golang: Provide way to wrap pyx/nogil-level chan[T] into pychan

This will be needed in Python-wrappers for C-level code which exposes
channels. For example in time package whose implementation will soon be
moved to nogil world fully.

README.rst

@@ -252,11 +252,12 @@ can be used to multiplex on several channels. For example::
              # default case
              ...
 
-Channels created from Python are represented by `pychan` cdef class. Python
+Python channels are represented by `pychan` cdef class. Python
 channels that carry non-Python elements (`pychan.dtype != DTYPE_PYOBJECT`) can
 be converted to Cython/nogil `chan[T]` via `pychan.chan_*()`.
-This provides interaction mechanism in between *nogil* and Python worlds.
-For example::
+Similarly Cython/nogil `chan[T]` can be wrapped into `pychan` via
+`pychan.from_chan_*()`. This provides interaction mechanism
+in between *nogil* and Python worlds. For example::
 
    def myfunc(pychan pych):
       if pych.dtype != DTYPE_INT:
@@ -268,6 +269,17 @@ For example::
          # send/receive on the channel simultaneously.
          ...
 
+   def mytick(): # -> pychan
+      cdef chan[int] ch
+      with nogil:
+         # create a channel that is connected to some nogil task of the program
+         ch = ...
+
+      # wrap the channel into pychan. Both Python and nogil parts can
+      # send/receive on the channel simultaneously.
+      cdef pychan pych = pychan.from_chan_int(ch)  # pychan <- chan[int]
+      return pych
+
 
 `panic` stops normal execution of current goroutine by throwing a C-level
 exception. On Python/C boundaries C-level exceptions have to be converted to

golang/_golang.pxd

@@ -165,3 +165,14 @@ cdef class pychan:
         chan[cbool]     chan_bool       (pychan pych)
         chan[int]       chan_int        (pychan pych)
         chan[double]    chan_double     (pychan pych)
+
+    # pychan.from_chan_X returns pychan wrapping pyx/nogil-level chan[X].
+    # X can be any C-level type, but not PyObject.
+    @staticmethod
+    cdef pychan from_chan_structZ   (chan[structZ] ch)
+    @staticmethod
+    cdef pychan from_chan_bool      (chan[cbool] ch)
+    @staticmethod
+    cdef pychan from_chan_int       (chan[int] ch)
+    @staticmethod
+    cdef pychan from_chan_double    (chan[double] ch)

golang/_golang.pyx

@@ -335,10 +335,33 @@ cdef class pychan:
             pychan_asserttype(pych, DTYPE_DOUBLE)
             return _wrapchan[double] (pych._ch)
 
+    # pychan <- chan[X]
+    @staticmethod
+    cdef pychan from_chan_structZ   (chan[structZ] ch):
+        return pychan_from_raw(ch._rawchan(),   DTYPE_STRUCTZ)
+
+    @staticmethod
+    cdef pychan from_chan_bool      (chan[cbool] ch):
+        return pychan_from_raw(ch._rawchan(),   DTYPE_BOOL)
+
+    @staticmethod
+    cdef pychan from_chan_int       (chan[int] ch):
+        return pychan_from_raw(ch._rawchan(),   DTYPE_INT)
+
+    @staticmethod
+    cdef pychan from_chan_double    (chan[double] ch):
+        return pychan_from_raw(ch._rawchan(),   DTYPE_DOUBLE)
+
 cdef void pychan_asserttype(pychan pych, DType dtype) nogil:
     if pych.dtype != dtype:
         panic("pychan: channel type mismatch")
 
+cdef pychan pychan_from_raw(_chan *_ch, DType dtype):
+    cdef pychan pych = pychan.__new__(pychan)
+    pych.dtype = dtype
+    pych._ch   = _ch; _chanxincref(_ch)
+    return pych
+
 
 # pydefault represents default case for pyselect.
 pydefault  = object()