If someone else built and marked one of our dependencies, then that
dependency would show up as *clean* in a later redo-ifchange, so other
dependents of that file wouldn't be rebuilt.
We actually have to track two session-specific variables: whether the file
has been checked, and whether it was rebuilt. (Or alternatively, whether it
was dirty when we checked it the first time. But we store the former.)
If a depends on b depends on c, then if when we consider building a, we have
to check b and c. If we then are asked about a2 which depends on b, there
is no reason to re-check b and its dependencies; we already know it's done.
This takes the time to do 'redo t/curse/all' the *second* time down from
1.0s to 0.13s. (make can still do it in 0.07s.)
'redo t/curse/all' the first time is down from 5.4s to to 4.6s. With -j4,
from 3.0s to 2.5s.
Now t/curse passes again when parallelized (except for the countall
mismatch, since we haven't fixed the source of that problem yet). At least
it's consistent now.
There's a bunch of stuff rearranged in here, but the actual important
problem was that we were doing unlink() on the lock fifo even if ENXIO,
which meant a reader could connect in between ENXIO and unlink(), and thus
never get notified of the disconnection. This would cause the build to
randomly freeze.
