Fixed Interval classification needs a safe-guard against huge number of classes

[kannes]

What is the bug or the crash?

The new Fixed Interval classification is awesome.
But it makes it really easy to crash the whole system if the number of classes is huge.

There needs to be a safeguard that halts before creating thousands or millions of classes and asks the user if that is what they really want with a warning (best not just about time but about memory and potentially crashing).

Steps to reproduce the issue

Warning, this might make your whole system become unresponsible, requiring a hard reboot, losing all your work.

1. Load a dataset that has a big numeric range for a field, e. g. Natural Earth 50m countries with its GDP_MD field (-99 to 21433226)
2. Classify by that field (GDP_MD) using Fixed Interval classification
3. QGIS will freeze, the system might freeze, you might need to force an unexpected reboot

Versions

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS-Version	3.26.0-Buenos Aires	QGIS-Codeversion	0aece28
Qt-Version	5.12.8
Python-Version	3.8.10
GDAL-Version	3.0.4
PROJ-Version	6.3.1
EPSG-Registraturdatenbankversion	v9.8.6 (2020-01-22)
Kompiliert mit GEOS	3.8.0-CAPI-1.13.1	Läuft mit GEOS	3.8.0-CAPI-1.13.1
SQLite-Version	3.31.1
PDAL-Version	2.0.1
PostgreSQL-Client-Version	12.11 (Ubuntu 12.11-0ubuntu0.20.04.1)
SpatiaLite-Version	4.3.0a
QWT-Version	6.1.4
QScintilla2-Version	2.11.2
BS-Version	Ubuntu 20.04.4 LTS
Aktive Python-Erweiterungen
processing	2.12.99
sagaprovider	2.12.99
grassprovider	2.12.99
MetaSearch	0.3.6
db_manager	0.1.20

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

As other classification methods emit warnings before the user creates a big number of classes and because this is something that can wreck total havoc by eating all available RAM, crashing QGIS or more likely the whole system, I'd consider it a bug (oversight), not a feature request. Hope that's alright! :)

[uclaros]

Fixed in #57722

[kannes]

Nice! Thanks @uclaros and @nyalldawson