sync: Move/Port sync.WorkGroup to C++/Pyx nogil

Provide sync.WorkGroup that can be used directly from C++ and Pyx/nogil codes.
Python-level sync.WorkGroup becomes a wrapper around pyx/nogil one.

Like with context (2a359791 "context: Move/Port context package to
C++/Pyx nogil"), timers (b073f6df "time: Move/Port timers to C++/Pyx
nogil") and interfaces (5a99b769 "libgolang: Start providing
interfaces") memory for on-heap allocated WorkGroup is managed
automatically.

Python-level tests should be enough to cover C++/Pyx functionality at
zero-level approximation.

golang/_sync.pxd

@@ -19,6 +19,7 @@
 # See https://www.nexedi.com/licensing for rationale and options.
 """Package sync mirrors and amends Go package sync.
 
+ - `WorkGroup` allows to spawn group of goroutines working on a common task(*).
  - `Once` allows to execute an action only once.
  - `WaitGroup` allows to wait for a collection of tasks to finish.
  - `Sema`(*) and `Mutex` provide low-level synchronization.
@@ -28,6 +29,9 @@ See also https://golang.org/pkg/sync for Go sync package documentation.
 (*) not provided in Go version.
 """
 
+from golang cimport error, refptr
+from golang cimport context
+
 cdef extern from "golang/sync.h" namespace "golang::sync" nogil:
     cppclass Sema:
         Sema()
@@ -45,3 +49,15 @@ cdef extern from "golang/sync.h" namespace "golang::sync" nogil:
         void done()
         void add(int delta)
         void wait()
+
+    # WorkGroup
+    cppclass _WorkGroup:
+        void  go(...)                       # ... = func<error(context::Context)>
+        error wait()
+
+    cppclass WorkGroup (refptr[_WorkGroup]):
+        # WorkGroup.X = WorkGroup->X in C++.
+        void  go    "_ptr()->go"    (...)   # ... = func<error(context::Context)>
+        error wait  "_ptr()->wait"  ()
+
+    WorkGroup NewWorkGroup(context.Context ctx)

golang/_sync.pyx

@@ -23,7 +23,17 @@ from __future__ import print_function, absolute_import
 
 from cython  cimport final
 from cpython cimport PyObject
-from golang  cimport topyexc
+from golang  cimport nil, newref, topyexc
+from golang  cimport context
+from golang.pyx cimport runtime
+ctypedef runtime._PyError* runtime_pPyError # https://github.com/cython/cython/issues/534
+
+# internal API sync.h exposes only to sync.pyx
+cdef extern from "golang/sync.h" namespace "golang::sync" nogil:
+    context.Context _WorkGroup_ctx(_WorkGroup *_wg)
+
+from libcpp.cast cimport dynamic_cast
+
 
 @final
 cdef class PySema:
@@ -124,6 +134,102 @@ cdef class PyWaitGroup:
             waitgroup_wait_pyexc(&pywg.wg)
 
 
+@final
+cdef class PyWorkGroup:
+    """WorkGroup is a group of goroutines working on a common task.
+
+    Use .go() to spawn goroutines, and .wait() to wait for all of them to
+    complete, for example:
+
+      wg = WorkGroup(ctx)
+      wg.go(f1)
+      wg.go(f2)
+      wg.wait()
+
+    Every spawned function accepts context related to the whole work and derived
+    from ctx used to initialize WorkGroup, for example:
+
+      def f1(ctx):
+          ...
+
+    Whenever a function returns error (raises exception), the work context is
+    canceled indicating to other spawned goroutines that they have to cancel their
+    work. .wait() waits for all spawned goroutines to complete and returns/raises
+    error, if any, from the first failed subtask.
+
+    WorkGroup is modelled after https://godoc.org/golang.org/x/sync/errgroup but
+    is not equal to it.
+    """
+    cdef WorkGroup         wg
+    cdef context.PyContext _pyctx   # PyContext wrapping wg._ctx
+
+    def __init__(PyWorkGroup pywg, context.PyContext pyctx):
+        with nogil:
+            pywg.wg = workgroup_new_pyexc(pyctx.ctx)
+        pywg._pyctx = context.PyContext.from_ctx(_WorkGroup_ctx(pywg.wg._ptr()))
+
+    def __dealloc__(PyWorkGroup pywg):
+        pywg.wg = NULL
+
+    def go(PyWorkGroup pywg, f, *argv, **kw):
+        # run f(._pyctx, ...) via _PyCtxFunc whose operator()(ctx)
+        # verifies that ctx == ._pyctx.ctx and tails to pyrunf().
+        def pyrunf():
+            f(pywg._pyctx, *argv, **kw)
+        with nogil:
+            workgroup_go_pyctxfunc_pyexc(pywg.wg, pywg._pyctx.ctx, <PyObject*>pyrunf)
+
+    def wait(PyWaitGroup g):
+        cdef error err
+        with nogil:
+            err = workgroup_wait_pyexc(g.wg)
+
+        if err == nil:
+            return
+
+        # check that err is python error
+        cdef runtime._PyError *_pyerr = dynamic_cast[runtime_pPyError](err._ptr())
+        cdef runtime.PyError   pyerr  = newref(_pyerr)
+        if pyerr == nil:
+            # NOTE this also includes runtime.ErrPyStopped
+            raise AssertionError("non-python error: " + err.Error())
+
+        # reraise pyerr with original traceback
+        pyerr_reraise(pyerr)
+
+# _PyCtxFunc complements PyWorkGroup.go() : it's operator()(ctx) verifies that
+# ctx is expected context and further calls python function without any arguments.
+# PyWorkGroup.go() arranges to use python functions that are bound to PyContext
+# corresponding to ctx.
+cdef extern from * nogil:
+    """
+    using namespace golang;
+    struct _PyCtxFunc : pyx::runtime::PyFunc {
+        context::Context _ctx;  // function is bound to this context
+
+        _PyCtxFunc(context::Context ctx, PyObject *pyf)
+                : PyFunc(pyf) {
+            this->_ctx = ctx;
+        }
+
+        // dtor - default is ok
+        // copy - default is ok
+
+        // WorkGroup calls f(ctx). We check that ctx is expected WorkGroup._ctx
+        // and call pyf() instead (which PyWorkgroup setup to be closure to call f(pywg._pyctx)).
+        error operator() (context::Context ctx) {
+            if (this->_ctx != ctx)
+                panic("_PyCtxFunc: called with unexpected context");
+            return PyFunc::operator() ();
+        }
+    };
+    """
+    cppclass _PyCtxFunc (runtime.PyFunc):
+        __init__(context.Context ctx, PyObject *pyf)
+        error operator() ()
+
+
+
 # ---- misc ----
 
 cdef nogil:
@@ -144,3 +250,13 @@ cdef nogil:
         wg.add(delta)
     void waitgroup_wait_pyexc(WaitGroup *wg)                except +topyexc:
         wg.wait()
+
+    WorkGroup workgroup_new_pyexc(context.Context ctx)      except +topyexc:
+        return NewWorkGroup(ctx)
+    void workgroup_go_pyctxfunc_pyexc(WorkGroup wg, context.Context ctx, PyObject *pyf) except +topyexc:
+        wg.go(_PyCtxFunc(ctx, pyf))
+    error workgroup_wait_pyexc(WorkGroup wg)                except +topyexc:
+        return wg.wait()
+
+    void pyerr_reraise(runtime.PyError pyerr)   except *:
+        runtime.PyErr_ReRaise(pyerr)

golang/sync.cpp

@@ -96,4 +96,55 @@ void WaitGroup::wait() {
     done.recv();
 }
 
+// WorkGroup
+_WorkGroup::_WorkGroup()  {}
+_WorkGroup::~_WorkGroup() {}
+void _WorkGroup::decref() {
+    if (__decref())
+        delete this;
+}
+
+WorkGroup NewWorkGroup(context::Context ctx) {
+    WorkGroup g = adoptref(new _WorkGroup());
+
+    tie(g->_ctx, g->_cancel) = context::with_cancel(ctx);
+    return g;
+}
+
+void _WorkGroup::go(func<error(context::Context)> f) {
+    // NOTE = refptr<_WorkGroup> because we pass ref to g to spawned worker.
+    WorkGroup g = newref(this);
+
+    g->_wg.add(1);
+
+    golang::go([g, f]() {       // NOTE g ref passed to spawned worker
+        defer([&]() {
+            g->_wg.done();
+        });
+
+        error err = f(g->_ctx); // TODO consider also propagating panic
+        if (err == NULL)
+            return;
+
+        g->_mu.lock();
+        defer([&]() {
+            g->_mu.unlock();
+        });
+
+        if (g->_err == NULL) {
+            // this goroutine is the first failed task
+            g->_err = err;
+            g->_cancel();
+        }
+    });
+}
+
+error _WorkGroup::wait() {
+    _WorkGroup& g = *this;
+
+    g._wg.wait();
+    g._cancel();
+    return g._err;
+}
+
 }}  // golang::sync::

