Similarly to build_ext we need ccan/config.h to be present for dso to
build. It was not the case and so pip install wendelin.core was failing:

    $ pip install wendelin.core-2.0a2.tar.gz
    Processing ./wendelin.core-2.0a2.tar.gz
      Installing build dependencies ... done
      Getting requirements to build wheel ... done
        Preparing wheel metadata ... done
    Collecting ZODB>=4
    ...
    Building wheels for collected packages: wendelin.core
      Building wheel for wendelin.core (PEP 517) ... error
      ERROR: Command errored out with exit status 1:
      ...
      running build_dso
      Building DSOs
      building 'wendelin.bigfile.libvirtmem' DSO as build/lib.linux-x86_64-2.7/wendelin/bigfile/liblibvirtmem.so
      creating build/temp.linux-x86_64-2.7
      creating build/temp.linux-x86_64-2.7/bigfile
      creating build/temp.linux-x86_64-2.7/lib
      x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -ffile-prefix-map=/build/python2.7-vgIf7a/python2.7-2.7.18=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -D_GNU_SOURCE -I/tmp/pip-build-env-lfVr7E/overlay/lib/python2.7/site-packages -I. -I./include -I./3rdparty/ccan -I./3rdparty/include -Ibuild/lib.linux-x86_64-2.7/. -c bigfile/pagefault.c -o build/temp.linux-x86_64-2.7/bigfile/pagefault.o -fno-strict-aliasing -std=gnu99 -fplan9-extensions -Wno-declaration-after-statement -Wno-error=declaration-after-statement
      In file included from ./include/wendelin/list.h:11,
                       from ./include/wendelin/bigfile/virtmem.h:50,
                       from bigfile/pagefault.c:29:
      ./3rdparty/ccan/ccan/array_size/array_size.h:4:10: fatal error: config.h: Нет такого файла или каталога
          4 | #include "config.h"
            |          ^~~~~~~~~~
      compilation terminated.
      error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
      ----------------------------------------
      ERROR: Failed building wheel for wendelin.core
    Failed to build wendelin.core
    ERROR: Could not build wheels for wendelin.core which use PEP 517 and cannot be installed directly

-> Fix it by making build_dso also first come through `make all`.

NOTE we cannot fix it in exactly the same way as for build_ext: if we split
build_dso into build_dso and ll_build_dso, `make all` will still go to infinite
recursion: build_dso -> ll_build_dso -> build_dso (not ll_build_dso, this is controlled by setuptools_dso) -> oops.

Starting from version 0.1 pygolang provides File out of the box:

nexedi/pygolang@4690460b
https://pypi.org/project/pygolang/#pygolang-change-history

-> Use it and remove our custom File implementation that originally
served as POC for that pygolang functionality.

Don't use golang::* namespaces to avoid clashes with pygolang adding
something in there and getting compilation error due to conflict, when
e.g. nexedi/pygolang!17 lands.

-> use xgolang:: as top-level namespace for what was previously living
in golang:: shipped in wendelin.core.

Inside that xgolang:: namespace don't use the same package names that
might be used inside golang:: in pygolang. This avoids ambiguation and
compile error in the future on e.g. os::AfterFork - is `using namespace
golang` and `using namespace xgolang` were both activated.

-> Prefix all namespaces inside xgolang:: also with "x".

See nexedi/pygolang!17

The problem is similar to a7bf0311 (wcfs: Fix crash if on invalidation
handledδZ needs to access ZODB) - I forgot to put zhead's transaction into
context.

