This reverts commit 9e7fa9b859.
CI had some unexpected results with this new version overnight. As a
result and until I investigate this, let go back to 3.1.0.
A subcustodian was incorrectly registered as weak for its parent,
which means that an unreferenced custodian could get lost when
shutting down an ancestor.
Register the port, not the file descriptor, especially since a TCP
connection can have ports that share a file descriptor. Also, I think
a weak reference in the custodian doesn't work as intended (visible
through finalization) if the file descriptor is referenced with a
callback that closes over the port.
No `syntax-protect` is needed for `define/private`, etc., because no
new identifiers or expressions are introduced. Adding extra dye packs
can interfere with other macros that pull apart syntax (although maybe
the macros shouldn't do that without using `syntax-disarm`).
A custodian doesn't provide any order on shutting down the objects
that it manages (I was confused about some past experiments), so
avoid that assumption.
Getting the current CPU time is relatively expensive, so get it only
on thread swaps where a thread used its full quantum or 1/100 swaps
otherwise. This approximation should work because thread-specific CPU
times are rarely requested, and they make the most sense for threads
that don't constantly swap out due to synchronization.
Formerly, an expression like `(arity-at-least-value 7)` could crash,
because the `arity-at-least-value` accessor is created in unsafe mode,
and the slow path to accessor errors attempted to use the accessor to
provoke an error message. Instead of using a potentially unsafe
accessor, have the slow path raise an error explicitly with
`raise-argument-arror`. That change has the added benefit of making
error messages mach traditional Racket (at least for structure types
that are not declared as "authentic").
The problem was exposed by tests added in 55dcdf5538.
Instead of a separate hash table mapping continuations to
linklet-instance names, use a continuation mark. That's faster,
because capturing a continuation means copying part of it on continue.
