Validation error for custom_build_dir parameter in runner 17.9

Summary

After upgrading to 17.9, I am getting validation errors about custom_build_dir:

There might be a problem with your config based on jsonschema annotations in common/config.go (experimental feature):
Feb 28 15:02:32 perception-ci3 gitlab-runner[1643839]: jsonschema: '/runners/0/custom_build_dir/enabled' does not validate with https://gitlab.com/gitlab-org/gitlab-runner/common/config#/$ref/properties/runners/items/$ref/properties/custom_build_dir/$ref/properties/enabled/type: expected boolean, but got null

My config contained:

  # ...
  executor = "shell"
  [runners.custom_build_dir]
  [runners.cache]
  # ...

As far as I know this is what was generated for me, I did not add this empty runners.custom_build_dir section. I get the validation error even if I omit the section entirely:

  # ...
  executor = "shell"
  [runners.cache]
  # ...

The only way I can stop the validation error is by defining a value for enabled:

  # ...
  executor = "shell"
  [runners.custom_build_dir]
    enabled = false
  [runners.cache]
  # ...

Steps to reproduce

Edit config.toml to include an empty runners.custom_build_dir section or omit it entirely. Tail gitlab-runner systemd service logs and check for validation errors.

Actual behavior

Validation error when my config.toml contains an empty runners.custom_build_dir section or does not contain a runners.custom_build_dir section.

Expected behavior

No validation error unless I've passed an incorrect value for enabled.

Relevant logs and/or screenshots

job log

There might be a problem with your config based on jsonschema annotations in common/config.go (experimental feature):
Feb 28 15:02:32 perception-ci3 gitlab-runner[1643839]: jsonschema: '/runners/0/custom_build_dir/enabled' does not validate with https://gitlab.com/gitlab-org/gitlab-runner/common/config#/$ref/properties/runners/items/$ref/properties/custom_build_dir/$ref/properties/enabled/type: expected boolean, but got null

Environment description

Custom shell executor installation on Ubuntu 22.04.

config.toml contents

concurrent = 1
check_interval = 0
shutdown_timeout = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "***"
  url = "https://gitlab.com"
  id = ***
  token = "glrt-****"
  pre_build_script = "source hil pre-build-script"
  post_build_script = "hil post-build-script"
  token_obtained_at = 2024-12-11T16:53:25Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "shell"
  shell = "bash"
  output_limit = 102401
  [runners.cache]
    MaxUploadedArchiveSize = 0
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]

Used GitLab Runner version

Version:      17.9.0
Git revision: c4cbe9dd
Git branch:   17-9-stable
GO version:   go1.23.2 X:cacheprog
Built:        2025-02-20T00:52:34Z
OS/Arch:      linux/amd64