Fix ignored exception in tracer shutdown related to module deletion (#5806)

With the tracer on, modules at shutdown can be destroyed in an order
that prevents the `trace()` function from working. This was currently
the case with Python 3.13 where the function was still being called
during the `xonsh.tracer` module's destruction.

This change adds a test to prevent this from happening in the future,
and works around the issue by creating local references to necessary
functions that were otherwise set to None at shutdown.

news/fix-tracer-shutdown.rst

@@ -0,0 +1,5 @@
+**Fixed:**
+
+* Using ``trace on`` with ``.xsh`` scripts could previously lead to
+  a spurious ignored exception showing up on stderr. This is now fixed.
+

tests/built_ins/test_tracer.py

@@ -1,10 +1,18 @@
+import os
 import re
+import subprocess
+import sys
+from pathlib import Path
+from textwrap import dedent
 
 import pytest
 
 from xonsh.procs.specs import cmds_to_specs
 from xonsh.tracer import tracermain
 
+# Minimum set of environment variables on Windows
+W_ENV = "SYSTEMDRIVE SYSTEMROOT ALLUSERSPROFILE HOMEDRIVE HOMEPATH APPDATA LOCALAPPDATA"
+
 
 def test_tracer_help(capsys, xsh_with_aliases):
     """verify can invoke it, and usage knows about all the options"""
@@ -17,3 +25,31 @@ def test_tracer_help(capsys, xsh_with_aliases):
     assert m[1]
     verbs = {v.strip().lower() for v in m[1].split(",")}
     assert verbs == {"rm", "start", "add", "on", "off", "del", "color", "stop", "ls"}
+
+
+def test_trace_in_script():
+    CURRENT_DIR = Path(__file__).parent
+    cmd = [sys.executable, "-m", "xonsh", str(CURRENT_DIR / "tracer" / "example.xsh")]
+    env = {"XONSH_SHOW_TRACEBACK": "True"}
+    if sys.platform == "win32":
+        # required for an empty environment on Windows. see python/cpython#120836
+        for ev in W_ENV.split():
+            env[ev] = os.environ[ev]
+    expected = dedent(
+        """\
+        Some output!
+        tests/built_ins/tracer/example.xsh:3:variable = ""
+        tests/built_ins/tracer/example.xsh:4:for part in parts:
+        tests/built_ins/tracer/example.xsh:5:    variable += part
+        tests/built_ins/tracer/example.xsh:4:for part in parts:
+        tests/built_ins/tracer/example.xsh:5:    variable += part
+        tests/built_ins/tracer/example.xsh:4:for part in parts:
+        tests/built_ins/tracer/example.xsh:5:    variable += part
+        tests/built_ins/tracer/example.xsh:4:for part in parts:
+        tests/built_ins/tracer/example.xsh:6:echo Some @(variable)
+        """
+    ).replace("/", os.sep)
+    proc = subprocess.run(cmd, capture_output=True, encoding="utf8", env=env)
+    assert proc.returncode == 0
+    assert proc.stderr == ""
+    assert proc.stdout == expected

tests/built_ins/tracer/example.xsh

@@ -0,0 +1,6 @@
+parts = ["out", "put", "!"]
+trace on
+variable = ""
+for part in parts:
+    variable += part
+echo Some @(variable)

xonsh/tracer.py

@@ -81,7 +81,7 @@ class TracerType:
                     frame.f_trace = self.prev_tracer
             self.prev_tracer = DefaultNotGiven
 
-    def trace(self, frame, event, arg):
+    def trace(self, frame, event, arg, *, find_file=find_file, print_color=print_color):
         """Implements a line tracing function."""
         if event not in self.valid_events:
             return self.trace