golang/sync.h

@@ -22,6 +22,7 @@
 
 // Package sync mirrors and amends Go package sync.
 //
+//  - `WorkGroup` allows to spawn group of goroutines working on a common task(*).
 //  - `Once` allows to execute an action only once.
 //  - `WaitGroup` allows to wait for a collection of tasks to finish.
 //  - `Sema`(*) and `Mutex` provide low-level synchronization.
@@ -38,6 +39,7 @@
 // (*) not provided in Go version.
 
 #include <golang/libgolang.h>
+#include <golang/context.h>
 
 // ---- C-level API ----
 
@@ -137,6 +139,72 @@ private:
     WaitGroup(WaitGroup&&);         // don't move
 };
 
+// WorkGroup is a group of goroutines working on a common task.
+//
+// Use .go() to spawn goroutines, and .wait() to wait for all of them to
+// complete, for example:
+//
+//   sync::WorkGroup wg = sync::NewWorkGroup(ctx);
+//   wg->go(f1);
+//   wg->go(f2);
+//   error err = wg->wait();
+//
+// Every spawned function accepts context related to the whole work and derived
+// from ctx used to initialize WorkGroup, for example:
+//
+//   error f1(context::Context ctx) {
+//       ...
+//   }
+//
+// Whenever a function returns error, the work context is canceled indicating
+// to other spawned goroutines that they have to cancel their work. .wait()
+// waits for all spawned goroutines to complete and returns error, if any, from
+// the first failed subtask.
+//
+// NOTE if spawned function panics, the panic is currently _not_ propagated to .wait().
+//
+// WorkGroup is modelled after https://godoc.org/golang.org/x/sync/errgroup but
+// is not equal to it.
+typedef refptr<class _WorkGroup> WorkGroup;
+class _WorkGroup : public object {
+    context::Context    _ctx;
+    func<void()>        _cancel;
+    WaitGroup           _wg;
+    Mutex               _mu;
+    error               _err;
+
+    // don't new - create only via NewWorkGroup()
+private:
+    _WorkGroup();
+    ~_WorkGroup();
+    friend WorkGroup NewWorkGroup(context::Context ctx);
+public:
+    LIBGOLANG_API void decref();
+
+public:
+    LIBGOLANG_API void go(func<error(context::Context)> f);
+    LIBGOLANG_API error wait();
+
+private:
+    _WorkGroup(const _WorkGroup&);  // don't copy
+    _WorkGroup(_WorkGroup&&);       // don't move
+
+    // internal API used by sync.pyx
+    friend inline context::Context _WorkGroup_ctx(_WorkGroup *_wg);
+};
+
+// NewWorkGroup creates new WorkGroup working under ctx.
+//
+// See WorkGroup documentation for details.
+LIBGOLANG_API WorkGroup NewWorkGroup(context::Context ctx);
+
+// sync.pyx uses WorkGroup._ctx directly for efficiency.
+#ifdef _LIBGOLANG_SYNC_INTERNAL_API
+inline context::Context _WorkGroup_ctx(_WorkGroup *_wg) {
+    return _wg->_ctx;
+}
+#endif
+
 }}   // golang::sync::
 #endif  // __cplusplus
 

