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.
If no body is defined, the email is rendered according to a template. Clearing requirements prevent unnecessary styles/scripts to be included in the html (and that needs to be processed/stripped down the line).
Fixes regression from 3.x, where sessions where lazy started as required:
Either because an existing session identifier was sent through with the request,
or because new session data needed to be persisted as part of the request execution.
Without this lazy starting, *every* request will get a session,
which makes all those responses uncacheable by HTTP layers.
Note that 4.x also changed the $data vs. $changedData payloads:
In 3.x, they both contained key/value pairs.
In 4.x, $data contains key/value, while $changedData contains key/boolean to declare isChanged.
While this reduces duplication in the class, it also surfaced a bug which was latent in 3.x:
When an existing session is lazily resumed via start(), $data is set back to an empty array.
In 3.x, any changed data before this point was *also* retained in $changedData,
ensuring it gets merged into existing $_SESSION data.
In 4.x, this clears out data - hence the need for a more complex merge logic.
Since isset($this->data) is no longer an accurate indicator of a started session,
we introduce a separate $this->started flag.
Note that I've chosen not to make lazy an opt-in (e.g. via start($request, $lazy=false)).
We already have a distinction between lazy starting via init(), and force starting via start().
