History tables: no history tables for table names longer than 63 chars

This makes sure errors due to Postgres clipping identifier names after
63 characters are found early. This shouldn't matter for us in practice,
but ideally we wouldn't include the target table name in generated
names (e.g. the update function, the history table and the time table).

django/applications/catmaid/migrations/0006_add_history_table_support.py

@@ -225,6 +225,14 @@
         -- History tables will be named like the live table plus a '_history' suffix
         history_table_name = history_table_name(live_table_name);
 
+        -- Make sure the history table name is not longer than 63 characters, a
+        -- limit that Postgres defaults to and which causes identifier names to
+        -- become shortened silently.
+        IF (LENGTH(get_history_update_fn_name_regular(live_table_name)) > 63) THEN
+            RAISE EXCEPTION 'Can''t create history table with name longer than '
+                '63 characters: %', history_table_name;
+        END IF;
+
         -- Don't do anything if there is already a history table registered with this name.
         IF EXISTS(SELECT 1 FROM catmaid_history_table cht
                   WHERE cht.history_table_name = outerblock.history_table_name) THEN

django/applications/catmaid/tests/test_history_tables.py

@@ -1,6 +1,6 @@
 import time
 
-from django.db import connection, transaction
+from django.db import connection, transaction, InternalError
 from django.test import TestCase, TransactionTestCase
 from guardian.shortcuts import assign_perm
 from catmaid import history
@@ -28,6 +28,22 @@ def run(self, *args):
         history.enable_history_tracking()
         return super(HistoryTableTests, self).run(*args)
 
+    def test_name_length_limits(self):
+        """ Make sure an exception is raised if the history table name for a live
+        table exceeds identifier limits imposed by Postgres.
+        """
+        cursor = connection.cursor()
+        with self.assertRaises(InternalError):
+            cursor.execute("""
+                CREATE TABLE
+                    a_very_very_long_table_name_which_is_pretty_close_to_63_chars (
+                        test    text
+                    );
+                SELECT create_history_table(
+                    'a_very_very_long_table_name_which_is_pretty_close_to_63_chars'::regclass)
+            """)
+
+
     def test_history_table_existence(self):
         """Test if all catmaid tables have a history table"""
         expected_tables_with_history = (