If a session already exists, and Session::start() isn’t called until
after a large enough block of content is output, then headers_sent()
will be false. The previous code prevented the session from being
started in this case. That might makes sense for the creation of a new
session, but it prevent legitimate access to an existing session.
This mostly manifested when running debugging tools such as showqueries,
which may output content before the session is started.
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