It's a little distressing how often we have historically coded S32 or U32 to
pass a length or index.
There are more such assumptions in other viewer subdirectories, but this is a
start.
A typical WorkQueue has a string name, which can be used to find it to post
work to it. "Work" is a nullary callable.
WorkQueue is a multi-producer, multi-consumer thread-safe queue: multiple
threads can service the WorkQueue, multiple threads can post work to it.
Work can be scheduled in the future by submitting with a timestamp. In
addition, a given work item can be scheduled to run on a recurring basis.
A requesting thread servicing a WorkQueue of its own, such as the viewer's
main thread, can submit work to another WorkQueue along with a callback to be
passed the result (of arbitrary type) of the first work item. The callback is
posted to the originating WorkQueue, permitting safe data exchange between
participating threads.
Methods are provided for different kinds of servicing threads. runUntilClose()
is useful for a simple worker thread. runFor(duration) devotes no more than a
specified time slice to that WorkQueue, e.g. for use by the main thread.
ThreadSafeSchedule::tryPopUntil() (and therefore tryPopFor()) was simply
delegating to LLThreadSafeQueue::tryPopUntil(), with an adjusted timeout since
we want to wake up as soon as the head item, if any, becomes ready. But then
we have to loop back to retry the pop to actually deal with that head item.
In addition, ThreadSafeSchedule::popWithTime() was spinning rather than
properly blocking on a timed condition variable. Fixed.
ThreadSafeSchedule orders its items by timestamp, which can be passed either
implicitly or explicitly. The timestamp specifies earliest delivery time: an
item cannot be popped until that time.
Add initial tests.
Tweak the LLThreadSafeQueue base class to support ThreadSafeSchedule:
introduce virtual canPop() method to report whether the current head item is
available to pop. The base class unconditionally says yes, ThreadSafeSchedule
says it depends on whether its timestamp is still in the future.
This replaces the protected pop_() overload accepting a predicate. Rather than
explicitly passing a predicate through a couple levels of function call, use
canPop() at the level it matters. Runtime behavior that varies depending on
an object's leaf class is what virtual functions were invented for.
Give pop_() a three-state enum return so pop() can distinguish between "closed
and empty" (throws exception) versus "closed, not yet drained because we're
not yet ready to pop the head item" (waits).
Also break out protected tryPopUntil_() method, the body logic of
tryPopUntil(). The public method locks the data structure, the protected
method requires that its caller has already done so.
Add chrono.h with a more full-featured LL::time_point_cast() function than the
one found in <chrono>, which only converts between time_point durations, not
between time_points based on different clocks.
Andrey Lihatskiy