Fix compilation with Cython 3.

The double-dunder name changes broke some things.

Fixes #1599

.travis.yml

@@ -174,7 +174,7 @@ jobs:
         # First, the build dependencies (see setup.cfg)
         # so that we don't have to use build isolation and can better use the cache;
         # Note that we can't use -U for cffi and greenlet on PyPy.
-        - &build-gevent-deps pip install -U setuptools wheel twine && pip install -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"' 'cffi;platform_python_implementation=="CPython"' cython 'greenlet;platform_python_implementation=="CPython"'
+        - &build-gevent-deps pip install -U setuptools wheel twine && pip install -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"' 'cffi;platform_python_implementation=="CPython"' 'cython>=3.0a4' 'greenlet;platform_python_implementation=="CPython"'
         # Next, build the wheel *in place*. This helps ccache, and also lets us cache the configure
         # output (pip install uses a random temporary directory, making this difficult)
         - python setup.py bdist_wheel

appveyor.yml

@@ -172,7 +172,7 @@ build_script:
   # Build the compiled extension
   # Try to get some things that don't wind up in the pip cache as
   # built wheels if they're built during an isolated build.
-  - "%CMD_IN_ENV% %PYEXE% -m pip install -U wheel cython setuptools cffi"
+  - "%CMD_IN_ENV% %PYEXE% -m pip install -U --pre wheel cython setuptools cffi"
   - if not "%GWHEEL_ONLY%"=="true" %PYEXE% -m pip install -U -e .[test]
 
 test_script:

docs/changes/1599.bugfix

+gevent can now be built using Cython 3.0a2 and newer.
+
+The libev extension was incompatible with this. As part of this,
+certain internal, undocumented names have been changed.

pyproject.toml

@@ -13,9 +13,8 @@ requires = [
      # name to be created so that we can have both foo.py and _foo.so
      # at the same time. 0.29 fixes some issues with Python 3.7,
      # and adds the 3str mode for transition to Python 3. 0.29.14+ is
-     # required for Python 3.8. As of now, we can't build with Cython 3.0a3
-     # https://github.com/gevent/gevent/issues/1599
-     "Cython >= 0.29.14, < 3",
+     # required for Python 3.8.
+     "Cython >= 3.0a4",
      # See version requirements in setup.py
      "cffi >= 1.12.3 ; platform_python_implementation == 'CPython'",
      # Python 3.7 requires at least 0.4.14, which is ABI incompatible with earlier

src/gevent/libev/corecffi.py

@@ -255,7 +255,7 @@ class loop(AbstractLoop):
             ptr = libev.ev_loop_new(c_flags)
             if not ptr:
                 raise SystemError("ev_loop_new(%s) failed" % (c_flags, ))
-        if default or globals()["__SYSERR_CALLBACK"] is None:
+        if default or SYSERR_CALLBACK is None:
             set_syserr_cb(self._handle_syserr)
 
         # Mark this loop as being used.
@@ -299,7 +299,7 @@ class loop(AbstractLoop):
         if self._ptr:
             super(loop, self).destroy()
             # pylint:disable=comparison-with-callable
-            if globals()["__SYSERR_CALLBACK"] == self._handle_syserr:
+            if globals()["SYSERR_CALLBACK"] == self._handle_syserr:
                 set_syserr_cb(None)
 
 
@@ -425,23 +425,23 @@ class loop(AbstractLoop):
 def _syserr_cb(msg):
     try:
         msg = ffi.string(msg)
-        __SYSERR_CALLBACK(msg, ffi.errno)
+        SYSERR_CALLBACK(msg, ffi.errno)
     except:
         set_syserr_cb(None)
         raise  # let cffi print the traceback
 
 
 def set_syserr_cb(callback):
-    global __SYSERR_CALLBACK
+    global SYSERR_CALLBACK
     if callback is None:
         libev.ev_set_syserr_cb(ffi.NULL)
-        __SYSERR_CALLBACK = None
+        SYSERR_CALLBACK = None
     elif callable(callback):
         libev.ev_set_syserr_cb(libev._syserr_cb)
-        __SYSERR_CALLBACK = callback
+        SYSERR_CALLBACK = callback
     else:
         raise TypeError('Expected callable or None, got %r' % (callback, ))
 
-__SYSERR_CALLBACK = None
+SYSERR_CALLBACK = None
 
 LIBEV_EMBED = libev.LIBEV_EMBED