[BUG] _validate_dir requires every configured directory to be writable — breaks read-only container and input directories at 2.0.0

[dadamsncsa]

Summary

Config._validate_dir on main requires every configured directory to be
writable. Three of the seven it is applied to are legitimately read-only, so a
site install with an admin-managed container directory — and the read-only input
directory that #121's changelog entry advertises — both fail at Config()
construction. v1.0.0 has no such check, so this breaks working setups on
upgrade.

Reproducer 1 — admin-managed container directory

Our Lmod module for the deployed llmflux/1.0.0 sets

setenv("LLMFLUX_CONTAINERS_DIR", "/sw/llmhub/llmflux/containers/1.0.0")

pointing at a versioned, service-account-owned directory holding
llm_processor.sif, mode 0755, deliberately not user-writable. Against main
@ 64686c9:

$ LLMFLUX_CONTAINERS_DIR=/sw/llmhub/llmflux/containers/1.0.0 \
    llmflux run --model Qwen2.5-0.5B-Instruct --input p.jsonl --output o.json \
                --account <acct> --partition <part>
Traceback (most recent call last):
  File ".../llmflux/cli.py", line 1054, in main
    return args.func(args)
  File ".../llmflux/cli.py", line 295, in _run_command
    config = Config()
  File ".../llmflux/core/config.py", line 219, in __init__
    self.containers_dir = str(self._validate_dir(Path(containers_dir or os.getenv('LLMFLUX_CONTAINERS_DIR') or self.workspace / "containers").expanduser().resolve()))
  File ".../llmflux/core/config.py", line 257, in _validate_dir
    raise OSError(f"Cannot use directory '{path}': {e}") from e
OSError: Cannot use directory '/sw/llmhub/llmflux/containers/1.0.0': Directory is not writable: /sw/llmhub/llmflux/containers/1.0.0

An unhandled traceback, before any work is attempted.

Reproducer 2 — the read-only input directory #121 advertises

import os, tempfile
os.environ['LLMFLUX_WORKSPACE'] = tempfile.mkdtemp()
os.environ['LLMFLUX_DATA_INPUT_DIR'] = '<a 0555 directory>'
from llmflux.core.config import Config
Config()
# OSError: Cannot use directory '...': Directory is not writable: ...

The [Unreleased] changelog entry for #121 says the input/output split exists so
a user can have "a read-only input location on one filesystem and results
written to another". That configuration is currently fatal.

The change

v1.0.0, src/llmflux/core/config.py:214 — no validation:

self.containers_dir = containers_dir or os.getenv('LLMFLUX_CONTAINERS_DIR') or str(self.workspace / "containers")

main, src/llmflux/core/config.py:240-258:

def _validate_dir(self, path: Path) -> Path:
    """Ensure a directory exists and is writable, creating it if necessary."""
    try:
        path.mkdir(parents=True, exist_ok=True)
        if not os.access(path, os.W_OK):
            raise PermissionError(f"Directory is not writable: {path}")
    except (OSError, PermissionError) as e:
        raise OSError(f"Cannot use directory '{path}': {e}") from e
    return path

applied at config.py:211,214-219 to workspace, data_dir, data_input_dir,
data_output_dir, models_dir, logs_dir and containers_dir.

Which of the seven actually need write access

Directory	Written to by llmflux?	Writability check correct?
workspace	yes	yes
data_dir	yes	yes
data_output_dir	yes	yes
logs_dir	yes	yes
containers_dir	no — only $CONTAINERS_DIR/llm_processor.sif is read	no
data_input_dir	no — read-only input is a documented use case (#121)	no
models_dir	no, where a site stages a shared read-only cache	no

Impact

Any site shipping LLMFlux as a module with an admin-managed container directory
breaks on upgrade to 2.0.0 — module load llmflux && llmflux run fails at
Config(). We have two clusters in that position (x86_64 A100/H200 and aarch64
GH200); both fail. The only workaround is a user-writable directory holding a
symlink to the deployed .sif, which defeats the point of the module setting
LLMFLUX_CONTAINERS_DIR at all and pushes per-user setup onto every user.

Suggested fix

Validate for the access each directory actually needs:

self.containers_dir  = str(self._validate_dir(..., need_write=False))
self.data_input_dir  = str(self._validate_dir(..., need_write=False))
self.models_dir      = str(self._validate_dir(..., need_write=False))
# workspace, data_dir, data_output_dir, logs_dir keep need_write=True

where need_write=False means: must exist (do not mkdir a path the user
cannot create), and must be readable and traversable — os.access(path, os.R_OK | os.X_OK). That keeps the diagnostic value the check was added for
while permitting the read-only configurations the docs and site deployments rely
on. Exempting containers_dir alone unbreaks the module case, but
data_input_dir still needs it for #121's stated use case.

Tests worth adding: containers_dir and data_input_dir at 0555 construct
successfully; logs_dir at 0555 still raises; a non-existent
containers_dir raises with a message naming the variable rather than being
silently created.

Affected

main @ 64686c9 (version = "2.0.0", unreleased). Not present in v1.0.0.
Happy to open a PR for the need_write variant if that shape is agreeable.