See added comments to wcfs_test.py for details on how that can happen.
Fixes test segmentation faults like

    $ WENDELIN_CORE_TEST_DB="<zeo>" python -m pytest -vs -k test_wcfs_watch_vs_access

    wcfs_test.py::test_wcfs_watch_vs_access
    ------------------------------- live log setup --------------------------------
    INFO     ZEO.ClientStorage:ClientStorage.py:263 ('localhost', 20106) ClientStorage (pid=36942) created RW/normal for storage: '1'
    INFO     ZEO.cache:cache.py:217 created temporary cache file '<fdopen>'
    INFO     ZEO.ClientStorage:ClientStorage.py:574 ('localhost', 20106) Testing connection <ManagedClientConnection ('127.0.0.1', 20106)>
    INFO     ZEO.zrpc.Connection('C'):connection.py:365 (127.0.0.1:20106) received handshake 'Z4'
    INFO     ZEO.ClientStorage:ClientStorage.py:580 ('localhost', 20106) Server authentication protocol None
    INFO     ZEO.ClientStorage:ClientStorage.py:640 ('localhost', 20106) Connected to storage: ('localhost', 20106)
    INFO     ZEO.ClientStorage:ClientStorage.py:1326 ('localhost', 20106) No verification necessary -- empty cache
    INFO     ZEO.ClientStorage:ClientStorage.py:728 ('localhost', 20106) Disconnected from storage: "('localhost', 20106)"
    -------------------------------- live log call --------------------------------
    INFO     ZEO.ClientStorage:ClientStorage.py:263 ('localhost', 20106) ClientStorage (pid=36942) created RW/normal for storage: '1'
    INFO     ZEO.cache:cache.py:217 created temporary cache file '<fdopen>'
    INFO     ZEO.ClientStorage:ClientStorage.py:574 ('localhost', 20106) Testing connection <ManagedClientConnection ('127.0.0.1', 20106)>
    INFO     ZEO.zrpc.Connection('C'):connection.py:365 (127.0.0.1:20106) received handshake 'Z4'
    INFO     ZEO.ClientStorage:ClientStorage.py:580 ('localhost', 20106) Server authentication protocol None
    INFO     ZEO.ClientStorage:ClientStorage.py:640 ('localhost', 20106) Connected to storage: ('localhost', 20106)
    INFO     ZEO.ClientStorage:ClientStorage.py:1326 ('localhost', 20106) No verification necessary -- empty cache
    INFO     root:__init__.py:294 wcfs: starting for zeo://localhost:20106 ...
    wcfs: 2021/08/13 02:27:40 zodb: FIXME: open zeo://localhost:20106: raw cache is not ready for invalidations -> NoCache forced
    INFO     root:__init__.py:335 wcfs: started pid37431 @ /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76

    M: commit -> @at0 (03e2107fabf002ee)

    M: commit -> @at1 (03e2107fac097466)
    M:      f<0000000000000002>     [2]

    M: commit -> @at2 (03e2107fac3df2aa)
    M:      f<0000000000000002>     [2, 3, 5]

    M: commit -> @at3 (03e2107fac5ef011)
    M:      f<0000000000000002>     [2, 5]

    C: setup watch f<0000000000000002> @at3 (03e2107fac5ef011)
    #  pinok: {}

    C: setup watch f<0000000000000002> @at3 (03e2107fac5ef011)
    #  pinok: {}

    C: setup watch f<0000000000000002> @at2 (03e2107fac3df2aa)
    #  pinok: {2: @at2 (03e2107fac3df2aa)}

    M: commit -> @at4 (03e2107face33c77)
    M:      f<0000000000000002>     [2, 5, 6]

    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e2107fabf002ee)
            @at1 (03e2107fac097466)     2
            @at2 (03e2107fac3df2aa)     2 3   5
            @at3 (03e2107fac5ef011)     2     5
            @at4 (03e2107face33c77)     2     5 6

    INFO     ZEO.ClientStorage:ClientStorage.py:728 ('localhost', 20106) Disconnected from storage: "('localhost', 20106)"
    INFO     root:__init__.py:401 wcfs: unmount/stop wcfs pid37431 @ /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76
    WARNING  root:__init__.py:548 fuse_unmount /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76: failed: fusermount: failed to unmount /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76: Device or resource busy
    WARNING  root:__init__.py:533 # lsof /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76
    WARNING  root:__init__.py:541
    WARNING  root:__init__.py:543 (lsof failed)
    WARNING  root:__init__.py:461 -> kill -TERM wcfs.go ...
    WARNING  root:__init__.py:464 -> abort FUSE connection ...
    Segmentation fault: read @00007f6e36bfe000
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(dump_traceback+0x1b)[0x7f6f80844e4b]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(+0x3956)[0x7f6f80841956]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x12730)[0x7f6f83117730]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/internal/wcfs_test.so(+0x10860)[0x7f6e3e2eb860]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7(PyEval_EvalFrameEx+0x7b5)[0x4d2dc5]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7(PyEval_EvalCodeEx+0x2cc)[0x4d1abc]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7[0x51b92e]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/_golang.so(+0xc8b0)[0x7f6f8182b8b0]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/_golang.so(+0x14ab4)[0x7f6f81833ab4]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7[0x54bbb4]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x7fa3)[0x7f6f8310cfa3]
    /lib/x86_64-linux-gnu/libc.so.6(clone+0x3f)[0x7f6f82eae4cf]
    Segmentation fault (core dumped)

