@ixv recently uncovered a bug (#2180) in Ford that caused certain
rebuilds to crash. @Fang- and I believe this change should fix the bug,
and we have confirmed that the reproduction that used to fail about two
thirds of the time now has not failed at all in the ten or so times
we've run it since then. @Fang- is still running more tests to confirm
the fix with more certainty.
It turned out the cause was that (depending on the rebuild order, which
is unspecified and should not need to be specified), Ford could enqueue
a provisional sub-build to be run but then, later in the same +gather
call, discover that the sub-build was in fact an orphan and delete it
from builds.state accordingly. Then when Ford tried to run the
sub-build, it would have already been deleted from the state, so Ford
would crash when trying to process its result in +reduce.
The fix was to make sure that when we discover a provisional sub-build
is orphaned, dequeue it from candidate-builds and next-builds to make
sure we don't try to run it. I'm about 95% sure this fix completely
solves the bug.
