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gh-139145: fix spinning event loop in tkinter #139180

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gh-139145: fix spinning event loop in tkinter #139180

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6d0489a
bpo-139145: fix spinning event loop in tkinter
Sep 20, 2025
5dcb99f
Adding test exposing the bug. The test mimics a user running Python i…
Sep 20, 2025
49258d1
📜🤖 Added by blurb_it.
blurb-it[bot] Sep 20, 2025
4c4e4b5
fixing lint
Sep 20, 2025
dafea18
Merge branch 'issue139145_bugfix' of github.com:mdehoon/cpython into …
Sep 20, 2025
99086de
fix blurb
Sep 20, 2025
add49bd
simplify test
Oct 1, 2025
35758f1
be sure to delete the file handler also in case of errorInCmd
Oct 1, 2025
a3d07d4
only relevant on Windows
Oct 1, 2025
e8fc833
trivial change to trigger another round of testing
Oct 1, 2025
72b04da
Merge branch 'main' into issue139145_bugfix
Oct 15, 2025
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7 changes: 3 additions & 4 deletions Modules/_tkinter.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
Comment on lines +3336 to +3338
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Suggested change
int *tfile = clientData;
stdin_ready = 1;
Tcl_DeleteFileHandler(*tfile);
int tfile = (int)(Py_intptr_t)clientData;
stdin_ready = 1;
Tcl_DeleteFileHandler(tfile);

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Thanks for looking into this. This is an important point.

The while-loop in EventHook can only be exited if either stdin_ready is 1 or errorInCmd is 1.
stdin_ready is set to 1 only in MyFileProc, which also calls Tcl_DeleteFileHandler. As the file handler is deleted, there won't be a dangling pointer.

But I was missing a call to Tcl_DeleteFileHandler for the case in which errorInCmd is 1, and MyFileProc was never called. Then then file handler will stay alive and will keep a dangling pointer to tfile after EventHook exits.

I fixed this by adding a call to Tcl_DeleteFileHandler to the if (errorInCmd) {... to ensure that the file handler is deleted before EventHook exits, and no pointer to tfile remains,

}
#endif

Expand All @@ -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);
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Suggested change
Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, &tfile);
Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, (void *)(Py_intptr_t)tfile);

#endif
while (!errorInCmd && !stdin_ready) {
int result;
Expand All @@ -3398,9 +3400,6 @@ EventHook(void)
if (result < 0)
break;
}
#ifndef MS_WINDOWS
Tcl_DeleteFileHandler(tfile);
#endif
if (errorInCmd) {
errorInCmd = 0;
PyErr_SetRaisedException(excInCmd);
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