track item: thread mapping for PYTEST_CURRENT_TEST

Author: Liam-DeVoe

src/_pytest/main.py

@@ -16,6 +16,8 @@
 import os
 from pathlib import Path
 import sys
+import threading
+from threading import Thread
 from typing import final
 from typing import Literal
 from typing import overload
@@ -581,6 +583,8 @@ def __init__(self, config: Config) -> None:
         self._initial_parts: list[CollectionArgument] = []
         self._collection_cache: dict[nodes.Collector, CollectReport] = {}
         self.items: list[nodes.Item] = []
+        # track the thread in which each item started execution in
+        self._item_to_thread: dict[nodes.Item, Thread] = {}
 
         self._bestrelpathcache: dict[Path, str] = _bestrelpath_cache(config.rootpath)
 
@@ -643,6 +647,22 @@ def startpath(self) -> Path:
         """
         return self.config.invocation_params.dir
 
+    def _readable_thread_id(self, thread: Thread) -> int:
+        # Returns a 0-indexed id of `thread`, corresponding to the order in
+        # which we saw that thread start executing tests. The main thread always
+        # has value 0, so non-main threads start at 1, even if the first thread
+        # we saw was a non-main thread, or even if we never saw the main thread
+        # execute tests.
+
+        # relying on item_to_thread to be sorted for stable ordering
+        threads = list(self._item_to_thread.values())
+        assert thread in threads
+
+        if thread is threading.main_thread():
+            return 0
+
+        return threads.index(thread) + 1
+
     def _node_location_to_relpath(self, node_path: Path) -> str:
         # bestrelpath is a quite slow function.
         return self._bestrelpathcache[node_path]

src/_pytest/runner.py

@@ -8,6 +8,7 @@
 import dataclasses
 import os
 import sys
+import threading
 import types
 from typing import cast
 from typing import final
@@ -127,6 +128,7 @@ def runtestprotocol(
         # This only happens if the item is re-run, as is done by
         # pytest-rerunfailures.
         item._initrequest()  # type: ignore[attr-defined]
+    item.session._item_to_thread[item] = threading.current_thread()
     rep = call_and_report(item, "setup", log)
     reports = [rep]
     if rep.passed:
@@ -201,20 +203,21 @@ def _update_current_test_var(
 
     If ``when`` is None, delete ``PYTEST_CURRENT_TEST`` from the environment.
     """
+    thread = item.session._item_to_thread[item]
+    readable_id = item.session._readable_thread_id(thread)
+    # main thread (aka humanid == 0) gets the plain var. Other threads
+    # append their id.
     var_name = "PYTEST_CURRENT_TEST"
+    if readable_id != 0:
+        var_name += f"_THREAD_{readable_id}"
+
     if when:
         value = f"{item.nodeid} ({when})"
         # don't allow null bytes on environment variables (see #2644, #2957)
         value = value.replace("\x00", "(null)")
         os.environ[var_name] = value
     else:
-        # 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:  # pragma: no cover # can be removed when #13768 is farther along
-            pass
+        os.environ.pop(var_name)
 
 
 def pytest_report_teststatus(report: BaseReport) -> tuple[str, str, str] | None: