FUSE puts X as st_dev's minor, which, for minors <= 255 is the same as st_dev.
However when there are many connections, and minor goes after 255, minor becomes != st_dev:

    In [2]: os.makedev(0, 254)
    Out[2]: 254

    In [3]: os.makedev(0, 255)
    Out[3]: 255

    In [5]: os.makedev(0, 256)
    Out[5]: 1048576

As a result we were constructing wrong patch, and if wcfs was failing we were
also failing to kill it with something like:

    t = <wcfs.wcfs_test.tDB object at 0x7fef78043260>

        @func
        def __init__(t):
            t.root = testdb.dbopen()
            def _(): # close/unlock db if __init__ fails
                exc = sys.exc_info()[1]
                if exc is not None:
                    dbclose(t.root)
            defer(_)

            assert not os.path.exists(testmntpt)
            t.wc = wcfs.join(testzurl, autostart=True)
            assert os.path.exists(testmntpt)
            assert is_mountpoint(testmntpt)

            # force-unmount wcfs on timeout to unstuck current test and let it fail.
            # Force-unmount can be done reliably only by writing into
            # /sys/fs/fuse/connections/<X>/abort. For everything else there are
            # cases, when wcfs, even after receiving `kill -9`, will be stuck in kernel.
            # ( git.kernel.org/linus/a131de0a482a makes in-kernel FUSE client to
            #   still wait for request completion even after fatal signal )
    >       t._wcfuseabort   = open("/sys/fs/fuse/connections/%d/abort" % os.stat(testmntpt).st_dev, "w")
    E       IOError: [Errno 2] No such file or directory: '/sys/fs/fuse/connections/2097264/abort'

    wcfs/wcfs_test.py:236: IOError

In the above failure st_dev=2097264 corresponds to X=624:

    In [6]: os.minor(2097264)
    Out[6]: 624

Without special care a forked child may interfere in parent-wcfs
exchange via Python GC -> PyFileH.__del__ -> FileH.close -> message to
WCFS sent from the child. This actually happens for real when running
test.py/neo-wcfs because NEO test cluster spawns master and storage
nodes with just fork without exec.

-> detach from wcfs in child right after fork and deactivate all
mappings in order not to provide stale data. See top-level comments
added to wcfs/client/wcfs.cpp for details.

Currently in wcfs_test.py there is only waiting for a proc
(subprocess.Popen instance) to become ready. However in the next patch
we'll need to wait via polling for another condition.

-> Generalize the pollwait code into waitfor* variants, and make
procwait* use waitfor* internally.

Currently the code to convert `int err` or errno into string is usde
only in _pathError, but in the next patches we'll need it to also handle
error from pthread_atfork. -> Factor-out to separate function.

ZEO4 can have ._addr as either ("host", port) or "path".

People don't want to raise up soft defaults
nexedi/slapos.package!130 (comment 118373)
nexedi/slapos.package!130 (comment 118405)

Previously if an assert or something failed in spawned thread, the main
thread was usually spinning indefinitely = tests hang. -> Switch all
threading places to use sync.WorkGroup and this way if a thread fails,
all other threads are canceled and the exception is reported back to
wg.wait in main thread.