fix: don't assume we have a current frame. #2005

Author: nedbat

CHANGES.rst

@@ -62,6 +62,9 @@ Unreleased
 - We no longer ship a PyPy-specific wheel. PyPy will install the pure-Python
   wheel.  Closes `issue 2001`_.
 
+- In the very unusual situation of not having a current frame, coverage no
+  longer crashes when using the sysmon core, fixing `issue 2005`_.
+
 .. _issue 310: https://github.com/nedbat/coveragepy/issues/310
 .. _issue 312: https://github.com/nedbat/coveragepy/issues/312
 .. _issue 367: https://github.com/nedbat/coveragepy/issues/367
@@ -73,7 +76,7 @@ Unreleased
 .. _issue 1941: https://github.com/nedbat/coveragepy/issues/1941
 .. _pull 1998: https://github.com/nedbat/coveragepy/pull/1998
 .. _issue 2001: https://github.com/nedbat/coveragepy/issues/2001
-
+.. _issue 2005: https://github.com/nedbat/coveragepy/issues/2005
 
 .. start-releases
 

coverage/sysmon.py

@@ -318,10 +318,12 @@ def sysmon_py_start(
             filename = code.co_filename
             disp = self.should_trace_cache.get(filename)
             if disp is None:
-                frame = inspect.currentframe().f_back  # type: ignore[union-attr]
-                if LOG:
-                    # @panopticon adds a frame.
-                    frame = frame.f_back  # type: ignore[union-attr]
+                frame = inspect.currentframe()
+                if frame is not None:
+                    frame = inspect.currentframe().f_back  # type: ignore[union-attr]
+                    if LOG:
+                        # @panopticon adds a frame.
+                        frame = frame.f_back  # type: ignore[union-attr]
                 disp = self.should_trace(filename, frame)  # type: ignore[arg-type]
                 self.should_trace_cache[filename] = disp