gh-154167: Fix test suite hang when SIGINT is ignored (GH-154168)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/libregrtest/setup.py

@@ -50,6 +50,14 @@ def setup_process() -> None:
         for signum in signals:
             faulthandler.register(signum, chain=True, file=stderr_fd)
 
+    # Restore the default SIGINT handler if there is no Python-level
+    # handler.  Python inherits the SIG_IGN disposition when the test
+    # suite runs as a shell background job; then asyncio.Runner does not
+    # install its own handler and _thread.interrupt_main() is a no-op,
+    # which makes some tests in test_asyncio and test_threading hang.
+    if signal.getsignal(signal.SIGINT) in (signal.SIG_IGN, signal.SIG_DFL):
+        signal.signal(signal.SIGINT, signal.default_int_handler)
+
     adjust_rlimit_nofile()
 
     support.record_original_stdout(sys.stdout)

Misc/NEWS.d/next/Tests/2026-07-19-20-15-00.gh-issue-154167.sigint.rst

@@ -0,0 +1,3 @@
+The test runner (regrtest) now restores the default SIGINT handler if it
+was inherited as ignored, so the test suite no longer hangs when run as
+a shell background job.