Looks like the way you're adding to preexec functions is different than precmd. Shouldn't this be like:

preexec_functions+=(__bp_original_debug_trap)

Oof, good catch. I realize my test is also missing the point, since the DEBUG trap is invoked immediately. I've got a better test pending.

This would also have been caught if we didn't silently skip missing functions - I'd been thinking about suggesting changing that, I'll file a bug unless you'd prefer to leave it be.

I think skipping missing functions is more pragmatic (although you can get silent failures you might want to know) and is the actual behavior by preexec/precmd in zsh. I think we should be trying to emulate that experience.

Modified your change via my comment below then found this issue.

I think there's an edge case here where a trap contains output. If I set a simple debug trap with an echo it's pulling in the output that is produced from the debug trap here. and setting it in the original debug trap function. e.g:

$ trap
trap -- '' SIGTSTP
trap -- '' SIGTTIN
trap -- '' SIGTTOU

$ trap 'echo "from trap"' DEBUG

$ source bash-preexec.sh 
from trap
from trap
from trap
from trap
from trap
from trap
from trap
from trap
from trap

$ type __bp_original_debug_trap
from trap
__bp_original_debug_trap is a function
__bp_original_debug_trap () 
{ 
    from trap;
    from trap;
    echo "from trap"
}

This causes __bp_original_debug_trap to error with the output 'from trap' in the function.

That's really interesting. I think this is due to set -T, I'll experiment some more but I may have to go back to something like your __bp_trap_install_string solution if set -T won't work. I'll get back to you.

sounds good.

Ok, this is fixed. It seems set -T is a non-starter, since it causes the DEBUG trap to be run from within command substitutions:

$ trap 'echo "from trap"' DEBUG
$ trap_cmd=$(trap -p DEBUG)
from trap
$ set -T
from trap
$ trap_t_cmd=$(trap -p DEBUG)
from trap
$ trap DEBUG
from trap
$ :
$ echo "$trap_cmd"
trap -- 'echo "from trap"' DEBUG
$ echo "$trap_t_cmd"
from trap
trap -- 'echo "from trap"' DEBUG

But the pattern I'm using now (capture the trap and clear it in PROMPT_COMMAND, then install the new trap in a function) appears to work.