Without the fix added test fails as:

    wcfs_test.py::test_wcfs_crash_old_data
    ---------------- live log call -----------------
    WARNING  ZODB.FileStorage:FileStorage.py:413 Ignoring index for /tmp/testdb_fs.OV0rS6/1.fs

    M: commit -> @at0 (03e5a3342bc5ab22)

    M: commit -> @at1 (03e5a3342bc88899)
    M:      f<0000000000000002>     [0]
    INFO     wcfs:__init__.py:293 starting for file:///tmp/testdb_fs.OV0rS6/1.fs ...
    I0120 17:12:10.274379  704327 wcfs.go:2393] start "/dev/shm/wcfs/556fa61a9f9675f34c6b44e1f978842c37176c59" "file:///tmp/testdb_fs.OV0rS6/1.fs"
    I0120 17:12:10.274409  704327 wcfs.go:2399] (built with go1.17.6)
    W0120 17:12:10.274560  704327 storage.go:152] zodb: FIXME: open file:///tmp/testdb_fs.OV0rS6/1.fs: raw cache is not ready for invalidations -> NoCache forced
    INFO     wcfs:__init__.py:334 started pid704327 @ /dev/shm/wcfs/556fa61a9f9675f34c6b44e1f978842c37176c59

    C: setup watch f<0000000000000002> @at1 (03e5a3342bc88899)
    #  pinok: {}

    M: commit -> @at2 (03e5a3342c895777)
    M:      f<0000000000000002>     [1]

    M: commit -> @at3 (03e5a3342ca5ef55)
    M:      f<0000000000000002>     [0]

    C: setup watch f<0000000000000002> @at2 (03e5a3342c895777)
    #  pinok: {0: @at1 (03e5a3342bc88899)}
    panic: transaction: no current transaction

    goroutine 88 [running]:
    lab.nexedi.com/kirr/neo/go/transaction.currentTxn({0x969718, 0xc0000b6240})
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/transaction.go:59 +0x77
    lab.nexedi.com/kirr/neo/go/transaction.Current(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/api.go:206
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).checkTxnCtx(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:374
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).Get(0xc0000c25a0, {0x969718, 0xc0000b6240}, 0x4)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:331 +0x73
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.(*ΔFtail).BlkRevAt(0xc00009dd40, {0x969718, 0xc0000b6240}, 0xc000100540, 0x30, 0x3e5a3342c895777)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/δftail.go:1140 +0x39d
    main.(*WatchLink).setupWatch(0xc0000120a0, {0x969718, 0xc0000b6240}, 0x2, 0x3e5a3342c895777)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1754 +0xe3f
    main.(*WatchLink)._handleWatch(0x0, {0x969718, 0xc0000b6240}, {0xc0000a0122, 0x0})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1973 +0x65
    main.(*WatchLink).handleWatch(0x0, {0x969718, 0xc0000b6240}, 0x0, {0xc0000a0122, 0x28})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1955 +0x10c
    main.(*WatchLink)._serve.func3({0x969718, 0xc0000b6240})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1944 +0x3c
    lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go.func1()
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:86 +0x68
    created by lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:83 +0x92
    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e5a3342bc5ab22)
            @at1 (03e5a3342bc88899) 0
            @at2 (03e5a3342c895777)   1
            @at3 (03e5a3342ca5ef55) 0

    ----------------------------------------

            # wcfs was crashing in setting up watch because of "1" and "2" from above, and
            # 3. setupWatch was calling ΔFtail.BlkRevAt without putting zhead's transaction into ctx.
            wl2 = t.openwatch()
    >       wl2.watch(zf, at2, {0:at1})

- put into if block to avoid collision with already-defined-elsewhere blkv
- show revisions in symbolic form

Noticed while working on recent change to allow ΔFtail/ΔBtail
point-queries with at=tail.

Watching with at=tail is inevitable as explained in the previous patch.

This is needed because when e.g. wcfs is just started the coverage of
ΔFtail is (head,head] i.e. empty, and if user wants to setup a watch
with at=head, it becomes watch with at=tail. Then that at is used in a
query and if point-queries with at=tail are disallowed it panics with
"at out of bounds".

This fixes crashes in test_wcfs_watch_setup (see 339f1884 "wcfs: tests:
Always start tDB with ZBigFile pre-created before WCFS startup") and in
test_wcfs_crash_old_data (see 97ce5105 "wcfs: tests: Add test do
demonstrate "at out of bounds" crash on readPinWatchers ->
ΔFtail.BlkRevAt")

For the reference zodb.ΔTail already allows point queries with at=tail:

https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L202-206
https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L225-228

The codepath that sends pin messages to watchers on FUSE READ, similarly
to what was showed in 339f1884 is also vulnerable to "at out of bounds"
panic if at=ΔFtail.tail:

    wcfs_test.py::test_wcfs_crash_old_data
    ---------------- live log call -----------------
    WARNING  ZODB.FileStorage:FileStorage.py:413 Ignoring index for /tmp/testdb_fs.nbSKXu/1.fs

    M: commit -> @at0 (03e5a31e5e5ef6bb)

    M: commit -> @at1 (03e5a31e5e63fa77)
    M:      f<0000000000000002>     [0]
    INFO     wcfs:__init__.py:293 starting for file:///tmp/testdb_fs.nbSKXu/1.fs ...
    I0120 16:50:22.136098  697106 wcfs.go:2393] start "/dev/shm/wcfs/93026d44ef96f87df2cc0e2e451c5aabee91b652" "file:///tmp/testdb_fs.nbSKXu/1.fs"
    I0120 16:50:22.136127  697106 wcfs.go:2399] (built with go1.17.6)
    W0120 16:50:22.136233  697106 storage.go:152] zodb: FIXME: open file:///tmp/testdb_fs.nbSKXu/1.fs: raw cache is not ready for invalidations -> NoCache forced
    INFO     wcfs:__init__.py:334 started pid697106 @ /dev/shm/wcfs/93026d44ef96f87df2cc0e2e451c5aabee91b652

    C: setup watch f<0000000000000002> @at1 (03e5a31e5e63fa77)
    #  pinok: {}
    panic: at out of bounds: at: @03e5a31e5e63fa77,  (tail, head] = (@03e5a31e5e63fa77, @03e5a31e5e63fa77]

    goroutine 7 [running]:
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.panicf(...)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/misc.go:47
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.(*ΔFtail).BlkRevAt(0xc0000a5d40, {0x969718, 0xc000076140}, 0xc0001a22a0, 0xc0001c0200, 0x3e5a31e5e63fa77)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/δftail.go:1077 +0xa45
    main.(*BigFile).readPinWatchers(0xc0001d0200, {0x969718, 0xc000076140}, 0x0, 0xffffffffffffffff)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1559 +0x2a5
    main.(*BigFile).readBlk(0xc0001d0200, {0x969718, 0xc000076140}, 0x0, {0xc000320000, 0x200000, 0x0})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1281 +0x4d2
    main.(*BigFile).Read.func1({0x969718, 0xc000076140})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1223 +0x71
    lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go.func1()
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:86 +0x68
    created by lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:83 +0x92
    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e5a31e5e5ef6bb)
            @at1 (03e5a31e5e63fa77) 0

    ...

        @func
        def test_wcfs_crash_old_data():
            # start wcfs with ΔFtail/ΔBtail not covering that initial data.
            t = tDB(old_data=[{0:'a'}]); zf = t.zfile; at1 = t.head
            defer(t.close)

            f = t.open(zf)

            # ΔFtail coverage is currently (at1,at1]
            wl = t.openwatch()
            wl.watch(zf, at1, {})

            # wcfs is crashing on readPinWatcher -> ΔFtail.BlkRevAt with
            #   "at out of bounds: at: @at1,  (tail,head] = (@at1,@at1]
            # because BlkRevAt(at=tail) query was disallowed.
    >       f.assertBlk(0, 'a')          # [0] becomes tracked

Still also crashing in test_wcfs_watch_setup.

Soon this test will also exercise functionality from isolation protocol
as well and so it will stop to be basic.

Move plus rename test_wcfs_basic_invalidation_wo_dFtail_coverage ->
test_wcfs_crash_old_data.

Still crashing in test_wcfs_watch_setup.

This semantically moves initialization code from
test_wcfs_basic_invalidation_wo_dFtail_coverage (see a7bf0311 "wcfs: Fix
crash if on invalidation handledδZ needs to access ZODB") to tDB itself,
and will be useful to exercise similar scenarios in other tests.

