Loki 3.0.0: local ruler-storage now needlessly requires the rules directory to be writable

[whentze]

Describe the bug
Loki v3.0.0, when configured to use the local ruler-storage backend and given a read-only rule directory, crashes at startup.

This regression seems to have been introduced in #12019, in which the EnsureDirectory utility was extended with an additional check that the directory be writable. That check is needed for the bloomstore but not for all the other users of that utility function, such as ruler-storage.

To Reproduce
Steps to reproduce the behavior:

1. Configured local ruler-storage backend with a read-only directory
2. Started loki v3.0.0
3. It crashed (see below for log messages)

Expected behavior
Loki should start normally. The local ruler-storage backend is documented as read-only so it should work if that directory is read-only (as it did in 2.9.x).

Environment:

• Infrastructure: Linux VM
• Deployment tool: NixOS

Note: This issue also occurs in containerized setups in which the ruler storage is mounted in as a readonly volume.

Screenshots, Promtail config, or terminal output

insufficient permissions: /nix/store/1rnvm2hh2qmk37plaryxqpy8y2jzv2ri-loki-rules dr-xr-xr-x
error initialising module: ruler-storage
github.com/grafana/dskit/modules.(*Manager).initModule
        github.com/grafana/[email protected]/modules/modules.go:138
github.com/grafana/dskit/modules.(*Manager).InitModuleServices
        github.com/grafana/[email protected]/modules/modules.go:108
github.com/grafana/loki/v3/pkg/loki.(*Loki).Run
        github.com/grafana/loki/v3/pkg/loki/loki.go:453
main.main
        github.com/grafana/loki/v3/cmd/loki/main.go:122
runtime.main
        runtime/proc.go:271
runtime.goexit
        runtime/asm_amd64.s:1695

[owen-d]

Ugh, sorry -- I pushed a PR to address this and make it less likely in the future.

[whentze]

No worries, and thank you for the very quick fix! 👍🏼

Could you cut a patch release for this, too?