From 14b7fd1c981f087934a878b7044fe8e6d04c86c4 Mon Sep 17 00:00:00 2001
From: Abdessamad Derraz <3028866+Abdess@users.noreply.github.com>
Date: Wed, 28 Oct 2026 03:56:25 +0100
Subject: [PATCH] fix: one thread at a time in the campaign cwd

---
 scripts/campaign.py    | 22 +++++++++++++++-------
 tests/test_campaign.py |  3 +++
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/scripts/campaign.py b/scripts/campaign.py
index 5648cb23d..4537075f7 100644
--- a/scripts/campaign.py
+++ b/scripts/campaign.py
@@ -69,6 +69,7 @@ import stat
 import subprocess
 import sys
 import tarfile
+import threading
 import time
 import urllib.error
 import urllib.request
@@ -454,15 +455,22 @@ def copy_regular(src: Path, dest: Path) -> None:
             shutil.copyfileobj(handle, out)
 
 
+# The working directory belongs to the process. Two passes in threads that
+# each saved and restored it could leave the process in the other's root,
+# then in a folder deleted with its workspace.
+_CWD_LOCK = threading.Lock()
+
+
 @contextlib.contextmanager
 def working_directory(path: Path) -> Iterator[None]:
-    """contextlib.chdir, which Python 3.9 lacks."""
-    before = Path.cwd()
-    os.chdir(path)
-    try:
-        yield
-    finally:
-        os.chdir(before)
+    """contextlib.chdir, which Python 3.9 lacks, one thread at a time."""
+    with _CWD_LOCK:
+        before = Path.cwd()
+        os.chdir(path)
+        try:
+            yield
+        finally:
+            os.chdir(before)
 
 
 @contextlib.contextmanager
diff --git a/tests/test_campaign.py b/tests/test_campaign.py
index fee154c1c..947ad3ade 100644
--- a/tests/test_campaign.py
+++ b/tests/test_campaign.py
@@ -1170,10 +1170,13 @@ class FakeCorePasses(Workspace):
             c.host.run = timed
             passes.append(c)
         threads = [threading.Thread(target=c.run, args=(set(), None)) for c in passes]
+        cwd = Path.cwd()
         for thread in threads:
             thread.start()
         for thread in threads:
             thread.join()
+        # Each pass moved into its own root and back: the process ends where it began.
+        self.assertEqual(Path.cwd(), cwd)
         ends = {name: sorted(end for _, end in runs) for name, runs in spans.items()}
         self.assertGreater(len(ends["a"]), 2)
         self.assertGreater(len(ends["b"]), 2)