golang: Fix recover to clear current exception

Else the exception, even if it was recovered from, will be included as
cause for next raised exception. See added test for details.

golang/__init__.py

@@ -39,6 +39,7 @@ from golang._gopath import gimport  # make gimport available from golang
 import inspect, sys
 import decorator, six
 
+from golang._golang import _pysys_exc_clear as _sys_exc_clear
 
 # @func is a necessary decorator for functions for selected golang features to work.
 #
@@ -176,6 +177,7 @@ def recover():
     if exc is not None:
         goframe.recovered = True
         # recovered: clear current exception context
+        _sys_exc_clear()
         if six.PY2:
             goframe.exc_ctx    = None
             goframe.exc_ctx_tb = None

golang/_golang.pyx

@@ -75,6 +75,33 @@ cdef extern from "golang/libgolang.h" nogil:
     const char *recover_ "golang::recover" () except +
 
 
+# pyrecover needs to clear sys.exc_info().
+# py2 has sys.exc_clear() but it was removed in py3.
+# provide our own _pysys_exc_clear().
+cdef extern from *:
+    """
+    static void XPySys_ExcClear() {
+    #if PY_VERSION_HEX >= 0x03000000 || defined(PYPY_VERSION)
+        PyErr_SetExcInfo(NULL, NULL, NULL);
+    #else
+        PyThreadState *ts;
+        PyObject *exc_type, *exc_value, *exc_traceback;
+
+        ts = PyThreadState_GET();
+        exc_type        = ts->exc_type;         ts->exc_type        = NULL;
+        exc_value       = ts->exc_value;        ts->exc_value       = NULL;
+        exc_traceback   = ts->exc_traceback;    ts->exc_traceback   = NULL;
+
+        Py_XDECREF(exc_type);
+        Py_XDECREF(exc_value);
+        Py_XDECREF(exc_traceback);
+    #endif
+    }
+    """
+    void XPySys_ExcClear()
+def _pysys_exc_clear():
+    XPySys_ExcClear()
+
 # ---- go ----
 
 # go spawns lightweight thread.

golang/golang_test.py

@@ -1295,6 +1295,46 @@ def test_defer_excchain():
     tb = Traceback(e.__traceback__)
     assert tb[-1].name == "_"
 
+# verify that recover breaks exception chain.
+@mark.xfail('PyPy' in sys.version and sys.version_info >= (3,),
+                reason="https://bitbucket.org/pypy/pypy/issues/3096")
+def test_defer_excchain_vs_recover():
+    @func
+    def _():
+        def p1():
+            raise RuntimeError(1)
+        defer(p1)
+        def p2():
+            raise RuntimeError(2)
+        defer(p2)
+        def _():
+            r = recover()
+            assert r == "aaa"
+        defer(_)
+        defer(lambda: panic("aaa"))
+
+    with raises(RuntimeError) as exci:
+        _()
+
+    e1 = exci.value
+    assert type(e1) is RuntimeError
+    assert e1.args == (1,)
+    assert e1.__cause__     is None
+    assert e1.__context__   is not None
+    if six.PY3: # .__traceback__ of top-level exception
+        assert e1.__traceback__ is not None
+        tb1 = Traceback(e1.__traceback__)
+        assert tb1[-1].name == "p1"
+
+    e2 = e1.__context__
+    assert type(e2) is RuntimeError
+    assert e2.args == (2,)
+    assert e2.__cause__     is None
+    assert e2.__context__   is None         # not chained to panic
+    assert e2.__traceback__ is not None
+    tb2 = Traceback(e2.__traceback__)
+    assert tb2[-1].name == "p2"
+
 # verify that traceback.{print_exception,format_exception} work on chained
 # exception correctly.
 def test_defer_excchain_traceback():