`bst shell` is giving a bug trace when using `--mount`

Reproduce the failure with bst shell components/python3-mkdocs.bst --mount /src/tests/ -- mkdocs build. Start at buildstream/element.py in _shell, then follow the mount handling into buildstream/sandbox/sandbox.py at mark_directory; compare this path with the CLI shell entry point in buildstream/_frontend/cli.py. Done means the reported mount command runs without the assertion or bug trace.

I'm trying to work out good ways to add some tests to a bst project

I thought about using some of the software inside a shell, this works for basic command --help tests.

toolchain-2026-06-03 src # bst shell components/python3-mkdocs.bst -- mkdocs --help
[--:--:--][        ][    main:core activity                 ] START   Loading elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Loading elements
[--:--:--][        ][    main:core activity                 ] START   Resolving elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Resolving elements
[--:--:--][        ][    main:core activity                 ] START   Initializing remote caches
[00:00:02][        ][    main:core activity                 ] SUCCESS Initializing remote caches
[--:--:--][        ][    main:core activity                 ] START   Query cache
[00:00:00][        ][    main:core activity                 ] SUCCESS Query cache
[--:--:--][d37eff3b][    main:components/python3-mkdocs.bst ] START   Staging dependencies
[00:00:00][d37eff3b][    main:components/python3-mkdocs.bst ] SUCCESS Staging dependencies
[--:--:--][d37eff3b][    main:components/python3-mkdocs.bst ] START   Integrating sandbox
[00:00:00][d37eff3b][    main:components/python3-mkdocs.bst ] SUCCESS Integrating sandbox
[--:--:--][d37eff3b][    main:components/python3-mkdocs.bst ] STATUS  Running command

    mkdocs --help

Usage: mkdocs [OPTIONS] COMMAND [ARGS]...

  MkDocs - Project documentation with Markdown.

Options:
  -V, --version         Show the version and exit.
  -q, --quiet           Silence warnings
  -v, --verbose         Enable verbose output
  --color / --no-color  Force enable or disable color and wrapping for the output. Default is auto-detect.
  -h, --help            Show this message and exit.

Commands:
  build      Build the MkDocs documentation.
  get-deps   Show required PyPI packages inferred from plugins in mkdocs.yml.
  gh-deploy  Deploy your documentation to GitHub Pages.
  new        Create a new MkDocs project.
  serve      Run the builtin development server.
dcs-toolchain-2026-06-03 src # 

but when i try to add a example project to use i need to use --mount which triggers a bug for me

toolchain-2026-06-03 src # ls /src/tests/
config.yml  index.md

toolchain-2026-06-03 src # bst shell components/python3-mkdocs.bst --mount /src/tests/ -- mkdocs build
[--:--:--][        ][    main:core activity                 ] START   Loading elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Loading elements
[--:--:--][        ][    main:core activity                 ] START   Resolving elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Resolving elements
[--:--:--][        ][    main:core activity                 ] START   Initializing remote caches
[00:00:02][        ][    main:core activity                 ] SUCCESS Initializing remote caches
[--:--:--][        ][    main:core activity                 ] START   Query cache
[00:00:00][        ][    main:core activity                 ] SUCCESS Query cache
[--:--:--][d37eff3b][    main:components/python3-mkdocs.bst ] START   Staging dependencies
[00:00:00][d37eff3b][    main:components/python3-mkdocs.bst ] SUCCESS Staging dependencies
[--:--:--][d37eff3b][    main:components/python3-mkdocs.bst ] START   Integrating sandbox
[00:00:00][d37eff3b][    main:components/python3-mkdocs.bst ] SUCCESS Integrating sandbox
[--:--:--][        ][    main:core activity                 ] BUG     The directories marked in the sandbox must be absolute paths

    Traceback (most recent call last):
      File "/usr/sbin/bst", line 8, in <module>
        sys.exit(cli())
                 ~~~^^
      File "/usr/lib/python3.13/site-packages/click/core.py", line 1442, in __call__
        return self.main(*args, **kwargs)
               ~~~~~~~~~^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/buildstream/_frontend/cli.py", line 282, in override_main
        original_main(self, args=args, prog_name=prog_name, complete_var=None, standalone_mode=standalone_mode, **extra)
        ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/click/core.py", line 1363, in main
        rv = self.invoke(ctx)
      File "/usr/lib/python3.13/site-packages/click/core.py", line 1830, in invoke
        return _process_result(sub_ctx.command.invoke(sub_ctx))
                               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/click/core.py", line 1226, in invoke
        return ctx.invoke(self.callback, **ctx.params)
               ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/click/core.py", line 794, in invoke
        return callback(*args, **kwargs)
      File "/usr/lib/python3.13/site-packages/click/decorators.py", line 46, in new_func
        return f(get_current_context().obj, *args, **kwargs)
      File "/usr/lib/python3.13/site-packages/buildstream/_frontend/cli.py", line 774, in shell
        exitcode = app.stream.shell(
            target,
        ...<9 lines>...
            ignore_project_source_remotes=ignore_project_source_remotes,
        )
      File "/usr/lib/python3.13/site-packages/buildstream/_stream.py", line 360, in shell
        return element._shell(
               ~~~~~~~~~~~~~~^
            scope, mounts=mounts, isolate=isolate, prompt=prompt(element), command=command, usebuildtree=usebuildtree
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        )
        ^
      File "/usr/lib/python3.13/site-packages/buildstream/element.py", line 2099, in _shell
        sandbox.mark_directory(mount.path)
        ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/buildstream/sandbox/sandbox.py", line 168, in mark_directory
        assert directory.startswith("/"), "The directories marked in the sandbox must be absolute paths"
               ~~~~~~~~~~~~~~~~~~~~^^^^^
    AssertionError: The directories marked in the sandbox must be absolute paths