gh-139391: properly handle signal.signal() in UnixConsole when called from a non-main thread

Lib/_pyrepl/unix_console.py

@@ -390,7 +390,15 @@ def restore(self):
             os.write(self.output_fd, b"\033[?7h")
 
         if hasattr(self, "old_sigwinch"):
-            signal.signal(signal.SIGWINCH, self.old_sigwinch)
+            # Only restore signal handler if we're in the main thread
+            # signal.signal() only works in the main thread of the main interpreter
+            try:
+                signal.signal(signal.SIGWINCH, self.old_sigwinch)
+            except ValueError:
+                # This can happen when called from a non-main thread
+                # (e.g., asyncio REPL). In this case, we skip signal restoration
+                # to avoid the "signal only works in main thread" error.
+                pass
             del self.old_sigwinch
 
     def push_char(self, char: int | bytes) -> None:

Lib/test/test_pyrepl/test_unix_console.py

@@ -317,6 +317,24 @@ def test_restore_with_invalid_environ_on_macos(self, _os_write):
             console.prepare()  # needed to call restore()
             console.restore()  # this should succeed
 
+    def test_restore_in_thread(self, _os_write):
+        # for gh-139391
+        import threading
+        console = unix_console([])
+        console.old_sigwinch = signal.SIG_DFL
+        exception_caught = []
+        def thread_target():
+            try:
+                console.restore()
+            except ValueError as e:
+                if "signal only works in main thread" in str(e):
+                    exception_caught.append(e)
+        thread = threading.Thread(target=thread_target)
+        thread.start()
+        thread.join()
+        self.assertEqual(len(exception_caught), 0,
+                        "restore() should not raise ValueError in non-main thread")
+
 
 @unittest.skipIf(sys.platform == "win32", "No Unix console on Windows")
 class TestUnixConsoleEIOHandling(TestCase):

Misc/NEWS.d/next/Library/2025-09-28-16-34-11.gh-issue-139391.nRFnmx.rst

@@ -0,0 +1,2 @@
+Fix crash in PyREPL asyncio mode when using Ctrl+Z (suspend) followed by
+``fg`` (resume).