sync: Move/Port Once to C++/Pyx nogil

Provide sync.Once that can be used directly from C++ and Pyx/nogil worlds.
Python-level sync.Once becomes small wrapper around pyx/nogil sync.Once .

golang/_golang_test.pyx

@@ -178,6 +178,7 @@ cdef extern from * nogil:
     extern void _test_select_win_while_queue();
     extern void _test_select_inplace();
     extern void _test_defer();
+    extern void _test_sync_once_cpp();
     """
     void _test_chan_cpp_refcount()              except +topyexc
     void _test_chan_cpp()                       except +topyexc
@@ -188,6 +189,7 @@ cdef extern from * nogil:
     void _test_select_win_while_queue()         except +topyexc
     void _test_select_inplace()                 except +topyexc
     void _test_defer()                          except +topyexc
+    void _test_sync_once_cpp()                  except +topyexc
 def test_chan_cpp_refcount():
     with nogil:
         _test_chan_cpp_refcount()
@@ -215,6 +217,9 @@ def test_select_inplace():
 def test_defer():
     with nogil:
         _test_defer()
+def test_sync_once_cpp():   # TODO move -> _sync_test.pyx
+    with nogil:
+        _test_sync_once_cpp()
 
 
 # helpers for pychan(dtype=X)  py <-> c  tests.

golang/_sync.pxd

@@ -33,3 +33,6 @@ cdef extern from "golang/libgolang.h" namespace "golang::sync" nogil:
     cppclass Mutex:
         void lock()
         void unlock()
+
+    cppclass Once:
+        void do "do_" (...)     # ... = std::function

golang/_sync.pyx

@@ -22,6 +22,8 @@
 from __future__ import print_function, absolute_import
 
 from cython  cimport final
+from cpython cimport PyObject
+from golang  cimport topyexc
 
 @final
 cdef class PySema:
@@ -63,9 +65,45 @@ cdef class PyMutex:
         pymu.unlock()
 
 
-# ---- misc ----
+@final
+cdef class PyOnce:
+    """Once allows to execute an action only once.
 
-from golang cimport topyexc
+    For example:
+
+      once = Once()
+      ...
+      once.do(doSomething)
+    """
+    cdef Once once
+
+    # FIXME cannot catch/pyreraise panic of .once ctor
+    # https://github.com/cython/cython/issues/3165
+
+    def do(PyOnce pyonce, object f):
+        with nogil:
+            _once_pydo(&pyonce.once, <PyObject *>f)
+
+cdef void _once_pydo(Once *once, PyObject *f) nogil except *:
+    __once_pydo(once, f)
+
+cdef extern from * nogil:
+    """
+    static void __pyx_f_6golang_5_sync__pycall_fromnogil(PyObject *);
+    static void __once_pydo(golang::sync::Once *once, PyObject *f) {
+        once->do_([&]() {
+            __pyx_f_6golang_5_sync__pycall_fromnogil(f);
+        });
+    }
+    """
+    void __once_pydo(Once *once, PyObject *f) except +topyexc
+
+cdef void _pycall_fromnogil(PyObject *f) nogil except *:
+    with gil:
+        (<object>f)()
+
+
+# ---- misc ----
 
 cdef nogil:
 

golang/_sync_test.pyx

@@ -147,3 +147,29 @@ cdef nogil:
 def test_sema_wakeup():
     with nogil:
         _test_sema_wakeup()
+
+
+# verify Once
+cdef nogil:
+    int  _once_ncall = 0
+    void _once_call():
+        global _once_ncall
+        _once_ncall += 1
+
+cdef void _test_once() nogil except +topyexc:
+    cdef sync.Once once
+    if not (_once_ncall == 0):
+        panic("once @0: ncall != 0")
+    once.do(_once_call);
+    if not (_once_ncall == 1):
+        panic("once @1: ncall != 1")
+    once.do(_once_call);
+    if not (_once_ncall == 1):
+        panic("once @2: ncall != 1")
+    once.do(_once_call);
+    if not (_once_ncall == 1):
+        panic("once @3: ncall != 1")
+
+def test_once():
+    with nogil:
+        _test_once()

golang/libgolang.h

@@ -509,6 +509,26 @@ private:
     Mutex(Mutex&&);         // don't move
 };
 
+// Once allows to execute an action only once.
+//
+// For example:
+//
+//   sync::Once once;
+//   ...
+//   once.do_(doSomething);
+class Once {
+    Mutex _mu;
+    bool  _done;
+
+public:
+    LIBGOLANG_API Once();
+    LIBGOLANG_API ~Once();
+    LIBGOLANG_API void do_(const std::function<void(void)> &f);
+
+private:
+    Once(const Once&);      // don't copy
+    Once(Once&&);           // don't move
+};
 
 }   // golang::sync::
 

golang/runtime/libgolang.cpp

@@ -157,7 +157,7 @@ void _semarelease(_sema *sema) {
     _runtime->sema_release((_libgolang_sema *)sema);
 }
 
-// golang::sync::
+// golang::sync::  (only Sema and Mutex)
 namespace sync {
 
 Sema::Sema() {
@@ -1234,6 +1234,33 @@ double now() {
 }}  // golang::time::
 
 
+// ---- golang::sync:: (except Sema and Mutex) ----
+
+namespace golang {
+namespace sync {
+
+Once::Once() {
+    Once *once = this;
+    once->_done = false;
+}
+
+Once::~Once() {}
+
+void Once::do_(const std::function<void(void)> &f) {
+    Once *once = this;
+    once->_mu.lock();
+    defer([&]() {
+        once->_mu.unlock();
+    });
+
+    if (!once->_done) {
+        once->_done = true;
+        f();
+    }
+}
+
+}}  // golang::sync::
+
 // ---- misc ----
 
 // zalloc allocates zeroed memory.

golang/runtime/libgolang_test.cpp

@@ -475,3 +475,26 @@ void _test_defer() {
     __test_defer(&called);
     ASSERT(called);
 }
+
+
+// ---- sync:: ----
+
+// verify that sync::Once works.
+void _test_sync_once_cpp() {
+    sync::Once once;
+    int ncall = 0;
+    ASSERT(ncall == 0);
+    once.do_([&]() {
+        ncall++;
+    });
+    ASSERT(ncall == 1);
+    once.do_([&]() {
+        ncall++;
+    });
+    ASSERT(ncall == 1);
+    once.do_([&]() {
+        ncall++;
+        panic("should not panic");
+    });
+    ASSERT(ncall == 1);
+}

golang/sync.py

@@ -34,26 +34,8 @@ import six
 
 from golang._sync import \
     PySema      as Sema,    \
-    PyMutex     as Mutex    \
-
-
-# Once allows to execute an action only once.
-#
-# For example:
-#
-#   once = Once()
-#   ...
-#   once.do(doSomething)
-class Once(object):
-    def __init__(once):
-        once._mu    = Mutex()
-        once._done  = False
-
-    def do(once, f):
-        with once._mu:
-            if not once._done:
-                once._done = True
-                f()
+    PyMutex     as Mutex,   \
+    PyOnce      as Once     \
 
 
 # WaitGroup allows to wait for collection of tasks to finish.