Civix fails to create session when trying to add API function

I'm having trouble adding an API function to a module. It's failing, and I'm not clear on why.

Errors as follows:

<pre>
root@sandbox-006:/var/local/www/xxxxxx/htdocs/sites/all/civicrm/extensions/xxxxxx.mailingstats# civix generate:api MailingStatsReport Update
Write /var/local/www/xxxxxx/htdocs/sites/all/civicrm/extensions/xxxxxx.mailingstats/api/v3/MailingStatsReport/Update.php

[ErrorException]
Warning: ini_set(): A session is active. You cannot change the session module's ini settings at this time in /home/xxxxxx/civix/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NativeFileSessionHandler.php line 56

generate:api [--schedule[="..."]] <EntityName> <ActionName>
</pre>

I'm running:
Debian 6.0 (squeeze).

• Civicrm 4.2.9 (with some backports from 4.2.10),
• PHP 5.3.3-7+squeeze17 with Suhosin-Patch v0.9.32.1
• civix/composer.lock referes to Symfony 2.1.7.

Looking at strace output, I can see where it loads civix/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NativeSessionHandler.php and just afterwards runs stat64 on civix/app/cache/dev/sessions, which has session files in it from earlier use of civix, but no new ones. I don't see any system call that tries to create a session file, or otherwise access the directory after stat'ing it.

I'm not all that clear on how some of the fields for this issue. Please correct as appropriate