Adding test exposing the bug. The test mimics a user running Python interactively.

Author: Michiel Jan Laurens de Hoon

Lib/test/test_tkinter/test_vwait_busyloop_stdin.py

@@ -0,0 +1,87 @@
+# test_tkinter_vwait_mainloop_stdin.py
+import unittest
+import subprocess
+import sys
+import time
+from test import support
+from test.support import import_helper
+
+tkinter = import_helper.import_module("tkinter")
+
+
+@unittest.skipUnless(support.has_subprocess_support, "test requires subprocess")
+class TkVwaitMainloopStdinTest(unittest.TestCase):
+
+    def run_child(self):
+        # Full Python script to execute in the child
+        code = r"""
+import tkinter as tk
+import time
+
+interp = tk.Tcl()
+
+def do_vwait():
+    start_wall = time.time()
+    start_cpu = time.process_time()
+    interp.eval("vwait myvar")
+    end_cpu = time.process_time()
+    end_wall = time.time()
+    cpu_frac = (end_cpu - start_cpu) / (end_wall - start_wall)
+    print(f"CPU fraction during vwait: {cpu_frac:.2f}", flush=True)
+
+# Schedule vwait and release
+interp.after(100, do_vwait)
+interp.after(500, lambda: interp.setvar("myvar", "done"))
+# Schedule quit to stop mainloop
+interp.after(1000, interp.quit)
+
+interp.mainloop()
+"""
+
+        # Start child in interactive mode, but use -c to execute code immediately
+        proc = subprocess.Popen(
+            [sys.executable, "-i", "-c", code],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+        )
+        return proc
+
+    def test_vwait_stdin_busy_loop(self):
+        proc = self.run_child()
+
+        # Wait until vwait likely started
+        time.sleep(0.15)
+
+        # Send input to stdin to trigger the bug
+        proc.stdin.write(b"x\n")
+        proc.stdin.flush()
+
+        # Ensure child exits cleanly
+        proc.stdin.write(b"exit()\n")
+        proc.stdin.flush()
+
+        stdout, stderr = proc.communicate()
+        out = stdout.decode("utf-8", errors="replace")
+        err = stderr.decode("utf-8", errors="replace")
+
+        if proc.returncode != 0:
+            self.fail(f"Child exited with {proc.returncode}\nSTDOUT:\n{out}\nSTDERR:\n{err}")
+
+        # Extract CPU fraction printed by child
+        cpu_frac = None
+        for line in out.splitlines():
+            if line.startswith("CPU fraction during vwait:"):
+                cpu_frac = float(line.split(":")[1].strip())
+                break
+
+        self.assertIsNotNone(cpu_frac, "CPU fraction not printed by child")
+
+        # Fail if CPU fraction is too high (indicative of busy-loop)
+        self.assertLess(cpu_frac, 0.5,
+            f"CPU usage too high during vwait with stdin input (fraction={cpu_frac:.2f})")
+
+
+if __name__ == "__main__":
+    unittest.main()
+