youknowone · Oct 29, 2026 · Oct 29, 2026
diff --git a/Lib/test/test_free_threading/test_syslog.py b/Lib/test/test_free_threading/test_syslog.py
@@ -0,0 +1,44 @@
+import unittest
+import threading
+
+from test.support import import_helper, threading_helper
+from test.support.threading_helper import run_concurrently
+
+syslog = import_helper.import_module("syslog")
+
+NTHREADS = 32
+
+# Similar to Lib/test/test_syslog.py, this test's purpose is to verify that
+# the code neither crashes nor leaks.
+
+
+@threading_helper.requires_working_threading()
+class TestSyslog(unittest.TestCase):
+    def test_racing_syslog(self):
+        def worker():
+            """
+            The syslog module provides the following functions:
+            openlog(), syslog(), closelog(), and setlogmask().
+            """
+            thread_id = threading.get_ident()
+            syslog.openlog(f"thread-id: {thread_id}")
+            try:
+                for _ in range(5):
+                    syslog.syslog("logline")
+                    syslog.setlogmask(syslog.LOG_MASK(syslog.LOG_INFO))
+                    syslog.syslog(syslog.LOG_INFO, "logline LOG_INFO")
+                    syslog.setlogmask(syslog.LOG_MASK(syslog.LOG_ERR))
+                    syslog.syslog(syslog.LOG_ERR, "logline LOG_ERR")
+                    syslog.setlogmask(syslog.LOG_UPTO(syslog.LOG_DEBUG))
+            finally:
+                syslog.closelog()
+
+        # Run the worker concurrently to exercise all these syslog functions
+        run_concurrently(
+            worker_func=worker,
+            nthreads=NTHREADS,
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/crates/host_env/src/syslog.rs b/crates/host_env/src/syslog.rs
@@ -67,6 +67,9 @@ pub fn closelog() {
 
 #[must_use]
 pub fn setlogmask(maskpri: i32) -> i32 {
+    // libc setlogmask() is not thread-safe.
+    static SETLOGMASK: parking_lot::Mutex<()> = parking_lot::Mutex::new(());
+    let _guard = SETLOGMASK.lock();
     unsafe { libc::setlogmask(maskpri) }
 }