input() is blocking exceptions at some phase during execution

Bug report

Bug description:

The following loop will soon stuck at some value, due to some phase of built-in input() itself is blocking exceptions and will not pass the exception to upper scope until an ENTER is hit on keyboard.

import os, signal, threading

cnt = [0]

def interrupter(cnt):
	local_cnt = 0
	while 1:
		if local_cnt != cnt[0]:
			local_cnt = cnt[0]
			os.kill(os.getpid(), signal.SIGINT)

threading.Thread(target=interrupter, args=(cnt,), daemon=True).start()

while 1:
	cnt[0] += 1
	try:
		input(">")
	except KeyboardInterrupt:
		print(cnt[0])

CPython versions tested on:

3.8

Operating systems tested on:

macOS

I have come up with a solution, but I am not sure if it is robust enough：

diff --git a/Parser/myreadline.c b/Parser/myreadline.c
index 1825665..3d6abe5 100644
--- a/Parser/myreadline.c
+++ b/Parser/myreadline.c
@@ -12,6 +12,7 @@
 #include "Python.h"
 #include "pycore_fileutils.h"     // _Py_BEGIN_SUPPRESS_IPH
 #include "pycore_pystate.h"   // _PyThreadState_GET()
+#include <sys/types.h>
 #ifdef MS_WINDOWS
 #  ifndef WIN32_LEAN_AND_MEAN
 #    define WIN32_LEAN_AND_MEAN
@@ -331,8 +332,9 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
             return NULL;
         }
         p = pr;
-        int err = my_fgets(tstate, p + n, (int)incr, sys_stdin);
-        if (err == 1) {
+        int err = _Py_read(sys_stdin->_fileno, p + n, incr);
+        //int err = my_fgets(tstate, p + n, (int)incr, sys_stdin);
+        if (err == -1) {
             // Interrupt
             PyMem_RawFree(p);
             return NULL;
@@ -393,8 +395,8 @@ PyOS_Readline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
     }
 
     _PyOS_ReadlineTState = tstate;
-    Py_BEGIN_ALLOW_THREADS
-    PyThread_acquire_lock(_PyOS_ReadlineLock, 1);
+    //Py_BEGIN_ALLOW_THREADS
+    //PyThread_acquire_lock(_PyOS_ReadlineLock, 1);
 
     /* This is needed to handle the unlikely case that the
      * interpreter is in interactive mode *and* stdin/out are not
@@ -418,9 +420,9 @@ PyOS_Readline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
     else {
         rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout, prompt);
     }
-    Py_END_ALLOW_THREADS
+    //Py_END_ALLOW_THREADS
 
-    PyThread_release_lock(_PyOS_ReadlineLock);
+    //PyThread_release_lock(_PyOS_ReadlineLock);
 
     _PyOS_ReadlineTState = NULL;

This is a very interesting test case.

Unfortunately, the short answer is that using SIGINT like this is just inherently racy and unsafe. The main thread may receive it at any point after cnt[0] += 1. If that happens to be before the main thread calls fgets and blocks it will not be interrupted by the signal, and there we are.

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input() is blocking exceptions at some phase during execution #112585

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input() is blocking exceptions at some phase during execution #112585

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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TuringKi commented on Dec 29, 2023

duaneg commented on May 25, 2025

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