Cython speedups break debugging with Stackless (PY-23283)

The problem appears when compiling Cython extensions with Cython 0.25, and it doesn't appear when compiling them with Cython 0.24. It looks like it's Cython bug, but we can't use old version of Cython, because it isn't compatible with Python 3.6 on Windows. So we have to (temporarily) disable Cython extensions for Stackless despite the fact that its implementation is "CPython".

(cherry picked from commit 2ac81cb)

_pydevd_bundle/pydevd_constants.py

@@ -44,6 +44,7 @@ def get_frame():
     if sys.version_info[0] == 2 and sys.version_info[1] < 5:
         IS_JYTH_LESS25 = True
 
+IS_PYTHON_STACKLESS = "stackless" in sys.version.lower()
 CYTHON_SUPPORTED = False
 
 try:
@@ -52,7 +53,7 @@ def get_frame():
 except:
     pass
 else:
-    if python_implementation == 'CPython':
+    if python_implementation == 'CPython' and not IS_PYTHON_STACKLESS:
         # Only available for CPython!
         if (
             (sys.version_info[0] == 2 and sys.version_info[1] >= 7)

_pydevd_bundle/pydevd_trace_dispatch.py

@@ -2,8 +2,16 @@
 # Should give warning only here if cython is not available but supported.
 
 import os
-use_cython = os.getenv('PYDEVD_USE_CYTHON', None)
 import sys
+from _pydevd_bundle.pydevd_constants import CYTHON_SUPPORTED
+
+
+use_cython = os.getenv('PYDEVD_USE_CYTHON', None)
+dirname = os.path.dirname(os.path.dirname(__file__))
+# Do not show incorrect warning for .egg files for Remote debugger
+if not CYTHON_SUPPORTED or dirname.endswith('.egg'):
+    # Do not try to import cython extensions if cython isn't supported
+    use_cython = 'NO'
 
 
 def delete_old_compiled_extensions():
@@ -55,17 +63,10 @@ def trace_dispatch(py_db, frame, event, arg):
     except ImportError:
         from _pydevd_bundle.pydevd_additional_thread_info_regular import PyDBAdditionalThreadInfo  # @UnusedImport
         from _pydevd_bundle.pydevd_trace_dispatch_regular import trace_dispatch, global_cache_skips, global_cache_frame_skips  # @UnusedImport
-        from _pydevd_bundle.pydevd_constants import CYTHON_SUPPORTED
-
-        dirname = os.path.dirname(os.path.dirname(__file__))
-        if dirname.endswith('.egg'):
-            # Do not show incorrect warning for .egg files for Remote debugger
-            CYTHON_SUPPORTED = False
+        from _pydev_bundle.pydev_monkey import log_error_once
 
-        if CYTHON_SUPPORTED:
-            from _pydev_bundle.pydev_monkey import log_error_once
-            log_error_once("warning: Debugger speedups using cython not found. Run '\"%s\" \"%s\" build_ext --inplace' to build." % (
-                sys.executable, os.path.join(dirname, 'setup_cython.py')))
+        log_error_once("warning: Debugger speedups using cython not found. Run '\"%s\" \"%s\" build_ext --inplace' to build." % (
+            sys.executable, os.path.join(dirname, 'setup_cython.py')))
 
 else:
     raise RuntimeError('Unexpected value for PYDEVD_USE_CYTHON: %s (accepted: YES, NO)' % (use_cython,))