make PYTEST_CURRENT_TEST thread-safe

Author: Liam-DeVoe

.gitignore

@@ -16,7 +16,6 @@ include/
 *.class
 *.orig
 *~
-.hypothesis/
 
 # autogenerated
 src/_pytest/_version.py
@@ -51,6 +50,7 @@ coverage.xml
 .vscode
 __pycache__/
 .python-version
+.claude
 
 # generated by pip
 pip-wheel-metadata/

changelog/13837.improvement.rst

@@ -0,0 +1 @@
+Pytest setting :ref:`PYTEST_CURRENT_TEST <pytest current test env>` internally is now thread-safe.

src/_pytest/runner.py

@@ -208,7 +208,13 @@ def _update_current_test_var(
         value = value.replace("\x00", "(null)")
         os.environ[var_name] = value
     else:
-        os.environ.pop(var_name)
+        # under multithreading, this may have already been popped by another thread.
+        # Note that os.environ inherits from MutableMapping and therefore .pop(var_name, None)
+        # is not atomic or thread-safe, unlike e.g. popping from a builtin dict.
+        try:
+            os.environ.pop(var_name)
+        except KeyError:
+            pass
 
 
 def pytest_report_teststatus(report: BaseReport) -> tuple[str, str, str] | None: