We use &vector[0] in several places to get the pointer to vector data,
but std::vector[i] states invariant that i must be < vector.size(). If
the vector is not empty - then &vector[0] works ok. However if the
vector is empty - then the invariant is broken.

Normally it should not be a problem, since with empty vector we usually
use (&vector[0], vector.size()) pair, and for empty vector its size is
zero. In other words even if &vector[0] returns some not good pointer
with zero size that data should never be accessed. But a build of
libstdc++ could have asserts turned on and verify that i is actually <
size. For example with default install on Fedora the following program
with plain select()

    from golang import select
    select()

crashes with assert triggered on the breakage of that invariant

    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib64/libthread_db.so.1".
    /usr/include/c++/10/bits/stl_vector.h:1042: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = golang::_selcase; _Alloc = std::allocator<golang::_selcase>; std::vector<_Tp, _Alloc>::reference = golang::_selcase&; std::vector<_Tp, _Alloc>::size_type = long unsigned int]: Assertion '__builtin_expect(__n < this->size(), true)' failed.

    Program received signal SIGABRT, Aborted.
    0x00007ffff7e22a25 in raise () from /lib64/libc.so.6
    (gdb) bt
    #0  0x00007ffff7e22a25 in raise () from /lib64/libc.so.6
    #1  0x00007ffff7e0b895 in abort () from /lib64/libc.so.6
    #2  0x00007fffea638d6d in std::__replacement_assert (__condition=0x7fffea63ea88 "__builtin_expect(__n < this->size(), true)",
        __function=0x7fffea63eab8 "std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = golang::_selcase; _Alloc = std::allocator<golang::_selcase>; std::vector<_Tp, "..., __line=1042, __file=0x7fffea63ebe8 "/usr/include/c++/10/bits/stl_vector.h")
        at /usr/include/c++/10/x86_64-redhat-linux/bits/c++config.h:2552
    #3  std::vector<golang::_selcase, std::allocator<golang::_selcase> >::operator[] (this=<synthetic pointer>, __n=0)
        at /usr/include/c++/10/bits/stl_vector.h:1042
    #4  std::vector<golang::_selcase, std::allocator<golang::_selcase> >::operator[] (__n=0, this=<synthetic pointer>)
        at /usr/include/c++/10/bits/stl_vector.h:1040
    #5  __pyx_pf_6golang_7_golang_6pyselect (__pyx_self=<optimized out>, __pyx_v_pycasev=<optimized out>) at golang/_golang.cpp:7930
    #6  __pyx_pw_6golang_7_golang_7pyselect (__pyx_self=<optimized out>, __pyx_args=<optimized out>, __pyx_kwds=<optimized out>) at golang/_golang.cpp:7284
    #7  0x00007ffff7bceeab in cfunction_call_varargs (func=<built-in function pyselect>, args=(), kwargs=0x0)
        at /usr/src/debug/python3-3.8.10-1.fc32.x86_64/Objects/call.c:743
    ...

-> Fix it all via replacing &vector[0] with vector.data() everywhere.

No need to add new test since select() is used by test_blockforever()
and running that test on Fedora was crashing as well.

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!22

Since the beginning (9bf03d9c "py.bench: New command to benchmark python
code similarly to `go test -bench`") py.bench was automatically
discovering benchmarks in bench_*.py files only. This was inherited from
wendelin.core which keeps its benchmarks in those files.

However in pygolang, following Go convention(*), we already have several
benchmarks that reside together with tests in same *_test.py files. And
currently just running py.bench does not discover them.

-> Let's fix this and teach py.bench to automatically discover
benchmarks in the test files by default as well.

Pytest's default is to look for tests in test_*.py and *_test.py (+).
Add those patterns and also keep bench_*.py for backward compatibility.