Which looks under gdb as

    #0  on_pagefault (sig=<optimized out>, si=0x7f6dde7fb570, _uc=<optimized out>) at bigfile/pagefault.c:171
    #1  <signal handler called>
    #2  __pyx_pf_8wendelin_4wcfs_8internal_9wcfs_test_read_nogil (__pyx_self=<optimized out>, __pyx_v_mem=...) at wcfs/internal/wcfs_test.cpp:3103
    #3  __pyx_pw_8wendelin_4wcfs_8internal_9wcfs_test_1read_nogil (__pyx_self=<optimized out>, __pyx_arg_mem=<optimized out>) at wcfs/internal/wcfs_test.cpp:3029
    #4  0x00000000004d2dc5 in call_function (oparg=<optimized out>, pp_stack=0x7f6dde7fbc88) at Python/ceval.c:4364
    #5  PyEval_EvalFrameEx (f=<optimized out>, throwflag=<optimized out>) at Python/ceval.c:3013
    #6  0x00000000004d1abc in PyEval_EvalCodeEx (co=0x7f6f8094cbb0, globals=<optimized out>, locals=locals@entry=0x0, args=args@entry=0x7f6f82d72068, argcount=<optimized out>, kws=kws@entry=0x7f6f82d72068, kwcount=0, defs=0x0, defcount=0,
        closure=0x7f6e3c6e7110) at Python/ceval.c:3608
    #7  0x000000000051b92e in function_call (func=0x7f6e3c711150, arg=0x7f6f82d72050, kw=0x7f6e3c710b90) at Objects/funcobject.c:523
    #8  0x00007f6f8182b8b0 in __Pyx_PyObject_Call (func=0x7f6e3c711150, arg=<optimized out>, kw=<optimized out>) at golang/_golang.cpp:15660
    #9  0x00007f6f81833ab4 in __pyx_f_6golang_7_golang___goviac (__pyx_v_arg=0x7f6e3c70f5f0) at golang/_golang.cpp:3466
    #10 __pyx_f_6golang_7_golang__goviac (__pyx_v_arg=__pyx_v_arg@entry=0x7f6e3c70f5f0) at golang/_golang.cpp:3350
    #11 0x000000000054bbb4 in pythread_wrapper (arg=<optimized out>) at Python/thread_pthread.h:178
    #12 0x00007f6f8310cfa3 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
    #13 0x00007f6f82eae4cf in clone () from /lib/x86_64-linux-gnu/libc.so.6

This reverts commit 30740602.

libbracktrace is not always automatically installed.

Do what we can do without gdb and then tail to regular segmentation
fault. With core gdb can still be useed, but if we already get in the
log traceback of the crash automatically.

t(TestΔBTailRandom) ~320s -> ~100s.

* kirr/t:
  X Fix mlock2 build on Debian 8

* t2:
  X ΔFtail.SliceByFileRev: Fix untracked entries to be present uniformly in result
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  X test that shows problem of SliceByRootRev where untracked blocks are not added uniformly into whole history
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  X Size no longer tracks [0,∞) since we start tracking when zfile is non-empty
  X ΔFtail: `go test -failfast -short -v -run Random -randseed=1626793016249041295` discovered problems

X test that shows problem of SliceByRootRev where untracked blocks are not added uniformly into whole history

=== RUN   TestΔFtailSliceXXX
2021/08/05 18:07:35 zodb: FIXME: open /tmp/TestΔFtailSliceXXX2265944622/001/1.fs: raw cache is not ready for invalidations -> NoCache forced
    δftail_test.go:689: slice (@at0,@at2]:
        have: [@at1·{0 1}S @at2·{1}S]
        want: [@at1·{0 1}S @at2·{0 1}S]

-> Always track all blocks in blkTab.

