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bug#51037: [PATCH] Make `print-level` & `print-length` customizable in E

From: Michael
Subject: bug#51037: [PATCH] Make `print-level` & `print-length` customizable in ERT batch tests
Date: Tue, 26 Oct 2021 14:10:01 -0700
User-agent: mu4e 1.4.15; emacs 28.0.50

Lars Ingebrigtsen <larsi@gnus.org> writes:

Michael <sp1ff@runbox.com> writes:

The issue: stack traces are printed by the `backtrace`
package. backtrace.el was authored by Gemini (which is why I've
added him to this thread). backtrace.el doesn't directly work in
terms of `print-le{ngth,vel}`: it defines a custom variable
`backtrace-line-length` and then adjusts print-level &
print-length in let bindings in order to:

 1. try to respect the desired line length
 2. not trigger bug 31919 (Lisp Debugger doesn't work when at
 stack limit)


For myself, I was quite surprised to all learn this: I had to
spend a fair bit of time digging through the source to find out
what was happening to my settings for print-length &

Yeah, I didn't remember `backtrace-line-length' either. Perhaps it
should be linked to from the doc strings of print-length and

I was thinking to augment the docstring for
`backtrace-line-lenght' to note that print-level & print-length
will be overiden to honor it.

My personal incliniation is to remove the `backtrace-line-length`
variable entirely, and make the `debug` package responsible for
controlling print-level so as to avoid 31919. But that's me: is
there a compelling use-case for backtrace.el working in terms of limiting line length rather than just using `print-le{ve,ength}`?

Well... if we're talking in an ert context, it could increase both the backtrace-line-length and max-lisp-eval-depth variables when gathering
the backtrace, I think?

The problem is that I can't know a priori how big I have to set
them in order to honor print-level & print-length. I think I can
short-circuit this entirely by setting `backtrace-line-length' to
nil or zero before printing the backtrace from ert. eval-level
shouldn't enter into it I think; the concern was that if code
exceeded max-lisp-eval-depth, and the debugger was triggered,
*then* backtrace.el might have problems. Do you see a reason to
be concerned outside of that corner case?

Michael <sp1ff@runbox.com>

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