Still crashing in test_wcfs_watch_setup.

This should hopefully exercise codepaths in wcfs.go a bit more for
mistakes similar to a7bf0311 (wcfs: Fix crash if on invalidation
handledδZ needs to access ZODB) where the code on server side forgets to
put zhead's transaction into context.

Currently, because watching @tail is disallowed, this leads to panic triggered by test_wcfs_watch_setup:

    @at0 (03e59e3e606b89bb) -> @at1 (03e59e3e610692bb) -> @at2 (03e59e3e612a5811) -> @at3 (03e59e3e614fa9cc) -> @at4 (03e59e3e6189c3ee) -> @at5 (03e59e3e61af0baa)

    C: setup watch f<0000000000000002> @at0 (03e59e3e606b89bb)
    #  pinok: {0: @at0 (03e59e3e606b89bb), 2: @at0 (03e59e3e606b89bb), 3: @at0 (03e59e3e606b89bb), 5: @at0 (03e59e3e606b89bb)}
    panic: at out of bounds: at: @03e59e3e606b89bb,  (tail, head] = (@03e59e3e606b89bb, @03e59e3e61af0baa]

    goroutine 187 [running]:
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.panicf(...)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/misc.go:47
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.(*ΔFtail).BlkRevAt(0xc000077d40, {0x969718, 0xc000062940}, 0xc0003060c0, 0x4174f4, 0x3e59e3e606b89bb)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/δftail.go:1077 +0xa45
    main.(*WatchLink).setupWatch(0xc000108050, {0x969718, 0xc000062940}, 0x2, 0x3e59e3e606b89bb)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1754 +0xe3f
    main.(*WatchLink)._handleWatch(0x0, {0x969718, 0xc000062940}, {0xc00001c812, 0xa00000})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1973 +0x65
    main.(*WatchLink).handleWatch(0x74039b, {0x969718, 0xc000062940}, 0xc0000a4280, {0xc00001c812, 0x28})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1955 +0x10c
    main.(*WatchLink)._serve.func3({0x969718, 0xc000062940})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1944 +0x3c
    lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go.func1()
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:86 +0x68
    created by lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:83 +0x92
    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e59e3e606b89bb)
            @at1 (03e59e3e610692bb)     2
            @at2 (03e59e3e612a5811)     2 3 4 5
            @at3 (03e59e3e614fa9cc) 0   2     5
            @at4 (03e59e3e6189c3ee)     2   4 5
            @at5 (03e59e3e61af0baa)       3   5

However next we will anyway need to allow to setup watches @tail, and so
we will be fixing this and other errors in followup commits.

NOTE: we don't loose coverage for the case when ZBigFile is created after wcfs
startup due to test_wcfs_watch_2files, where that scenario is tested.

ΔFtail/ΔBtail tests also exercise ZBigFile/BTree epochs
(creation/deletion) well.

tDB.commit always creates only one transaction and so wcfs should be
expected to catch up with only that single one -> no need to loop.

No need to keep tDB._wc_zheadv as we have information about all
committed transactions in t.dFtail.

.commit is the only caller of ._wcsync. .commit is also the only place
via which tests are intended to modify ZODB.

- .commit performs ZODB commit and synchronizes WCFS to database changes;
- ._commit performs ZODB commit without WCFS synchronization.

We will soon need ._commit to create initial revisions for ZBigFile
while WCFS is not yet started.

If we do not setup logging explicitly, it will print only

    No handlers could be found for logger "wcfs"

instead of emitting useful details in before e.g.

    RuntimeError: fuse_unmount /dev/shm/wcfs/<X>: failed: fusermount: failed to unmount /dev/shm/wcfs/<X>: Device or resource busy
    (more details logged)

-> Fix it.