@rporchetto reports build failure on Debian 8 / Linux 3.16

    [2021-07-30 15:40:35,677] INFO     gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/srv/slapgrid/slappart19/srv/runner/shared/python2.7/3b7a074d1ded44046871b13502341482/include/python2.7 -c wcfs/internal/mm.c -o build/temp.linux-x86_64-2.7/wcfs/internal/mm.o
    [2021-07-30 15:40:35,677] INFO     wcfs/internal/mm.c: In function 'mlock2':
    [2021-07-30 15:40:35,677] INFO     wcfs/internal/mm.c:618:28: error: 'SYS_mlock2' undeclared (first use in this function); did you mean 'SYS_mlock'?
    [2021-07-30 15:40:35,677] INFO              long err = syscall(SYS_mlock2, addr, len, flags);
    [2021-07-30 15:40:35,677] INFO                                 ^~~~~~~~~~
    [2021-07-30 15:40:35,677] INFO                                 SYS_mlock

Fix the build.

NOTE mlock2 was added in Linux 4.3.

Similarly MCL_ONFAULT is not provided on that old glibc 2.19:

    [2021-07-30 15:40:35,677] INFO     wcfs/internal/mm.c:986:55: error: 'MCL_ONFAULT' undeclared here (not in a function); did you mean 'MLOCK_ONFAULT'?
    [2021-07-30 15:40:35,677] INFO        __pyx_e_8wendelin_4wcfs_8internal_2mm_MCL_ONFAULT = MCL_ONFAULT,
    [2021-07-30 15:40:35,678] INFO                                                            ^~~~~~~~~~~
    [2021-07-30 15:40:35,678] INFO                                                            MLOCK_ONFAULT

-> Comment MCL_ONFAULT for now since we do not actually use it anywhere yet.

X ΔFtail: `go test -failfast -short -v -run Random -randseed=1626793016249041295` discovered problems

=== RUN   TestΔFtailRandom
    δftail_test.go:141: # n=10 seed=1626793016249041295
2021/07/23 12:26:01 zodb: FIXME: open /tmp/TestΔFtailRandom1363232041/001/1.fs: raw cache is not ready for invalidations -> NoCache forced
    δftail_test.go:191: #   @at0 (03e19cb6064c58dd)
    δftail_test.go:203: # → @at1 (03e19cb6064ddd99)  t0:a Da:a,b:b,c:c,d:d,e:e,f:f,g:g,h:h,i:i,j:j      ; not-yet-tracked
    δftail_test.go:375: # → @at2 (03e19cb6064fc922)  δT2:i,3:c,5:d,9:c  δD{a b c d e f g h i}   ; t0:a,2:i,3:c,5:d,9:c Da:a2,b:b2,c:c2,d:d2,e:e2,f:f2,g:g2,h:h2,i:i2,j:j        δ{0 2 3 5 9}
    δftail_test.go:472: δf:
        have: &{03e19cb6064fc922 false {2 3 5 9} true}
        want: &{03e19cb6064fc922 false {0 2 3 5 9} true}
    δftail_test.go:499: .trackSetZBlk:
        ~have: map[c:{3 9} d:{5} i:{2}]
         want: map[a:{0} c:{3 9} d:{5} i:{2}]

    ...

- Reimplement ΔFtail queries via gluing ΔBtail and ΔZtail data on the fly.
  This helps to avoid implementing complex rebuild logic in ΔFtail.
  The only place that needs to have that complexity is now ΔBtail, and there it
  already works draftly.

- Add ΔFtail tests.

- Add notion of epochs to ΔFtail. Epochs correspond to ZBigFile objects changes
  (creation and deletion). Unfortunately handling ZBigFile object changes
  turned out to be necessary to keep wcfs tests in passing state.

- Move common testing infrastructure - that is used by both ΔBtail and ΔFtail - to xbtreetest package.

- Add tests for ΔBtail.SliceByRootRev aliasing

- Lazy rebuild is now on

- ΔBtail.GetAt reworked

...

* t2: (112 commits)
  X wcfs: v↑ NEO/go (checkpoint)
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  X ΔFtail: Rebuild vδE after first track
  .
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  ...

To pick up neo@bc3b5ec3

This is necessarry because previously both ΔFtail.BlkRevAt and
ΔBtail.GetAt required zfile/root checkouts from @head. However during
update process in wcfs it is ΔFtail that is updated first, and only
after some invalidations zconn is resynced to new @head. And if we
move zconn.resync to happen before ΔFtail.Update, that would break
the invariant that data in OS cache correspond to zconn.At.

The rework triggered and clarified logic of what ΔBtail.GetAt
responsibility and functionality should be. Now it computes query result
only based on ΔBtail data, and returns valueExact=false or
revExact=false, if those data is insufficient. Previously it was further
trying to load data from the database which is conflicting with

	// ΔBtail semantically consists of
	//
	//	[]δB			; rev ∈ (tail, head]

Now that database access moved to GetAt user - to ΔFtail.BlkRevAt.

WCFS tests - that were broken after ΔFtail rework - now finally pass again.

* t2+qoldhead:
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