With a bit of tweaking in the JavaScript plugin to support opaque types, this
will allow the delay functionality to work there, too.
Making the LogDelayToken an actual opaque seems reasonable, too. It's not
supposed to be user inspected.
This is a verbose, opinionated and fairly restrictive version of the log delay idea.
Main drivers are explicitly, foot-gun-avoidance and implementation simplicity.
Calling the new Log::delay() function is only allowed within the execution
of a Log::log_stream_policy() hook for the currently active log write.
Conceptually, the delay is placed between the execution of the global stream
policy hook and the individual filter policy hooks. A post delay callback
can be registered with every Log::delay() invocation. Post delay callbacks
can (1) modify a log record as they see fit, (2) veto the forwarding of the
log record to the log filters and (3) extend the delay duration by calling
Log::delay() again. The last point allows to delay a record by an indefinite
amount of time, rather than a fixed maximum amount. This should be rare and
is therefore explicit.
Log::delay() increases an internal reference count and returns an opaque
token value to be passed to Log::delay_finish() to release a delay reference.
Once all references are released, the record is forwarded to all filters
attached to a stream when the delay completes.
This functionality separates Log::log_stream_policy() and individual filter
policy hooks. One consequence is that a common use-case of filter policy hooks,
removing unproductive log records, may run after a record was delayed. Users
can lift their filtering logic to the stream level (or replicate the condition
before the delay decision). The main motivation here is that deciding on a
stream-level delay in per-filter hooks is too late. Attaching multiple filters
to a stream can additionally result in hard to understand behavior.
On the flip side, filter policy hooks are guaranteed to run after the delay
and can be used for further mangling or filtering of a delayed record.