The invalidation logic is generally right, but invalidateBlk -> ΔFtail.BlkRevAt
was being called with ctx without transaction. As the result it was
panicking as

    panic: transaction: no current transaction

    goroutine 41 [running]:
    lab.nexedi.com/kirr/neo/go/transaction.currentTxn({0x9696d8, 0xc0000d8080})
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/transaction.go:59 +0x77
    lab.nexedi.com/kirr/neo/go/transaction.Current(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/api.go:206
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).checkTxnCtx(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:374
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).Get(0xc00010c640, {0x9696d8, 0xc0000d8080}, 0x4)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:331 +0x73
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.(*ΔFtail).BlkRevAt(0xc000077d40, {0x9696d8, 0xc0000d8080}, 0xc000064f60, 0x0, 0x3e5983329bbd100)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/δftail.go:1140 +0x39d
    main.(*BigFile).invalidateBlk.func1(0xc000164400, {0x9696d8, 0xc0000d8080}, 0xc0005a0000, 0x200000, 0x200000, {0xc0005a0000, 0x200000, 0x200000})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1089 +0xb8
    main.(*BigFile).invalidateBlk(0xc000164400, {0x9696d8, 0xc0000d8080}, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1105 +0x3bb
    main.(*Root).handleδZ.func3({0x9696d8, 0xc0000d8080})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:898 +0x34
    lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go.func1()
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:86 +0x68
    created by lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:83 +0x92

on any new change to tracked file block whose previous history is not covered by ΔFtail/ΔBtail.

Problem reported by @Francois.

Fixes the following warning that started to appear:

    kirr@deca:~/src/wendelin/wendelin.core/t$ ./qemu-runlinux -g  /home/kirr/src/linux/obj-qemu_debug/arch/x86/boot/bzImage /bin/bash
    qemu-system-x86_64: warning: 9p: Multiple devices detected in same VirtFS export, which might lead to file ID collisions and severe misbehaviours on guest! You should either use a separate export for each device shared from host or use virtfs option 'multidevs=remap'!

See https://wiki.qemu.org/Documentation/9psetup for documentation of
multidevs option.

In 5c8340d2 we said:

    dbclose now uses defer almost everywhere - there are still few places in
    tests, where one test function is opening/closing test database multiple
    times - those were not (yet ?) converted.

Let's convert those remaining places now, because when wendelin.core
tests are run wrt plain ZODB4 (contrary to ZODB4-wc2), many tests fail
at fileh_open time, e.g.

        @func
        def test_bigfile_filezodb_fileh_gc():
            root1= dbopen()
            conn1= root1._p_jar
            db   = conn1.db()
            defer(db.close)
            root1['zfile4'] = f1 = ZBigFile(blksize)
            transaction.commit()

    >       fh1  = f1.fileh_open()

    bigfile/tests/test_filezodb.py:588:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    bigfile/file_zodb.py:603: in fileh_open
        fileh = _ZBigFileH(self, _use_wcfs)
    bigfile/file_zodb.py:664: in __init__
        self.zfileh = zfile._v_file.fileh_open(use_wcfs)
    bigfile/_file_zodb.pyx:112: in wendelin.bigfile._file_zodb._ZBigFile.fileh_open
        pywconn   = wczsync.pywconnOf(zconn)
    wcfs/client/_wczsync.pyx:56: in wendelin.wcfs.client._wczsync.pywconnOf
        wconn = wc.connect(zconn_at(zconn))
    lib/zodb.py:163: in zconn_at
        "ZODB!1")
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    patch = 'conn:MVCC-via-loadBefore-only', details_link = 'ZODB!1'

        def _zassertHasNXDPatch(patch, details_link):
            if not _zhasNXDPatch(patch):
                raise AssertionError(
                    "ZODB%s is not patched with required Nexedi patch %r\n\tSee %s for details" %
    >               (zmajor, patch, details_link))
    E           AssertionError: ZODB4 is not patched with required Nexedi patch 'conn:MVCC-via-loadBefore-only'
    E               See ZODB!1 for details

and DB is left unclosed.

This change should reduce, if not completely fix, the number of
leaked /tmp/testdb_* directories for Wendelin.core.UnitTest-ZODB4(xfail) testsuite.

