Why does bash print the debug trap after edit-and-execute-command

Question

I've found a behavior in bash (or is it readline?) that I cannot explain. After invoking the edit-and-execute-command Readline command, the debug trap's command is printed prior to execution. Is this a bug?

Here's how to reproduce the issue with a default bash install (no dotfiles besides those in /etc, and it happens on Linux and macOS with all the versionf of bash I could find):

bash-5.0$ trap date DEBUG
bash-5.0$ echo hi
Fri May  1 23:07:38 PDT 2020
hi
bash-5.0$ echo hi

At this point, type Control-X Control-E. The editor opens. Exit the editor. This is what you see next:

echo hi
date
Fri May  1 23:07:47 PDT 2020
hi
bash-5.0$ echo hi

My question is if there's any good reason for the shell to print date after exiting the editor.

The reason this is a problem for me is that I use the debug trap to run a command after the user enters a command but before it runs for iTerm2's shell integration using https://github.com/rcaloras/bash-preexec. AFAIK there is no other way to do this in bash (though I'm open to alternatives!)

For context, it originally came up here: https://gitlab.com/gnachman/iterm2/-/issues/8808#note_331367916

Answer 1

I got lucky and found an almost satisfying answer in the bash source code. It's detailed here: https://gitlab.com/gnachman/iterm2/-/issues/8808#note_335276685