$ go test -v -failfast -run 'TestΔBTail$/rebuild'
    ...
    PASS
    ok      lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree        176.661s

--- FAIL: TestΔBTail (0.74s)
    --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R/_→T1/T-T/B0:g-T/B1:e (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x632cc1]

goroutine 238 [running]:
testing.tRunner.func1.2(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000001b00)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1146 +0x4b6
panic(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/runtime/panic.go:965 +0x1b9
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.PPTreeSubSet.ApplyΔ(0xc00040f9b0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/pptreesubset.go:515 +0x41
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔTtail).rebuild(0xc00040f830, 0xc, 0xc00040f500, 0xc000142b40, 0x0, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:371 +0x145
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔBtail).rebuildAll(0xc00015dc00, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:319 +0x18c
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild_TR(0xc000001b00, 0xc000142b40, 0xc00015dc00, 0xc00006a960, 0xc, 0xc0004863c0, 0xc00040f3b0, 0xc00040f5c0, 0xc00040f650, 0xc00040f740, ...)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1127 +0x1b6
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild.func1.1.1(0xc000001b00)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1074 +0xafa
testing.tRunner(0xc000001b00, 0xc000555740)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
        /home/kirr/src/tools/go/go/src/testing/testing.go:1238 +0x2b3
exit status 2

Access to node entries is valid only after the node was activated.
And it fails otherwise.

--- FAIL: TestΔBTail (0.63s)
    --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R/_→T/T/B0:a (0.00s)
panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children [recovered]
        panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children

Rebuild tests were "surprisingly" failing when run as part of whole
testsuite, but succeeding when run separately.

So that even A \ A was not ø.

It was due to thinko in iterating through keys set - we were using key
ordinals instead of keys themselvs. Now it find such bugs:

=== RUN   TestΔBTailAllStructs/Update/T1,3/B0:d-B1:a-B3:d,4:g→T1,4/B0:e-B1:a-B4:c
    δbtail_test.go:839: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0}]
                want: map[000000000000000c:{pffffffffffffffff c3} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0} 0000000000000620:{p000000000000000c c0}]
    δbtail_test.go:708: T1,3/B0:d-B1:a-B3:d,4:g -> T1,4/B0:e-B1:a-B4:c  ; tracked={0 1} :

        d12:  map[0:{d e} 3:{d ø} 4:{g c}]
        δTok: map[0:{d e} 3:{d ø} 4:{g c}]
        δT:   map[0:{d e}]

        δZ:               {000000000000000c 0000000000000618 0000000000000620}
        Tracked^δZ:       {0 1}
        kadj[Tracked^δZ]: {0 1 3 4 9223372036854775807}
        kadj: map[0:{0} 1:{1 3 4 9223372036854775807} 3:{1 3 4 9223372036854775807} 4:{1 3 4 9223372036854775807} 9223372036854775807:{1 3 4 9223372036854775807}]

        δtkeycov wrong:
        have: {}
        want: {[3,∞)}
        δT    ∌  δTok[3]
        δT    ∌  δTok[4]

Currently fails with e.g.:

(neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree$ go test -short -v -failfast -run 'TestΔBTail$/Update/T3/B1:a-B8:c→T7/B1:a-B8:c'
=== RUN   TestΔBTail
2021/06/23 14:08:45 zodb: FIXME: open /tmp/δBTail926507346/1.fs: raw cache is not ready for invalidations -> NoCache forced
=== RUN   TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c
    δbtail_test.go:812: BUG: tkeyCov1 ⊄ tkeyCov2:
                tkeyCov1: {[3,∞)}
                tkeyCov2: {[7,∞)}
                tkeyCov1 \ tkeyCov2: {[3,7)}
    δbtail_test.go:708: T3/B1:a-B8:c -> T7/B1:a-B8:c  ; tracked={8} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 8 9223372036854775807} 8:{1 8 9223372036854775807} 9223372036854775807:{1 8 9223372036854775807}]

--- FAIL: TestΔBTail (0.84s)
    --- FAIL: TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c (0.00s)

=== RUN   TestΔBTail/Update/T7/B1:a,2:b-B8:c,9:d→T3/T-T/B1:a,2:b-B8:c,9:d
    δbtail_test.go:832: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 0000000000000041:{p0000000000000043 c0} 0000000000000042:{p0000000000000044 c0} 0000000000000043:{p000000000000000c c1} 0000000000000044:{p000000000000000c c1}]
                want: map[000000000000000c:{pffffffffffffffff c1} 0000000000000041:{p0000000000000043 c0} 0000000000000043:{p000000000000000c c1}]
    δbtail_test.go:708: T7/B1:a,2:b-B8:c,9:d -> T3/T-T/B1:a,2:b-B8:c,9:d  ; tracked={1} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c 0000000000000043 0000000000000044}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 2 8 9 9223372036854775807} 2:{1 2 8 9 9223372036854775807} 8:{1 2 8 9 9223372036854775807} 9:{1 2 8 9 9223372036854775807} 9223372036854775807:{1 2 8 9 9223372036854775807}]

        δtkeycov wrong:
        have: {[7,∞)}
        want: {}

* t:
  .
  X wcfs: Fix ZSync to close wconn on zdb.close, even if zconn stays alive
  X lib/zodb: Connection += onShutdownCallback
  .
  X wcfs: lsof +D misbehaves - don't use it
  X wcfs: _fuse_unmount: Try first `kill -TERM` before `kill -QUIT` wcfs
  X wcfs: Tune _fuse_unmount to include `fusermount -u` error message into raised exception
  X wcfs: Teach start to start successfully even after unclean wcfs shutdown
  fixup! X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
  X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
  X wcfs: Add start to spawn a Server that can be later stopped  (draft)

Even if ZODB DB is closed and releases zconn from its .pool, zconn can
still stay alive being referenced from any other live python object -
e.g. from a frame, a traceback etc. This situation in particular happens
under ERP5's runUnitTest.

As the result, if we don't hook into DB.close and close wconn only on
zconn GC, files opened on WCFS filesystem stay opened, and WCFS server
cannot be cleanly unmounted when test run completes.

-> Fix it.

P.S. ZODB@bbd03b3a is good, but does
not really solve this problem because, once again, zconn stays
referenced from objects besides DB.

In the next patch we'll need this functionality to subscribe ZSync into
db.close to know when ZODB Connection is shut down even if it stays
alive referenced by some other objects.