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

Here wcfs.go should have exited due to either unmount request, _or_
SIGTERM.

If first unmount fails, e.g. due to "device or resource is busy", we are
trying to unmount the filesystem the second time after force
kill/FUSE-abort (see 5f684a49 "wcfs: Server.stop: Make sure to remove
mount entry even if we had to use FUSE abort").

This way the caller of Server.stop should get an error only if that
second unmount fails, not on unmount-1 error, which should be considered
as internal to Server.stop implementation.

If we don't hide that unmount-1 error and raise it to the caller, from
outside it can confusingly look like "the server is successfully
stopped, but nevertheless we are raised with an error".

This way on plain ZODB4 the following non-wcfs tests will continue to
pass

   test.py/fs-!wcfs
   test.py/zeo-!wcfs
   test.py/neo-!wcfs

instead of failing as e.g. in here:

   https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706

On plain ZODB4 WCFS-related functionality - which uses zconn_at - will
continue to raise corresponding assertion in WCFS-related tests, as e.g. in

https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706/6

Server.stop currently tries to unmount, and if that fails invokes FUSE
abort and kills wcfs.go . However it does not call unmount the second
time after such abort, and this way the filesystem remains mounted (in
ENOTCONN state) and rmdir(mountpoint) fails.

-> Fix it by calling unmount the second time if we had to abort FUSE
connection. In that second try use lazy unmounting, because regular
unmount can still fail with "Device or resource busy" since there
could be still client file descriptors left pointing to the mounted
filesystem. With lazy mode unmounting + followup rmdir, hopefully,
always succeeds.

Here is example test run where one test timed out, FUSE connection was
aborted, but neither the filesystem was unmounted, nor mountpoint
directory was deleted, which led to all followup tests failing in setup
assert that testmountpoint does not exist:

https://nexedijs.erp5.net/#/test_result_module/20211112-1ACEA62D/22

This patch should fix those followup failures + fix another leakage of
WCFS mounts in real services.

By default every WCFS run creates several files in /tmp/wcfs.*.log.* and
without explicit cleanup those files are left hanging on testnodes. Over
last ~6 months we accumulated ~ 300K such files.

Don't allow those files to be leaked by instructing WCFS to log to
stderr during test run. This should be also useful to see details in the
test output.

With NEO we were creating test database on /tmp but we were not deleting
it in the end. As the result many /tmp/neo_XXXXXX non-empty directories
were being leaked.

-> Fix it by creating testdb directory outselves and removing it at the
end, similarly to FileStorage and ZEO.

Fixes: 7fc4ec66 (tests: Allow to test with ZEO & NEO ZODB storages)

We run tests with different GOMAXPROCS because some WCFS bugs are only
likely to trigger when there is only 1 or 2 main OS thread(s) in WCFS.

However test.go does not exercise filesystem functionality - it runs
unit tests for ZBlk decoding, ΔBtail and similar. At the same time
test.go:* currently occupies ~ 50% of whole time to run full testsuite
with the main consumer being ΔBtail random testing.

-> Run test.go only once. This should save ~ 1000s for each run and
lower whole time to run wendelin.core testsuite on testnode from
~60m -> to ~40 minutes.

Else every time test.py/wcfs is run several empty directories are left
in /dev/shm/wcfs - each corresponding to WCFS server that was
automatically spawned and stopped at the end of the test. Over time this
can accumulate to some big number as e.g. ~20000 of such directories
were left on the testnode during last 6 months.

This are the early days of WCFS - we want full details which in default
configuration might not be available to see if WCFS gets stuck for one
reason or another. See added comments for details.

