Add WhereDepth user preference to control depth of initial stack trace in break loops

[limakzi]

Adds a WhereDepth user preference that controls the default number of stack frames shown by Where and WhereWithVars when called without an explicit depth argument (i.e. in the default OnBreak handler).

Previously this was hardcoded to 5.
The default remains 5, so existing behaviour is unchanged.

Fixes #6210

[Copilot]

Pull request overview

Adds a new core GAP user preference (WhereDepth) to control the default stack depth used by Where / WhereWithVars when called with no explicit argument (notably via the default OnBreak handler), keeping the default behavior at 5.

Changes:

• Declare new WhereDepth user preference (default 5, non-negative integer).
• Use UserPreference("WhereDepth") as the default depth in Where and WhereWithVars.
• Add an installation test that verifies the preference exists and can be changed.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lib/init.g	Declares the new WhereDepth user preference with default/check logic.
lib/error.g	Switches Where / WhereWithVars default depth from constant 5 to UserPreference("WhereDepth").
tst/testinstall/error.tst	Adds a basic regression test for reading/setting WhereDepth.

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[fingolfin]

Nice, thanks!

[fingolfin]

Actually, this also needs to adjust the Where documentation, which currently states

If not given, nr defaults to 5.

[fingolfin]

This may fail if the user changed the preference

[limakzi]

It tests defaults.

[fingolfin]

No, it doesn't test default, it queries the current setting, which may or may not be the default value. On my computer it most likely will not be the default value 5 but rather something like 30

[fingolfin]

Isn't this really just testing the UserPreference system, not WhereDepth? that doesn't strike me as super useful.

Instead, the place to test this ought to be in tst/testspecial/backtrace.g or tst/testspecial/backtrace2.g or so (actually... probably in tst/testspecial/stack-trace-depth.g but that only gets added in PR #6263 -- so now I am thinking about adding it right away in a PR of its own, with the "old" Where, so that (a) you can use it here, (b) one sees how the output compares from old to new in my PR.

And that also reminds me of another issue: all those tests producing backtraces will break if the user running the test suite has a custom WhereDepth set. This will affect me as I will immediately set WhereDepth to 30 or so .

To solve this, tst/testspecial/run_gap.sh needs to be edited to do SetUserPreference("WhereDepth", 5); (it already changes UseColorsInTerminal so it should be easy to do this)