Generally allow the "nogil" decorator and context manager to accept boolean...

Generally allow the "nogil" decorator and context manager to accept boolean arguments like "@nogil(False)".
See #2579.

Cython/Shadow.py

@@ -123,8 +123,6 @@ overflowcheck.fold = optimization.use_switch = \
 
 final = internal = type_version_tag = no_gc_clear = no_gc = _empty_decorator
 
-gil = nogil = _EmptyDecoratorAndManager()
-
 
 _cython_inline = None
 def inline(f, *args, **kwds):
@@ -187,6 +185,25 @@ def declare(type=None, value=_Unspecified, **kwds):
     else:
         return value
 
+class _nogil(object):
+    """Support for 'with nogil' statement and @nogil decorator.
+    """
+    def __call__(self, x):
+        if callable(x):
+            # Used as function decorator => return the function unchanged.
+            return x
+        # Used as conditional context manager or to create an "@nogil(True/False)" decorator => keep going.
+        return self
+
+    def __enter__(self):
+        pass
+    def __exit__(self, exc_class, exc, tb):
+        return exc_class is None
+
+nogil = _nogil()
+gil = _nogil()
+del _nogil
+
 
 # Emulated types
 

tests/errors/pure_errors.py

@@ -18,19 +18,40 @@ def cdef_nogil(x):
     return x + 1
 
 
+@cython.cfunc
+@cython.nogil(True)
+@cython.locals(x=cython.int)
+@cython.returns(cython.int)
+def cdef_nogil_true(x):
+    return x + 1
+
+
+@cython.cfunc
+@cython.nogil(False)
+@cython.locals(x=cython.int)
+@cython.returns(cython.int)
+def cdef_nogil_false(x):
+    return x + 1
+
+
 @cython.locals(x=cython.int)
 def test_cdef_nogil(x):
     cdef_nogil(x)  # ok
+    cdef_nogil_false(x)  # ok
+    cdef_nogil_true(x)  # ok
     with cython.nogil:
-        cdef_needs_gil(x)  # not ok
+        cdef_nogil_true(x)   # ok
+        cdef_needs_gil(x)    # not ok
+        cdef_nogil_false(x)  # not ok
 
 
 @cython.nogil
-def pyfunc(x):
+def pyfunc(x):  # invalid
     return x + 1
 
 
 _ERRORS = """
-25:22: Calling gil-requiring function not allowed without gil
-29:0: Python functions cannot be declared 'nogil'
+44:22: Calling gil-requiring function not allowed without gil
+45:24: Calling gil-requiring function not allowed without gil
+49:0: Python functions cannot be declared 'nogil'
 """

tests/run/pure_py.py

@@ -275,14 +275,33 @@ def cdef_nogil(x):
     return x + 1
 
 
+@cython.cfunc
+@cython.nogil(True)
 @cython.locals(x=cython.int)
+@cython.returns(cython.int)
+def cdef_nogil_true(x):
+    return x + 1
+
+
+@cython.cfunc
+@cython.nogil(False)
+@cython.locals(x=cython.int)
+@cython.returns(cython.int)
+def cdef_nogil_false(x):
+    return x + 1
+
+
+@cython.locals(x=cython.int, result=cython.int)
 def test_cdef_nogil(x):
     """
     >>> test_cdef_nogil(5)
-    6
+    18
     """
     with cython.nogil:
         result = cdef_nogil(x)
+    with cython.nogil(True):
+        result += cdef_nogil_true(x)
+    result += cdef_nogil_false(x)
     return result