`python setup.py egg_info` stopped working after we added non-ASCII
files, e.g. δbtail.go in 2ab4be93 (wcfs: xbtree: ΔBtail) and δftail.go
in f980471f (wcfs: zdata: ΔFtail):

    (neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core$ python setup.py egg_info
    running egg_info
    writing requirements to wendelin.core.egg-info/requires.txt
    writing wendelin.core.egg-info/PKG-INFO
    writing top-level names to wendelin.core.egg-info/top_level.txt
    writing dependency_links to wendelin.core.egg-info/dependency_links.txt
    writing entry points to wendelin.core.egg-info/entry_points.txt
    package init file '__init__.py' not found (or not a regular file)
    /usr/lib/python2.7/distutils/filelist.py:64: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
      sortable_files.sort()
    Traceback (most recent call last):
      File "setup.py", line 416, in <module>
        """.splitlines()]
      File "/home/kirr/src/tools/go/pygolang/golang/pyx/build.py", line 118, in setup
        setuptools_dso.setup(**kw)
      File "/home/kirr/src/wendelin/venv/z-dev/lib/python2.7/site-packages/setuptools_dso/__init__.py", line 37, in setup
        _setup(**kws)
      File "/home/kirr/src/wendelin/venv/z-dev/lib/python2.7/site-packages/setuptools/__init__.py", line 162, in setup
        return distutils.core.setup(**attrs)
      File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
        dist.run_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
        self.run_command(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
        cmd_obj.run()
      File "/home/kirr/src/wendelin/venv/z-dev/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 296, in run
        self.find_sources()
      File "/home/kirr/src/wendelin/venv/z-dev/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 303, in find_sources
        mm.run()
      File "/home/kirr/src/wendelin/venv/z-dev/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 538, in run
        self.filelist.sort()
      File "/usr/lib/python2.7/distutils/filelist.py", line 64, in sort
        sortable_files.sort()
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xce in position 0: ordinal not in range(128)

This happens becuase by default setuptools collects filenames as str, not
unicode, and our git_lsfiles - also registered into setuptools.file_finders
entrypoint - collects filenames as unicode. Previously everything was working
because there was no on-ASCII filenames, and so unicode vs str coercion worked
automatically. But now, after there is filename like 'δbtail.go', it stopped to
work and raises UnicodeDecodeError.

-> Fix it by adjusting git_lsfiles to collect filenames as UTF-8 encoded
strings instead of unicode.

Fix last-minute error that crept in during
4af54da9 :

    (neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs$ go test
    # lab.nexedi.com/nexedi/wendelin.core/wcfs
    ./wcfs.go:957:4: Errorf format %s has arg sk of wrong type *lab.nexedi.com/nexedi/wendelin.core/wcfs.FileSock

Amends 4430de41.

Add two functions, that were developed during wendelin.core 2 α, to the
package for completeness:

- map_zero_into_ro complements map_zero_ro, but mmaps into user-provided buffer.
- sync calls msync on the provided memory.

Remove outdated TODO because test_wcfs_watch_before_create passes this
days. It was fixed after ΔFtail was taught about epochs and the fix was
reflected in 63ae8326.

Amends 10f7153a.

Fix how unpatched ZODB4 is reported to lack required patch:

Before:

    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 251, in test_zconn_at
        assert zconn_at(conn1) == at0
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 162, in zconn_at
        assert 'conn:MVCC-via-loadBefore-only' in ZODB.nxd_patches, \
    AttributeError: 'module' object has no attribute 'nxd_patches'

After:

    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 251, in test_zconn_at
        assert zconn_at(conn1) == at0
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 163, in zconn_at
        "nexedi/ZODB!1")
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 191, in _zassertHasNXDPatch
        (zmajor, patch, details_link))
    AssertionError: ZODB4 is not patched with required Nexedi patch 'conn:MVCC-via-loadBefore-only'
            See nexedi/ZODB!1 for details

Fixes 1f866c00 (lib/zodb: Teach zconn_at to work on ZODB4).

It is not possible for WCFS to access data of in-RAM storage of another
process. But without explicit explanation the error message is confusing
- it was something like:

    NotImplementedError: don't know how to extract zurl from <ZODB.MappingStorage.MappingStorage object at 0x7f28f04cea10>

which suggests it was just not implemented.

By using WCFS as mmap-overlay for base data(*). WCFS-mode is still opt-in
with default remaining to use old full user-space virtual memory manager
mode as initially introduced in 2015.

