ]
 
+        class BadValueException(Exception):
+            pass
+
         def conv(j, col):
             if j in bool_cols:
                 return bool(col)
+            elif isinstance(col, basestring) and "\0" in col:
+                logger.warn("DROPPING ROW: NUL value in table %s col %s: %r", table, headers[j], col)
+                raise BadValueException();
             return col
 
+        outrows = []
         for i, row in enumerate(rows):
-            rows[i] = tuple(
+            try:
+                outrows.append(tuple(
                     conv(j, col)
                     for j, col in enumerate(row)
                     if j > 0
-            )
+                ))
+            except BadValueException:
+                pass
+
+        return outrows
 
     @defer.inlineCallbacks
     def _setup_sent_transactions(self):
@@ -594,7 +606,7 @@ class Porter(object):
             "select", r,
         )
 
-        self._convert_rows("sent_transactions", headers, rows)
+        rows = self._convert_rows("sent_transactions", headers, rows)
 
         inserted_rows = len(rows)
         if inserted_rows: