In-project `--output_user_root` triggers the repo contents cache error, even though the target directory is excluded with `.bazelignore`

[yuyawk]

Description of the bug:

After upgrading bazel from 8.2.1 to 8.3.0, I started to see an error like

ERROR: The repo contents cache [/home/runner/work/rules_build_error/rules_build_error/.cache/bazel/cache/repos/v1/contents] is inside the workspace [/home/runner/work/rules_build_error/rules_build_error]. This can cause spurious failures. Disable the repo contents cache with `--repo_contents_cache=`, or specify `--repo_contents_cache=<path outside the workspace>`.

when setting --output_user_root to the directory inside the project. (I guess this was brought by this commit)

However, this error happens even when that cache directory is listed in .bazelignore, though the directory is ignored and therefore won't cause any spurious failures.

Which category does this issue belong to?

External Dependency

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

When using bazel 8.3.0,

(1) add a line

startup --output_user_root=.cache/bazel

to .bazelrc

(2) add a line

.cache

to .bazelignore

(3) execute a bazel command

(If I comment out the line in .bazelrc the error disappears)

Which operating system are you running Bazel on?

ubuntu 22.04, 24.04

What is the output of bazel info release?

release 8.3.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

N/A (using the released version)

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

ad74aa5

Have you found anything relevant by searching the web?

N/A

Any other information, logs, or outputs that you want to share?

Here I put some CI logs indicating the issue:

• https://github.com/yuyawk/rules_build_error/actions/runs/15871801864/job/44749934685
• https://github.com/yuyawk/rules_build_error/actions/runs/15845818368/job/44667642912
• https://github.com/yuyawk/libc_replacer/actions/runs/15871755131/job/44749774518
• https://github.com/yuyawk/libc_replacer/actions/runs/15845772215/job/44667489698

[fmeum]

@bazel-io fork 8.3.1

[fmeum]

@Wyverald

[Wyverald]

I'm a bit on the fence about whether this counts as a breaking change (especially as "output base in main repo" was never a sanctioned use case). We have a few options here (in the order of my personal preference):

1. Claim this is WAI -- the error message already mentions a fix: specify --repo_contents_cache=
2. On 8.x only, silently disable the repo contents cache if it's detected to be inside the main repo
3. On 8.x only, disable the repo contents cache by default
4. Check if the repo contents cache is also in .bazelignore or a subdirectory thereof; if so, suppress the error

@fmeum WDYT?