Wendelin.core should be draftly usable in WCFS mode now.

This patch is organized as follows:

- file_zodb.cpp provides mmap-overlay operations for WCFS implemented via
  WCFS client library.
- file_zodb.py is adjusted accordingly to use WCFS if requested.
  Low-level things specific to gluing to file_zodb.cpp are moved to _file_zodb.pyx.
- the rest of the changes are drive-by by main ones.

(*) see the following patches for what is mmap-overlay:

- fae045cc  (bigfile/virtmem: Introduce "mmap overlay" mode)
- 23362204  (bigfile/py: Allow PyBigFile backend to expose "mmap overlay" functionality)

Some preliminary history:

01916f09    X Draft demo that reading data through wcfs works
fd58082a    X Fix build on old GCC
f622e751    X tests: Stop wcfs spawned during tests
f118617b    X tests: Don't try to stop wcfs that is already exited

Provide integration with virtmem, so that WCFS Mapping can be associated
and managed under virtmem VMA. In other words provide support so that WCFS can
be used as ZBigFile backend in "mmap overlay" mode (see fae045cc "bigfile/virtmem:
Introduce "mmap overlay" mode" for description of mmap-overlay mode).

We'll need this functionality for ZBigFile + WCFS client integration.

Virtmem integration will be tested via running whole wendelin.core functional
testsuite in wcfs-mode after the next patch.

Quoting added description:

---- 8< ----

Integration with wendelin.core virtmem layer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This client package can be used standalone, but additionally provides
integration with wendelin.core userspace virtual memory manager: when a
Mapping is created, it can be associated as serving base layer for a
particular virtmem VMA via FileH.mmap(vma=...). In that case, since virtmem
itself adds another layer of dirty pages over read-only base provided by
Mapping(+)

                 ┌──┐                      ┌──┐
                 │RW│                      │RW│    ← virtmem VMA dirty pages
                 └──┘                      └──┘
                           +
                                                   VMA base = X@at view provided by Mapping:

                                          ___        /@revA/bigfile/X
        __                                           /@revB/bigfile/X
               _                                     /@revC/bigfile/X
                           +                         ...
     ───  ───── ──────────────────────────   ─────   /head/bigfile/X

the Mapping will interact with virtmem layer to coordinate
updates to mapping virtual memory.

How it works
~~~~~~~~~~~~

Wcfs client integrates with virtmem layer to support virtmem handle
dirtying pages of read-only base-layer that wcfs client provides via
isolated Mapping. For wcfs-backed bigfiles every virtmem VMA is interlinked
with Mapping:

      VMA     -> BigFileH -> ZBigFile -----> Z
       ↑↓                                    O
     Mapping  -> FileH    -> wcfs server --> DB

When a page is write-accessed, virtmem mmaps in a page of RAM in place of
accessed virtual memory, copies base-layer content provided by Mapping into
there, and marks that page as read-write.

Upon receiving pin message, the pinner consults virtmem, whether
corresponding page was already dirtied in virtmem's BigFileH (call to
__fileh_page_isdirty), and if it was, the pinner does not remmap Mapping
part to wcfs/@revX/f and just leaves dirty page in its place, remembering
pin information in fileh._pinned.

Once dirty pages are no longer needed (either after discard/abort or
writeout/commit), virtmem asks wcfs client to remmap corresponding regions
of Mapping in its place again via calls to Mapping.remmap_blk for previously
dirtied blocks.

The scheme outlined above does not need to split Mapping upon dirtying an
inner page.

See bigfile_ops interface (wendelin/bigfile/file.h) that explains base-layer
and overlaying from virtmem point of view. For wcfs this interface is
provided by small wcfs client wrapper in bigfile/file_zodb.cpp.

(+) see bigfile_ops interface (wendelin/bigfile/file.h) that gives virtmem
    point of view on layering.

----------------------------------------

Some preliminary history:

f330bd2f    X wcfs/client: Overview += interaction with virtmem layer