When WCFS-mmapped memory is accessed, it can get SIGBUS on IO error (and
automatically on WCFS crash), and SIGSEGV when accessed client mapping is closed.

tFile.assertBlk in wcfs_test.py already converts SIGSEGV into python
exception when accessing on-wcfs file's block. However
tMapping.assertBlk was not doing so, which, instead of providing proper
details, leads to test crashes if something goes wrong.

For example when wendelin.core tests are run wrt plain ZODB4 (contrary to
ZODB4-wc2, see nexedi/ZODB!1 and
nexedi/slapos@e256ed97), it first fails in
pinner and then gets SIGSEGV on data access, because, to mimic SIGBUS on
EIO, pinner shutdowns all mappings on its failure:

https://lab.nexedi.com/nexedi/wendelin.core/blob/49f826b1/wcfs/client/wcfs.cpp#L477-501
https://nexedijs.erp5.net/#/test_result_module/20211118-7C45220A/25

-> Fix it by wrapping test block access with appropriate read_exfault
variant.

Before this patch:

    .../wendelin.core$ WENDELIN_CORE_TEST_DB='<zeo>' WENDELIN_CORE_VIRTMEM='r:wcfs+w:uvmm' python -m pytest -vsx wcfs/ -k test_wcfs_client
    ...
    wcfs/client/client_test.py::test_wcfs_client
    -------------------- live log call ---------------------
    INFO     wcfs:__init__.py:293 starting for zeo://localhost:28866 ...
    I1122 19:17:14.376182  110032 wcfs.go:2384] start "/dev/shm/wcfs/ef87339c054c3e0e48d494fa584bb209518844b2" "zeo://localhost:28866"
    I1122 19:17:14.376291  110032 wcfs.go:2390] (built with go1.17.3)
    W1122 19:17:14.380882  110032 storage.go:152] zodb: FIXME: open zeo://localhost:28866: raw cache is not ready for invalidations -> NoCache forced
    INFO     wcfs:__init__.py:334 started pid110032 @ /dev/shm/wcfs/ef87339c054c3e0e48d494fa584bb209518844b2

    M: commit -> @at0 (03e452313dddbc00)

    M: commit -> @at1 (03e452313e0f3b99)
    M:      f<0000000000000002>     [2, 3]

    M: commit -> @at2 (03e452313e1adb55)
    M:      f<0000000000000002>     [2]

    M: commit -> @at3 (03e452313e3be500)
    M:      f<0000000000000002>     [3, 4]
    W1122 19:17:14.597654  110032 wcfs.go:2050] /@03e452313d343c88/bigfile: lookup "0000000000000002": bigfopen 0000000000000002 @03e452313d343c88: invalid argument: Get 0000000000000002: Get 03e452313d343c88:0000000000000002: zeo://localhost:28866: load 03e452313d343c88:0000000000000002: 0000000000000002: no such object
    E1122 19:17:14.597759  110032 wcfs.go:1220] /head/bigfile/0000000000000002: readblk #4: pin watchers: wlink1: f<0000000000000002>: pin #4 @03e452313d343c88: expect "ack"; got "nak: _remmapblk #4 @03e452313d343c88: open /dev/shm/wcfs/ef87339c054c3e0e48d494fa584bb209518844b2/@03e452313d343c88/bigfile/0000000000000002: Invalid argument"
    F1122 19:17:14.597803  110050 wcfs/client/wcfs.cpp:487] CRITICAL: pinner: pin f<0000000000000002> #4 @03e452313d343c88: _remmapblk #4 @03e452313d343c88: open /dev/shm/wcfs/ef87339c054c3e0e48d494fa584bb209518844b2/@03e452313d343c88/bigfile/0000000000000002: Invalid argument
    F1122 19:17:14.597835  110050 wcfs/client/wcfs.cpp:488] CRITICAL: wcfs server will likely kill us soon.
    CRITICAL: pinner: pin f<0000000000000002> #4 @03e452313d343c88: _remmapblk #4 @03e452313d343c88: open /dev/shm/wcfs/ef87339c054c3e0e48d494fa584bb209518844b2/@03e452313d343c88/bigfile/0000000000000002: Invalid argument
    CRITICAL: wcfs server will likely kill us soon.
    Segmentation fault: read @00007ff7b9534000
    /home/kirr/src/wendelin/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(dump_traceback+0x34)[0x7ff7d6b5c279]
    /home/kirr/src/wendelin/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(+0x27b0)[0x7ff7d6b577b0]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x14140)[0x7ff7da078140]
    python(PyString_FromStringAndSize+0x228)[0x5627feb96b58]
    python(PyEval_EvalFrameEx+0x603e)[0x5627febb7a4e]
    python(PyEval_EvalCodeEx+0x57c)[0x5627febb03cc]
    ...
    python(PyObject_Call+0x43)[0x5627feb9d903]
    python(+0x18a7e1)[0x5627fec5d7e1]
    python(Py_Main+0x3ad)[0x5627fec4b8ed]
    /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xea)[0x7ff7d9d59d0a]
    python(_start+0x2a)[0x5627fec4b46a]
    Ошибка сегментирования (стек памяти сброшен на диск)

After this patch:

    .../wendelin.core$ WENDELIN_CORE_TEST_DB='<zeo>' WENDELIN_CORE_VIRTMEM='r:wcfs+w:uvmm' python -m pytest -vsx wcfs/ -k test_wcfs_client
    ...
    wcfs/client/client_test.py::test_wcfs_client
    -------------------- live log call ---------------------
    INFO     wcfs:__init__.py:293 starting for zeo://localhost:22854 ...
    I1122 18:17:22.486445  102541 wcfs.go:2384] start "/dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98" "zeo://localhost:22854"
    I1122 18:17:22.486525  102541 wcfs.go:2390] (built with go1.17.3)
    W1122 18:17:22.489908  102541 storage.go:152] zodb: FIXME: open zeo://localhost:22854: raw cache is not ready for invalidations -> NoCache forced
    INFO     wcfs:__init__.py:334 started pid102541 @ /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98