Before this patch running py.bench inside pygolang repository does not
run any benchmark at all. After the patch py.bench runs all the
benchmarks by default:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ py.bench
    ========================= test session starts ==========================
    platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.10.0, pluggy-0.13.1
    rootdir: /home/kirr/src/tools/go/pygolang
    plugins: timeout-1.4.2, profiling-1.7.0, mock-2.0.0
    collected 18 items

    pymod: golang/golang_str_test.py
    Benchmarkstddecode              2000000 0.756 µs/op
    Benchmarkudecode                20000   74.359 µs/op
    Benchmarkstdencode              3000000 0.327 µs/op
    Benchmarkbencode                40000   32.613 µs/op

    pymod: golang/golang_test.py
    Benchmarkpyx_select_nogil       500000  2.051 µs/op
    Benchmarkpyx_go_nogil           90000   12.177 µs/op
    Benchmarkpyx_chan_nogil         600000  1.826 µs/op
    Benchmarkgo                     80000   13.267 µs/op
    Benchmarkchan                   500000  2.076 µs/op
    Benchmarkselect                 300000  3.835 µs/op
    Benchmarkdef                    30000000        0.035 µs/op
    Benchmarkfunc_def               40000   29.387 µs/op
    Benchmarkcall                   30000000        0.043 µs/op
    Benchmarkfunc_call              2000000 0.819 µs/op
    Benchmarktry_finally            20000000        0.096 µs/op
    Benchmarkdefer                  600000  1.755 µs/op

    pymod: golang/sync_test.py
    Benchmarkworkgroup_empty        40000   25.807 µs/op
    Benchmarkworkgroup_raise        40000   31.637 µs/op                     [100%]

    =========================== warnings summary ===========================

(*) see https://pkg.go.dev/cmd/go#hdr-Test_packages
(+) see https://docs.pytest.org/en/7.1.x/reference/reference.html#confval-python_files

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!20

We recently moved our custom UTF-8 encoding/decoding routines to Cython.
Now we can start taking speedup advantage on C level to make our own
UTF-8 decoder a bit less horribly slow on py2:

    name       old time/op  new time/op  delta
    stddecode   752ns ± 0%   743ns ± 0%   -1.19%  (p=0.000 n=9+10)
    udecode     216µs ± 0%    75µs ± 0%  -65.19%  (p=0.000 n=9+10)
    stdencode   328ns ± 2%   327ns ± 1%     ~     (p=0.252 n=10+9)
    bencode    34.1µs ± 1%  32.1µs ± 1%   -5.92%  (p=0.000 n=10+10)

So it is ~ 3x speedup for u(), but still significantly slower compared
to std unicode.decode('utf-8').

Only low-hanging fruit here to make _utf_decode_rune a bit more prompt,
since it sits in the most inner loop. In the future
_utf8_decode_surrogateescape might be reworked as well to avoid
constructing resulting unicode via py-level list of py-unicode character
objects. And similarly for _utf8_encode_surrogateescape.

On py3 the performance of std and u/b decode/encode is approximately the same.

/trusted-by @jerome
/reviewed-on nexedi/pygolang!19

Error rune (u+fffd) is returned by _utf8_decode_rune to indicate an
error in decoding. But the error rune itself is valid unicode codepoint:

   >>> x = u"�"
   >>> x
   u'\ufffd'
   >>> x.encode('utf-8')
   '\xef\xbf\xbd'

This way only (r=_rune_error, size=1) should be treated by the caller as
utf8 decoding error.

But e.g. strconv.quote was not careful to also inspect the size, and this way
was quoting � into just "\xef" instead of "\xef\xbf\xbd".
_utf8_decode_surrogateescape was also subject to similar error.

-> Fix it.

Without the fix e.g. added test for strconv.quote fails as

    >           assert quote(tin) == tquoted
    E           assert '"\xef"' == '"�"'
    E             - "\xef"
    E             + "�"

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18

So that those routines could be just called and do what is expected
without the caller caring whether it is py2 or py3. We will soon need to
use those routines from several callsites, and having that py2/py3
conditioning being spread over all usage places would be inconvenient.

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18

- Move _utf8_decode_rune, _utf8_decode_surrogateescape, _utf8_encode_surrogateescape out from strconv into _golang_str
- Factor _bstr/_ustr code into pyb/pyu. _bstr/_ustr become plain wrappers over pyb/pyu.
- work-around emerged golang ↔ strconv dependency with at-runtime import.

Moved routines belong to the main part of golang strings processing
-> their home should be in _golang_str.pyx

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18

We are going to significantly extend py-strings related functionality soon
- to the point where amount of strings related code will be
approximately the same compared to the amount of all other
python-related code inside golang module.

-> First move everything related to py strings to dedicated
_golang_str.pyx as a preparatory step.

Keep that new file included from _golang.pyx instead of being real new
module, because we want strings functionality to be provided by golang
main namespace itself, and to ease internal code interdependencies.

Plain code movement.

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18

On Python2 without .tp_print printing _pystr crashes as:

    pygolang$ ./golang/testprog/golang_test_str.py
    Traceback (most recent call last):
      File "./golang/testprog/golang_test_str.py", line 39, in <module>
        main()
      File "./golang/testprog/golang_test_str.py", line 34, in main
        print("print(qq(b)):", qq(sb))
    RuntimeError: print recursion

See added comments for details.

Add convenient utility to read whole file and return its content
similarly to Go. The code is taken from wendelin.core:

https://lab.nexedi.com/nexedi/wendelin.core/blob/wendelin.core-2.0.alpha1-18-g38dde766/wcfs/client/wcfs_misc.cpp#L246-281

Provide os/signal package that can be used to setup signal delivery to nogil
channels. This way for user code signal handling becomes regular handling of a
signalling channel instead of being something special or limited to only-main
python thread. The rationale for why we need it is explained below:

There are several problems with regular python's stdlib signal module:

1. Python2 does not call signal handler from under blocked lock.acquire.
   This means that if the main thread is blocked waiting on a semaphore,
   signal delivery will be delayed indefinitely, similarly to e.g. problem
   described in nexedi/nxdtest!14 (comment 147527)
   where raising KeyboardInterrupt is delayed after SIGINT for many,
   potentially unbounded, seconds until ~semaphore wait finishes.

   Note that Python3 does not have this problem wrt stdlib locks and
   semaphores, but read below for the next point.

2. all pygolang communication operations (channels send/recv, sync.Mutex,
   sync.RWMutex, sync.Sema, sync.WaitGroup, sync.WorkGroup, ...) run with
   GIL released, but if blocked do not handle EINTR and do not schedule
   python signal handler to run (on main thread).

   Even if we could theoretically adjust this behaviour of pygolang at python
   level to match Python3, there are also C++ and pyx/nogil worlds. And we want gil
   and nogil worlds to interoperate (see https://pypi.org/project/pygolang/#cython-nogil-api),
   so that e.g. if completely nogil code happens to run on the main thread,
   signal handling is still possible, even if that signal handling was setup at
   python level.

With signals delivered to nogil channels both nogil world and python
world can setup signal handlers and to be notified of them irregardles
of whether main python thread is currently blocked in nogil wait or not.

/reviewed-on nexedi/pygolang!17

To convert an object to str of current python.
It will be handy to use __pystr when implementing __str__ methods.

/reviewed-on nexedi/pygolang!17

Provide C++ package "os" with File, Pipe, etc similarly to what is
provided on Go side. The package works through IO methods provided by
runtimes.

We need IO facility because os/signal package will need to use
pipe in cooperative IO mode in its receiving-loop goroutine.

os.h and os.cpp are based on drafts from wendelin.core:

https://lab.nexedi.com/nexedi/wendelin.core/blob/wendelin.core-2.0.alpha1-18-g38dde766/wcfs/client/wcfs_misc.h
https://lab.nexedi.com/nexedi/wendelin.core/blob/wendelin.core-2.0.alpha1-18-g38dde766/wcfs/client/wcfs_misc.cpp

/reviewed-on nexedi/pygolang!17

Else as https://github.com/python-greenlet/greenlet/pull/285
demonstrates there can be segmentation faults and crashes due to
exceptions from one greenlet propagating to C stack of another greenlet.

No test here. I've tried to do it, but with gevent (contrary to plain
greenlets), spawning new task only schedules corresponding greenlet to
run in the end of current event loop cycle instead of switching to
created greenlet immediately. With this delaying, it was hard for me to
develop corresponding test in a reasonable time.

Hopefully having the test I've done for greenlet itself + hereby
protection is good enough.

/reviewed-on nexedi/pygolang!17

This package provides special kind of atomic that is automatically reset
to zero after fork in child. This kind of atomic will be used in os
package to implement IO that does not deadlock in Close after fork.

/reviewed-on nexedi/pygolang!17

This package provides wrappers to OS system calls.
Minimal functionality that will be used to implement os and os/signal pacakges.

/reviewed-on nexedi/pygolang!17

Package os will need to access runtime operations to implement IO.

/reviewed-on nexedi/pygolang!17

We will soon need to use common functionality(*) from both runtimes and
other packages. The "other packages" are all C++ and it is handy to keep
common functionality in C++ as well. While we could also maintain
`extern "C"` interface, it duplicates the work. Let's switch everything
to C++ to ease further maintenance.

(*) e.g. package internal/syscall from both runtimes and from package os.

/reviewed-on !17

Without defined std::hash it is not possible to use channels as keys in
dict or set. We will be using set<chan> in os/signal package
implementation.

/reviewed-on nexedi/pygolang!17

I forgot to include it in ad00be70 (libgolang: Introduce runtimes).

/reviewed-on nexedi/pygolang!17

/reviewed-on nexedi/pygolang!17

    $ check-manifest
    lists of files in version control and sdist do not match!
    missing from sdist:
      .nxdtest

/reviewed-on nexedi/pygolang!17

See https://foss.heptapod.net/pypy/pypy/-/issues/3610 for stdin case.
-m started to behave as CPython.

On py3 'ps1' and b'ps1' are different keys and it was failing on PyPy3 as follows:

            # -i after stdin (also tests interactive mode as -i forces interactive even on non-tty)
            d = {
                b'hellopy': b(hellopy),
                b'ps1':     b'' # cpython emits prompt to stderr
            }
            if is_pypy and not is_gpython:
                d['ps1'] = b'>>>> ' # native pypy emits prompt to stdout and >>>> instead of >>>
            _ = pyout(['-i'], stdin=b'import hello\n', cwd=testdata)
    >       assert _ == b"%(ps1)shello\nworld\n['']\n%(ps1)s"           % d
    E       assert b">>>> hello\...\n['']\n>>>> " == b"hello\nworld\n['']\n"
    E         At index 0 diff: b'>' != b'h'
    E         Full diff:
    E         - b"hello\nworld\n['']\n"
    E         + b">>>> hello\nworld\n['']\n>>>> "
    E         ?   +++++                    +++++

    gpython/gpython_test.py:200: AssertionError

Fix it.
Amends e205dbf6 (gpython: Implement -i (interactive inspect after program run) + promised interactive-mode tests)

When _BaseCtx is setting up cancel propagation it locks a parent,
checks for parent.err != nil, and, if it is, calls
ctx._cancel(parent.err) _with_ _holding_ parent.mu. Since _cancel
internally also goes through parents and locks them, this was deadlocking
on the second call to parent.mu.lock().

-> Fix it by calling ctx._cancel(err) in the constructor outside of
parent lock.

The bug was there from the beginning - from e9567c7b (context: New
package that mirrors Go's context).

/trusted-by @jerome
/reviewed-on !16

- _propagateCancel is used only in _BaseCtx constructor -> inline it
  there. Being run in the constructor makes it clear that this code
  works on new _BaseCtx object with empty set of children.

- since _cancelFrom interacts with the code moved from _propagateCancel,
  also move it to be close to cancel propagation setup.

No functional changes, just plain code movement.

/trusted-by @jerome
/reviewed-on !16

The function that verifies "no exception/panic" case was prepared but not called at all.
This oversight was there from 5146eb0b "Add support for defer & recover".

/reviewed-by @jerome
/reviewed-on !15

Because else sys.modules['__main__'] points to the module of gpython
itself, not user program. This difference can be important when e.g.
user defines classes in main module and tries to pickle them. Such
pickling will fail if gpython is not careful to run user's code in its
own main.

Without this patch added test fails as

      File "check_main.py", line 51, in <module>
        main()
      File "check_main.py", line 39, in main
        assert hasattr(mainmod, 'MyUniqueClassXYZ'),        dir(mainmod)
    AssertionError: ['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'main']

The problem was discovered while trying to run test program from
https://github.com/zopefoundation/ZEO/issues/166 :

