gh-139145: fix spinning event loop in tkinter

Lib/test/test_tkinter/test_vwait_busyloop_stdin.py

@@ -0,0 +1,75 @@
+# 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")
+@unittest.skipIf(sys.platform == "win32", "test is not supported on Windows")
+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(cpu_frac)
+
+# Schedule vwait and release
+interp.after(100, do_vwait)
+interp.after(500, lambda: interp.setvar("myvar", "done"))
+"""
+
+        # 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"\n")
+
+        # Ensure child exits cleanly
+        proc.stdin.write(b"exit()\n")
+
+        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 = float(out)
+
+        # 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()

Misc/NEWS.d/next/Library/2025-09-20-15-52-12.gh-issue-139145.1bMDI-.rst

@@ -0,0 +1,4 @@
+Prevents tkinter ending up in a busy loop when ``vwait`` (and other Tcl commands
+that run their own tight event loop) are used with Python in interactive mode,
+and the user types any input on stdin while ``vwait`` is still running. Patch
+by Michiel de Hoon.

Modules/_tkinter.c

@@ -3356,7 +3356,9 @@ static int stdin_ready = 0;
 static void
 MyFileProc(void *clientData, int mask)
 {
+    int *tfile = clientData;
     stdin_ready = 1;
+    Tcl_DeleteFileHandler(*tfile);
 }
 #endif
 
@@ -3373,7 +3375,7 @@ EventHook(void)
     errorInCmd = 0;
 #ifndef MS_WINDOWS
     tfile = fileno(stdin);
-    Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, NULL);
+    Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, &tfile);
 #endif
     while (!errorInCmd && !stdin_ready) {
         int result;
@@ -3398,14 +3400,14 @@ EventHook(void)
         if (result < 0)
             break;
     }
-#ifndef MS_WINDOWS
-    Tcl_DeleteFileHandler(tfile);
-#endif
     if (errorInCmd) {
         errorInCmd = 0;
         PyErr_SetRaisedException(excInCmd);
         excInCmd = NULL;
         PyErr_Print();
+#ifndef MS_WINDOWS
+        if (!stdin_ready) Tcl_DeleteFileHandler(tfile);
+#endif
     }
     PyEval_SaveThread();
     return 0;