gh-157870: Do not count Rosetta's thread when warning about fork() with threads

Lib/test/test_os/test_os.py

@@ -5568,6 +5568,25 @@ def test_fork(self):
         else:
             assert_python_ok("-c", code, PYTHONMALLOC="malloc_debug")
 
+    def test_fork_does_not_warn_when_single_threaded(self):
+        # gh-157870: on macOS, Rosetta 2 adds a thread of its own to every
+        # translated x86-64 process; it must not count as a thread of ours.
+        code = """if 1:
+            import os, warnings
+            with warnings.catch_warnings(record=True) as ws:
+                warnings.simplefilter("always", DeprecationWarning)
+                pid = os.fork()
+                if pid == 0:
+                    os._exit(0)  # child
+                # Waiting allows an error in the child to hit stderr.
+                exitcode = os.wait()[1]
+                assert exitcode == 0, f"child exited {exitcode}"
+            assert not ws, [str(w.message) for w in ws]
+        """
+        _, out, err = assert_python_ok("-c", code, PYTHONOPTIMIZE='0')
+        self.assertEqual(err.decode("utf-8"), "")
+        self.assertEqual(out.decode("utf-8"), "")
+
     @unittest.skipUnless(sys.platform in ("linux", "android", "darwin"),
                          "Only Linux and macOS detect this today.")
     @unittest.skipIf(_testcapi is None, "requires _testcapi")

Misc/NEWS.d/next/macOS/2026-10-01-03-18-15.gh-issue-157870.WQ1hGY.rst

@@ -0,0 +1,4 @@
+Fix a spurious :exc:`DeprecationWarning` about a multi-threaded process from
+:func:`os.fork` and :func:`os.forkpty` when an x86-64 build of Python runs
+under Rosetta 2 on Apple Silicon Macs. The thread that Rosetta adds to every
+translated process is no longer counted.

Modules/posixmodule.c

@@ -8565,6 +8565,23 @@ static Py_ssize_t get_number_of_os_threads(void)
         if (task_threads(macos_task, &macos_threads,
                          &macos_n_threads) == KERN_SUCCESS) {
             num_python_threads = macos_n_threads;
+#if defined(__x86_64__)
+            // Rosetta 2 runs a thread of its own in every translated
+            // process. It is not one of ours, so do not count it.
+            for (mach_msg_type_number_t i = 0; i < macos_n_threads; i++) {
+                thread_extended_info_data_t info;
+                mach_msg_type_number_t count = THREAD_EXTENDED_INFO_COUNT;
+                kern_return_t kr = thread_info(
+                    macos_threads[i], THREAD_EXTENDED_INFO,
+                    (thread_info_t)&info, &count);
+                if (kr == KERN_SUCCESS &&
+                    strncmp(info.pth_name, "com.apple.rosetta.exceptionserver",
+                            sizeof(info.pth_name)) == 0)
+                {
+                    num_python_threads--;
+                }
+            }
+#endif
         }
     }
 #elif defined(__linux__)