    $ gpython zopenrace-zeo.py
    Traceback (most recent call last):
      File "/home/kirr/tmp/trashme/Z/py2.venv/bin/gpython", line 8, in <module>
        sys.exit(main())
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 395, in main
        pymain(argv, init)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 266, in pymain
        run()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 172, in run
        _execfile(filepath, g)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 275, in _execfile
        six.exec_(code, globals, locals)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/six.py", line 719, in exec_
        exec("""exec _code_ in _globs_, _locs_""")
      File "<string>", line 1, in <module>
      File "zopenrace-zeo.py", line 205, in <module>
        main()
      File "zopenrace-zeo.py", line 190, in main
        init()
      File "zopenrace-zeo.py", line 126, in init
        transaction.commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_manager.py", line 257, in commit
        return self.manager.commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_manager.py", line 134, in commit
        return self.get().commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 282, in commit
        reraise(t, v, tb)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 273, in commit
        self._commitResources()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 456, in _commitResources
        reraise(t, v, tb)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 430, in _commitResources
        rm.commit(self)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 497, in commit
        self._commit(transaction)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 546, in _commit
        self._store_objects(ObjectWriter(obj), transaction)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 578, in _store_objects
        p = writer.serialize(obj)  # This calls __getstate__ of obj
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/serialize.py", line 430, in serialize
        return self._dump(meta, obj.__getstate__())
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/serialize.py", line 439, in _dump
        self._p.dump(state)
    PicklingError: Can't pickle <class '__main__.PInt'>: attribute lookup __main__.PInt failed

/reviewed-by @jerome
/reviewed-on !15

To match python behaviour:

    $ python >/dev/null
    Python 2.7.18 (default, Apr 20 2020, 20:30:41)
    [GCC 9.3.0] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> 1+1			# NOTE the prompt (printed to stderr)

    $ python 2>/dev/null
    >>> 1+1			# NOTE the prompt (printed to stdout)

Tests pending.

/reviewed-by @jerome
/reviewed-on !15

Not through interactive console, because it was printing prompts, while
`python <prog.py` does not emit anything:

    $ cat prog.py
    print 'Hello World'

    $ python <prog.py
    Hello World

    $ gpython <prog.py
    Python 2.7.18 (default, Apr 20 2020, 20:30:41)
    [GCC 9.3.0] [GPython 0.0.8] [gevent 20.9.0] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    Hello World

After the patch gpython output is the same as of python:

    $ gpython <prog.py
    Hello World

Test coverage for interactive mode is pending.
We'll add them soon in a follow-up patch after implementing -i.

For _interact - by this patch logic - we should be dropping custom
raw_input, since now _interact is called only when sys.stdin is tty.
But we'll soon be invoking _interact when stdin is not a tty (for
`gpython -i <prog.py`), so leave that logic in place as is.

/reviewed-by @jerome
/reviewed-on !15

We will soon need this to be run from several places when implementing
support for -i.

/reviewed-by @jerome
/reviewed-on !15

setuptools_dso 2 started to emit those autogenerated files.
See https://github.com/mdavidsaver/setuptools_dso/pull/15 for details.

Those are quote codes that Go strconv.Quote might produce. And even
though Python does not use them when quoting, it too handles those quote
codes when decoding:

    In [1]: '\r'
    Out[1]: '\r'

    In [2]: '\a\b\v\f'
    Out[2]: '\x07\x08\x0b\x0c'

https://github.com/python/cpython/blob/2.7.18-0-g8d21aa21f2c/Objects/stringobject.c#L677-L688

-> Teach strconv.unquote + friends to handle them as well.

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!14

https://bugs.python.org/issue20443

This broke test_defer_excchain_dump because
testprog/golang_test_defer_excchain.txt is prepared with output where
`python file` shows that file name in traceback as it was specified on
the command line, e.g.

    .../pygolang/golang/testprog$ python golang_test_defer_excchain.py
    Traceback (most recent call last):
      File ".../pygolang/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File "golang_test_defer_excchain.py", line 42, in main
        raise RuntimeError("err")
    RuntimeError: err

while with py39 it became

    .../pygolang/golang/testprog$ python golang_test_defer_excchain.py
    Traceback (most recent call last):
      File ".../pygolang/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File ".../pygolang/golang/testprog/golang_test_defer_excchain.py", line 42, in main
        raise RuntimeError("err")
    RuntimeError: err

(notice the difference related to "line 42")

-> Fix it:

- amend the test to conditionally prefix golang_test_defer_excchain.py
  in expected output with PYGOLANG/golang/testprog/ if it is Python >= 3.9.
- amend `gpython file` to match behaviour of underlying `python file`,
  so that the test passes unconditionally whether it is run by python or
  gpython.

--------

@jerome also says (!13 (comment 122826)):

FYI, buildout (and many zope packages) essentially use doctests for
testing and they had to deal with similar differences in output.
`zope.testing` comes with a doctest checker named "renormalizing" which
normalize output with regular expressions, see for example setup of some
buildout test [here](https://github.com/buildout/buildout/blob/db3d6e2fbf5d7ff2cc4b2507253c7a221cfc3e32/src/zc/buildout/tests.py#L3615-L3651).
We definitely don't need this here for the moment, but maybe one day it
can be useful.

/reviewed-on !13

Debian testing recently switched default python3 to be python3.9.
Let's make sure pygolang works with that python version.
Currently some tests fail - this will be addressed in the next patch.

/reviewed-on !13

So that it becomes possible to write

    with WorkGroup(ctx) as wg:
        wg.go(f1)
        wg.go(f2)

instead of

    wg = WorkGroup(ctx)
    defer(wg.wait)
    wg.go(f1)
    wg.go(f2)

or

    wg = WorkGroup(ctx)
    wg.go(f1)
    wg.go(f2)
    wg.wait()

This is sometimes handy and is referred to as "structured concurrency"
in Python world.

sync.Sema, sync.Mutex, sync.RWMutex already support "with".
sync.WaitGroup is imho too low-level, but we might consider adding
"with" support for it in the future as well.

In general pygolang way is to use defer instead of plugging all classes
with __enter__/__exit__ "with" support, but for small well-known class of
concurrency-related things its seems "with" support is worth it:

- having "with" for sync.Mutex+co allows it to be used as a drop-in
  replacement instead of threading.Lock+co, and
- having "with" for sync.WorkGroup - the most commonly-used tool to
  spawn jobs and wait for their completion - makes it on-par with
  "structured concurrency".

/reviewed-on !12

WorkGroup methods work on PyWorkGroup, not on PyWaitGroup.
This probably used to work because Cython ignores (?) provided type of self?

/reviewed-on !12

Similarly to test_pymain_syspath we cannot run this test on SlapOS because with
buildout raw underlying python interpreter does not have access to eggs with
which gpython script was generated.

See 0fa9d6e7 and 92bb5bcc for details.

Without this patch running pygolang tests fails on SlapOS as shown below:

    (pygolang-env) slapuser34@vifibcloud-rapidspace-hosting-007:~/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev$ gpython -m pytest -vsx -k relpath
    ================================================= test session starts ==================================================
    platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.9.0, pluggy-0.13.1 -- /srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/bin/gpython
    cachedir: .pytest_cache
    rootdir: /srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev
    collected 115 items / 114 deselected / 1 selected

    gpython/gpython_test.py::test_pymain_run_via_relpath Traceback (most recent call last):
      File "./__init__.py", line 511, in <module>
        main()
      File "./__init__.py", line 395, in main
        pymain(argv, init)
      File "./__init__.py", line 217, in pymain
        init()
      File "./__init__.py", line 364, in init
        import gevent
    ImportError: No module named gevent
    FAILED

    ======================================================= FAILURES =======================================================
    _____________________________________________ test_pymain_run_via_relpath ______________________________________________

        @gpython_only
        def test_pymain_run_via_relpath():
            argv = ['-c',  'import sys; print(sys.version)']
            out1 = pyout(                    argv, pyexe=sys.executable)
    >       out2 = pyout(['./__init__.py'] + argv, pyexe=sys._gpy_underlying_executable, cwd=here)

    gpython/gpython_test.py:301:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    golang/golang_test.py:1842: in pyout
        return pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    argv = ['./__init__.py', '-c', 'import sys; print(sys.version)'], stdin = None, stdout = '', stderr = None
    kw = {'cwd': '/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev/gpython', 'pyexe': '/srv/slapgrid/slappart34/srv//runner//shared/python2.7/44578f5389723084b136574c34bc8023/bin/python2.7'}
    retcode = 1

        def pyrun(argv, stdin=None, stdout=None, stderr=None, **kw):
            retcode, stdout, stderr = _pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
            if retcode:
    >           raise RuntimeError(' '.join(argv) + '\n' + (stderr and str(stderr) or '(failed)'))
    E           RuntimeError: ./__init__.py -c import sys; print(sys.version)
    E           (failed)

    golang/golang_test.py:1836: RuntimeError
    ======================================= 1 failed, 114 deselected in 0.73 seconds =======================================

Fixes 076cdd8f (gpython: Fix crash when invoked as e.g. ./bin/gpython).

/reviewed-on !11