golang: pyselect: Fix tx object reference leak on error exit

Before passing objects to _chanselect for send, pyselect increfs them, as just
send does, to indicate that one object reference is passed to channel buffer.
On exit, since only one case is actually executed by select, pyselect needs to
decref incref'ed object from not executed cases.

Pyselect already implements the latter cleanup, but currently the cleanup is
executed only if control flow reaches _chanselect at all. Which is a bug, since
pyselect can panic or raise an exception just in the middle of preparation phase.

-> Fix it by associating the finally-decref cleanup with whole
prepare+_chanselect code.

Without the fix, the second part of added test (abnormal exit) fails e.g. like:

        @mark.skipif(not hasattr(sys, 'getrefcount'),   # skipped e.g. on PyPy
                     reason="needs sys.getrefcount")
        def test_select_refleak():
            ch1 = chan()
            ch2 = chan()
            obj1 = object()
            obj2 = object()
            tx1 = (ch1.send, obj1)
            tx2 = (ch2.send, obj2)

            # normal exit
            gc.collect()
            nref1 = sys.getrefcount(obj1)
            nref2 = sys.getrefcount(obj2)
            _, _rx = select(
                tx1,        # 0
                tx2,        # 1
                default,    # 2
            )
            assert (_, _rx) == (2, None)
            gc.collect()
            assert sys.getrefcount(obj1) == nref1
            gc.collect()
            assert sys.getrefcount(obj1) == nref2

            # abnormal exit
            with raises(AttributeError) as exc:
                select(
                    tx1,        # 0
                    tx2,        # 1
                    'zzz',      # 2 causes pyselect to panic
                )
            assert exc.value.args == ("'str' object has no attribute '__self__'",)
            gc.collect()
    >       assert sys.getrefcount(obj1) == nref1
    E       assert 4 == 3
    E         -4
    E         +3

    golang/golang_test.py:690: AssertionError

The bug was introduced in 3b241983 (Port/move channels to C/C++/Pyx).

golang/_golang.pyx

@@ -272,11 +272,13 @@ pydefault  = object()
 #       ...
 def pyselect(*pycasev):
     cdef int i, n = len(pycasev), selected
-    cdef vector[_selcase] casev = vector[_selcase](n)
+    cdef vector[_selcase] casev = vector[_selcase](n, default)
     cdef pychan pych
     cdef PyObject *_rx = NULL # all select recvs are setup to receive into _rx
     cdef cbool rxok = False   # (its ok as only one receive will be actually executed)
 
+    selected = -1
+    try:
         # prepare casev for chanselect
         for i in range(n):
             pycase = pycasev[i]
@@ -322,10 +324,9 @@ def pyselect(*pycasev):
                 else:
                     pypanic("pyselect: recv expected: %r" % (pyrecv,))
 
-    selected = -1
-    try:
         with nogil:
             selected = _chanselect_pyexc(&casev[0], casev.size())
+
     finally:
         # decref not sent tx (see ^^^ send prepare)
         for i in range(n):

golang/golang_test.py

@@ -22,7 +22,7 @@ from __future__ import print_function, absolute_import
 
 from golang import go, chan, select, default, nilchan, _PanicError, func, panic, defer, recover
 from golang import sync
-from pytest import raises
+from pytest import raises, mark
 from os.path import dirname
 import os, sys, inspect, importlib
 from subprocess import Popen, PIPE
@@ -652,6 +652,46 @@ def test_select():
     assert len_sendq(ch2) == len_recvq(ch2) == 0
 
 
+# verify that select does not leak references to passed objects.
+@mark.skipif(not hasattr(sys, 'getrefcount'),   # skipped e.g. on PyPy
+             reason="needs sys.getrefcount")
+def test_select_refleak():
+    ch1 = chan()
+    ch2 = chan()
+    obj1 = object()
+    obj2 = object()
+    tx1 = (ch1.send, obj1)
+    tx2 = (ch2.send, obj2)
+
+    # normal exit
+    gc.collect()
+    nref1 = sys.getrefcount(obj1)
+    nref2 = sys.getrefcount(obj2)
+    _, _rx = select(
+        tx1,        # 0
+        tx2,        # 1
+        default,    # 2
+    )
+    assert (_, _rx) == (2, None)
+    gc.collect()
+    assert sys.getrefcount(obj1) == nref1
+    gc.collect()
+    assert sys.getrefcount(obj1) == nref2
+
+    # abnormal exit
+    with raises(AttributeError) as exc:
+        select(
+            tx1,        # 0
+            tx2,        # 1
+            'zzz',      # 2 causes pyselect to panic
+        )
+    assert exc.value.args == ("'str' object has no attribute '__self__'",)
+    gc.collect()
+    assert sys.getrefcount(obj1) == nref1
+    gc.collect()
+    assert sys.getrefcount(obj1) == nref2
+
+
 # benchmark sync chan send vs recv on select side.
 def bench_select(b):
     ch1  = chan()