bug#45380: 28.0.50; Error in coloured output in Emacs 28.05(AUR package: emacs-git)

[Morgan Smith]

Hey all,

I spent a good chunk of time debugging this but I'm pretty new to emacs devel 
so please let me know if I say anything funny.


How to reproduce: Just keep trying some long running colored commands in eshell 
like "git log --color". Eventually the logic will get messed up and it'll look 
like the previously posted screenshot. I've seen this bugs hundred of times 
from all sorts of different external commands. As long as the command has 
enough output it's not a question of "if" but "when" will the bug show.

I think the issue is in eshell-output-filter.  Specifically I think the issue 
is with how it sets the eshell-last-output-start and eshell-last-input-end 
markers.

If we add some advice around our coloring function we can see that the coloring 
regions "overlap".  I assume this overlap is because coloring the region is 
removing the control characters and making the region smaller.  I don't think 
this is actually an issue, it's just something that smells like it could create 
bugs.

;; How to see the marker values
(defun where-are-markers (marker1 marker2)
(message "%s and %s" marker1 marker2))
(advice-add 'ansi-color-apply-on-region :before 'where-are-markers)


Thankfully, we can test out what would happen if we colored each bit of output 
as it comes in without worrying about regions. All we have to do is run this 
first:

(delete 'eshell-handle-ansi-color eshell-output-filter-functions)
(add-to-list 'eshell-preoutput-filter-functions 'ansi-color-apply)

And voila! Problem gone! Also I feel like this might run faster then the 
current method of filtering but I might be wrong about that.

So in conclusion: I found a workaround that might be more performant then the 
current coloring method but also doesn't actually fix the bug so we should 
probably dig deeper.


Morgan