The `Cancelled` struct now reflects multiple potential reasons
for a query execution to be cancelled, including unexpected cycles.
It also gives information about the participants that lacked
recovery information.
Because it bugs me to clone the vector.
Maybe silly, I admit, since cycle recovery
is not the hot path.
But by that same token, we now spend only 1 word
for a null pointer instead of 4 words for a (usually empty) vector.
Currently, when one thread blocks on another, we clone the
stack from that task. This results in a lot of clones, but it also
means that we can't mark all the frames involved in a cycle atomically.
Instead, when we propagate information between threads, we also
propagate the participants of the cycle and so forth.
This branch *moves* the stack into the runtime while a thread
is blocked, and then moves it back out when the thread resumes.
This permits the runtime to mark all the cycle participants at once.
It also avoids cloning.
Instead of creating a future for each edge
in the graph, we now have all dependent queries
block in the runtime and wake up whenever results
are published to see if their results are ready.
We could certainly allocate a CondVar for each dependent
query if we found that spurious wakeups were a problem.
I consider this highly unlikely in practice.
Before we could not observe the case where:
* thread A is blocked on B
* cycle detected in thread B
* some participants are on thread A and have to be marked
(In particular, I commented out some code that seemed necessary and
didn't see any tests fail)
Rather than checking return value of from `Q::cycle_fallback`, we
now consult the computed recovery strategy to decide whether to
panic or to recover. We can thus assume that we will successfully
recover and don't need to check for `None` results anymore.
If I am not in an error, all current tests uses "static" cycles -- a
cycle always present. Let's spice that up by adding conditional cycles:
cycles that appear only for specific impls
