While in principle messages are "instantaneous" assertions, it invites
confusion to treat them as such in context of `(object)` because
assertions by default spawn nested facets, so it's too easy to
accidentally wrap a message handler in an unwanted facet. Being
explicit about messages means you never get an unwanted facet wrapper.
during/spawn used not to add linkage assertions to its
initial-assertion set. In addition, if a spawned actor died in its
initial boot procedure, its initial assertions would never be visible.
These two problems interlocked to cause a space leak in during/spawn,
where monitoring facets would never be cleaned up.
This change does two things:
- adds linkage assertions to the initial-assertion set in during/spawn
- properly briefly signals initial-assertions even when a new actor
immediately crashes.
Together, these repair the space leak in during/spawn with a crashy
child startup procedure.
The `add-endpoint!` call is changed in two ways:
- the old `assertion-fn` has become `update-fn`, yielding both
an assertion *and* an optional handler, because if the handler
depends on a field which changes, previously the handler wasn't
being updated
- a new parameter, `dynamic?`, can be set to #f (it's usually #t)
to ensure that the assertion and skeleton-interest are calculated
only once ever, and are not connected to the dataflow machinery.
The first change makes it possible for the `(later-than (deadline))`
pattern, where `deadline` is a field, to work; the second change makes
`during` and `during/spawn` work correctly in the face of field
updates.