    M: commit -> @at0 (03e451f560834477)

    M: commit -> @at1 (03e451f560a2aa77)
    M:      f<0000000000000002>     [2, 3]

    M: commit -> @at2 (03e451f560adafcc)
    M:      f<0000000000000002>     [2]

    M: commit -> @at3 (03e451f560d02111)
    M:      f<0000000000000002>     [3, 4]
    W1122 18:17:22.703710  102541 wcfs.go:2050] /@03e451f55fcc4c77/bigfile: lookup "0000000000000002": bigfopen 0000000000000002 @03e451f55fcc4c77: invalid argument: Get 0000000000000002: Get 03e451f55fcc4c77:0000000000000002: zeo://localhost:22854: load 03e451f55fcc4c77:0000000000000002: 0000000000000002: no such object
    E1122 18:17:22.703840  102541 wcfs.go:1220] /head/bigfile/0000000000000002: readblk #4: pin watchers: wlink1: f<0000000000000002>: pin #4 @03e451f55fcc4c77: expect "ack"; got "nak: _remmapblk #4 @03e451f55fcc4c77: open /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98/@03e451f55fcc4c77/bigfile/0000000000000002: Invalid argument"
    F1122 18:17:22.704380  102558 wcfs/client/wcfs.cpp:487] CRITICAL: pinner: pin f<0000000000000002> #4 @03e451f55fcc4c77: _remmapblk #4 @03e451f55fcc4c77: open /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98/@03e451f55fcc4c77/bigfile/0000000000000002: Invalid argument
    F1122 18:17:22.704639  102558 wcfs/client/wcfs.cpp:488] CRITICAL: wcfs server will likely kill us soon.
    CRITICAL: pinner: pin f<0000000000000002> #4 @03e451f55fcc4c77: _remmapblk #4 @03e451f55fcc4c77: open /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98/@03e451f55fcc4c77/bigfile/0000000000000002: Invalid argument
    CRITICAL: wcfs server will likely kill us soon.
    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e451f560834477)
            @at1 (03e451f560a2aa77)     2 3
            @at2 (03e451f560adafcc)     2
            @at3 (03e451f560d02111)       3 4

    INFO     wcfs:__init__.py:400 unmount/stop wcfs pid102541 @ /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98
    I1122 18:17:22.728452  102541 wcfs.go:2560] stop "/dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98" "zeo://localhost:22854"
    FAILED

    ======================= FAILURES =======================
    ___________________ test_wcfs_client ___________________

        @func
        def test_wcfs_client():
            t = tDB(); zf = t.zfile; at0=t.at0
            defer(t.close)
            pinned = lambda fh: fhpinned(t, fh)

            at1 = t.commit(zf, {2:'c1', 3:'d1'})
            at2 = t.commit(zf, {2:'c2'})

            wconn = t.wc.connect(at1)
            defer(wconn.close)

            fh = wconn.open(zf._p_oid)
            defer(fh.close)

            # create mmap with 1 block beyond file size
            m1 = fh.mmap(2, 3)
            defer(m1.unmap)

            assert m1.blk_start == 2
            assert m1.blk_stop  == 5
            assert len(m1.mem)  == 3*zf.blksize

            tm1 = tMapping(t, m1)

            assert pinned(fh) == {}

            # verify initial data reads
            tm1.assertBlk(2, 'c1',  {2:at1})
            tm1.assertBlk(3, 'd1',  {2:at1})
            tm1.assertBlk(4, '',    {2:at1})

            # commit with growing file size -> verify data read as the same, #3 pinned.
            # (#4 is not yet pinned because it was not accessed)
            at3 = t.commit(zf, {3:'d3', 4:'e3'})
            assert pinned(fh) == {2:at1}
            tm1.assertBlk(2, 'c1',  {2:at1})
            tm1.assertBlk(3, 'd1',  {2:at1, 3:at1})
            tm1.assertBlk(4, '',    {2:at1, 3:at1})

            # resync at1 -> at2:    #2 must unpin to @head; #4 must stay as zero
            wconn.resync(at2)
            assert pinned(fh) == {3:at1}
            tm1.assertBlk(2, 'c2',  {       3:at1})
            tm1.assertBlk(3, 'd1',  {       3:at1})
    >       tm1.assertBlk(4, '',    {       3:at1,  4:at0})     # XXX at0->ø ?

    wcfs/client/client_test.py:158:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    wcfs/client/client_test.py:86: in assertBlk
        _ = read_exfault_withgil(blkview[0:1])
    wcfs/internal/wcfs_test.pyx:90: in wendelin.wcfs.internal.wcfs_test.read_exfault_withgil
        return _read_exfault(mem, withgil=True)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    >   raise SegmentationFault()
    E   SegmentationFault

    wcfs/internal/wcfs_test.pyx:120: SegmentationFault
    ------------------ Captured log call -------------------
    INFO     wcfs:__init__.py:293 starting for zeo://localhost:22854 ...
    INFO     wcfs:__init__.py:334 started pid102541 @ /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98
    INFO     wcfs:__init__.py:400 unmount/stop wcfs pid102541 @ /dev/shm/wcfs/c818c147676f8d6f3b408b02f727aca5e3229e98