gh-139145: fix spinning event loop in tkinter by mdehoon · Pull Request #139180 · python/cpython · GitHub

This pull request fixes the bug described in issue #139145 by moving the call to Tcl_DeleteFileHandler from EventHook to MyFileProc.
The reasoning behind this is:

after the first call to MyFileProc, stdin_ready is already set to 1, so there is no reason to keep watching stdin and call MyFileProc again, so we may as well call Tcl_DeleteFileHandler` immediately;
more importantly, if EventHook started another event loop (e.g. the tight event loop started by wait_variable; see issue tkinter wait_variable can cause the event loop to spin #139145), then calling Tcl_DeleteFileHandler in MyFileProc (instead of waiting for control to return to EventHook) will ensure that the tight event loop won't be spinning its wheels by calling MyFileProc continuously.

Issue: tkinter wait_variable can cause the event loop to spin #139145

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…issue139145_bugfix

I am just a past contributor, not a Python member.

I think the issue description and proposed change to _tkinter.c make sense. I have not reviewed the unit test.

However, I notice tfile being passed by address to Tcl_CreateFileHandler(); is there a particular reason for doing so? To avoid any potential stack-use-after-scope errors, such as if the while loop in EventHook() were to end because of errorInCmd rather than stdin_ready, I think it is better to pass tfile by value, even though that requires casting to avoid compiler warnings/errors (unless CPython already has some PTR2INT()/INT2PTR()-style macros I am unaware of).

Suggested change

      
                Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, &tfile);
          
                Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, (void *)(Py_intptr_t)tfile);

Suggested change

      
                int *tfile = clientData;
          
                stdin_ready = 1;
          
                Tcl_DeleteFileHandler(*tfile);
          
                int tfile = (int)(Py_intptr_t)clientData;
          
                stdin_ready = 1;
          
                Tcl_DeleteFileHandler(tfile);

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,

Let's keep this PR open for now. In particular, it would be better to get PR #140147 #148819 through first, as it would let us run the tests also on Windows.

This PR is stale because it has been open for 30 days with no activity.

I opened #152257, which builds on this — it keeps your MyFileProc fix, adds @chrstphrchvz's value-packing of the fd into clientData, deletes the handler on every loop-exit path (the result < 0 case), and also folds in the related gh-139816 hang since it's the same function. Credited you both as co-authors.

Happy to go either way: land yours for gh-139145 and I'll rebase the rest on top, or take the combined PR. Thanks for digging into this.

@serhiy-storchaka

Thanks for looking into this.

Happy to go either way: land yours for #139145 and I'll rebase the rest on top, or take the combined PR. Thanks for digging into this.

I am fine either way too, so whatever is easiest for you is fine for me.

bpo-139145: fix spinning event loop in tkinter

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bedevere-app Bot added the awaiting review label Sep 20, 2025

bedevere-app Bot mentioned this pull request Sep 20, 2025

tkinter wait_variable can cause the event loop to spin #139145

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StanFromIreland reviewed Sep 20, 2025

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

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…nteractively.

blurb-it Bot and others added 4 commits September 20, 2025 15:52

📜🤖 Added by blurb_it.

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fixing lint

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Merge branch 'issue139145_bugfix' of github.com:mdehoon/cpython into …

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…issue139145_bugfix

fix blurb

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chrstphrchvz suggested changes Sep 30, 2025

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bedevere-app Bot added awaiting core review and removed awaiting review labels Sep 30, 2025

Michiel de Hoon and others added 5 commits October 1, 2025 16:07

simplify test

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be sure to delete the file handler also in case of errorInCmd

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only relevant on Windows

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trivial change to trigger another round of testing

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Merge branch 'main' into issue139145_bugfix

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github-actions Bot added the stale Stale PR or inactive for long period of time. label Apr 29, 2026

github-actions Bot removed the stale Stale PR or inactive for long period of time. label Jun 8, 2026

serhiy-storchaka self-requested a review June 25, 2026 10:29

serhiy-storchaka mentioned this pull request Jun 26, 2026

gh-139145: Fix tkinter event loop in interactive mode #152257

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serhiy-storchaka closed this Jun 26, 2026

mdehoon deleted the issue139145_bugfix branch June 26, 2026 14:50

               #ifndef MS_WINDOWS
                   tfile = fileno(stdin);
-                  Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, NULL);
+                  Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, &tfile);

gh-139145: fix spinning event loop in tkinter by mdehoon · Pull Request #139180 · python/cpython · GitHub

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	Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, &tfile);
	Tcl_CreateFileHandler(tfile, TCL_READABLE, MyFileProc, (void *)(Py_intptr_t)tfile);

gh-139145: fix spinning event loop in tkinter by mdehoon · Pull Request #139180 · python/cpython · GitHub

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