I don't think you can accomplish this with pyramid_tm being used.
My initial response was going to be this:
since session_1 is readonly, don't configure it to join the
pyramid_tm/zope transaction. Then just rollback session_1 when session_2
commits.
but session_2 is going to commit as part of the pyramid_tm tween - so you
don't have control over that.
you could have a tween running over pyramid_tm that applies a rollback onto
session_1, but the tweens happen after rendering in the pyramid
lifecycle... so that's not going to work either.
visually, here's the pyramid request lifecycle
- https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/router.html
the commit on session_2 is happening in the tween_egress, but the rendering
is happening in the view_pipeline above it.
i think the only way you can accomplish easily is without using the
transaction manger.
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