golang/sync.py

@@ -41,70 +41,5 @@ from golang._sync import \
     PySema      as Sema,        \
     PyMutex     as Mutex,       \
     PyOnce      as Once,        \
-    PyWaitGroup as WaitGroup    \
-
-
-# WorkGroup is a group of goroutines working on a common task.
-#
-# Use .go() to spawn goroutines, and .wait() to wait for all of them to
-# complete, for example:
-#
-#   wg = WorkGroup(ctx)
-#   wg.go(f1)
-#   wg.go(f2)
-#   wg.wait()
-#
-# Every spawned function accepts context related to the whole work and derived
-# from ctx used to initialize WorkGroup, for example:
-#
-#   def f1(ctx):
-#       ...
-#
-# Whenever a function returns error (raises exception), the work context is
-# canceled indicating to other spawned goroutines that they have to cancel their
-# work. .wait() waits for all spawned goroutines to complete and returns/raises
-# error, if any, from the first failed subtask.
-#
-# WorkGroup is modelled after https://godoc.org/golang.org/x/sync/errgroup but
-# is not equal to it.
-class WorkGroup(object):
-
-    def __init__(g, ctx):
-        g._ctx, g._cancel = context.with_cancel(ctx)
-        g._wg   = WaitGroup()
-        g._mu   = Mutex()
-        g._err  = None
-
-    def go(g, f, *argv, **kw):
-        g._wg.add(1)
-        go(lambda: g._run(f, *argv, **kw))
-
-    @func
-    def _run(g, f, *argv, **kw):
-        defer(g._wg.done)
-
-        try:
-            f(g._ctx, *argv, **kw)
-        except:
-            _, exc, tb = sys.exc_info()
-            with g._mu:
-                if g._err is None:
-                    # this goroutine is the first failed task
-                    g._err = exc
-                    if six.PY2:
-                        # py3 has __traceback__ automatically
-                        exc.__traceback__ = tb
-                    g._cancel()
-            exc = None
-            tb  = None
-
-
-    def wait(g):
-        g._wg.wait()
-        g._cancel()
-        if g._err is not None:
-            # reraise the exception so that original traceback is there
-            if six.PY3:
-                raise g._err
-            else:
-                six.reraise(g._err, None, g._err.__traceback__)
+    PyWaitGroup as WaitGroup,   \
+    PyWorkGroup as WorkGroup

golang/sync_test.py

@@ -26,7 +26,7 @@ from pytest import raises
 from golang.golang_test import import_pyx_tests, panics
 from golang.time_test import dt
 from six.moves import range as xrange
-import six
+import sys, six
 
 import_pyx_tests("golang._sync_test")
 
@@ -142,6 +142,13 @@ def test_waitgroup():
         wg.done()
 
 
+# PyErr_Restore_traceback_ok indicates whether python exceptions are restored with correct traceback.
+# It is always the case for CPython, but PyPy < 7.3 had a bug:
+# https://bitbucket.org/pypy/pypy/issues/3120/pyerr_restore-does-not-restore-traceback
+PyErr_Restore_traceback_ok = True
+if 'PyPy' in sys.version and sys.pypy_version_info < (7,3):
+    PyErr_Restore_traceback_ok = False
+
 def test_workgroup():
     ctx, cancel = context.with_cancel(context.background())
     mu = sync.Mutex()
@@ -187,6 +194,7 @@ def test_workgroup():
         wg.wait()
     assert exc.type       is MyError
     assert exc.value.args == ('aaa',)
+    if PyErr_Restore_traceback_ok:
         assert 'Iam__' in exc.traceback[-1].locals
         assert 'Iam_f' in exc.traceback[-2].locals
     assert l == [1, 2]
@@ -213,6 +221,7 @@ def test_workgroup():
         wg.wait()
     assert exc.type       is MyError
     assert exc.value.args == ('bbb',)
+    if PyErr_Restore_traceback_ok:
         assert 'Iam__' in exc.traceback[-1].locals
         assert 'Iam_f' in exc.traceback[-2].locals
     assert l == [1, 2]

setup.py

@@ -247,7 +247,9 @@ setup(
                         ['golang/_context.pyx']),
 
                     Ext('golang._sync',
-                        ['golang/_sync.pyx']),
+                        ['golang/_sync.pyx'],
+                        dsos = ['golang.runtime.libpyxruntime'],
+                        define_macros = [('_LIBGOLANG_SYNC_INTERNAL_API', None)]),
                     Ext('golang._sync_test',
                         ['golang/_sync_test.pyx']),