Use `platformdirs` for `.config.get_rt_dir()`

Thanks to the `tox`-dev community for such a lovely pkg which seems to
solves all the current cross-platform user-dir problems B)

Also this,
- now passes `platformdirs.user_runtime_dir(appname='tractor')`
  and allows caller to pass an optional `subdir` under `tractor/`
  if desired.
- drops the `.config._rtdir: Path` mod var.
- bumps the lock file with the new dep.

pyproject.toml

@@ -63,11 +63,11 @@ dev = [
   "stackscope>=0.2.2,<0.3",
   # ^ requires this?
   "typing-extensions>=4.14.1",
-
   "pyperclip>=1.9.0",
   "prompt-toolkit>=3.0.50",
   "xonsh>=0.19.2",
   "psutil>=7.0.0",
+  "platformdirs>=4.4.0",
 ]
 # TODO, add these with sane versions; were originally in
 # `requirements-docs.txt`..

tractor/_state.py

@@ -22,7 +22,6 @@ from __future__ import annotations
 from contextvars import (
     ContextVar,
 )
-import os
 from pathlib import Path
 from typing import (
     Any,
@@ -30,6 +29,7 @@ from typing import (
     TYPE_CHECKING,
 )
 
+import platformdirs
 from trio.lowlevel import current_task
 
 if TYPE_CHECKING:
@@ -172,22 +172,31 @@ def current_ipc_ctx(
     return ctx
 
 
-# std ODE (mutable) app state location
-_rtdir: Path = Path(os.environ['XDG_RUNTIME_DIR'])
-
 
 def get_rt_dir(
-    subdir: str = 'tractor'
+    subdir: str|Path|None = None,
 ) -> Path:
     '''
-    Return the user "runtime dir" where most userspace apps stick
-    their IPC and cache related system util-files; we take hold
-    of a `'XDG_RUNTIME_DIR'/tractor/` subdir by default.
+    Return the user "runtime dir", the file-sys location where most
+    userspace apps stick their IPC and cache related system
+    util-files.
+
+    On linux we take use a `'${XDG_RUNTIME_DIR}/tractor/` subdir by
+    default but equivalents are mapped for each platform using
+    the lovely `platformdirs`.
 
     '''
-    rtdir: Path = _rtdir / subdir
+    _rt_dir: Path = Path(
+        platformdirs.user_runtime_dir(
+            appname='tractor',
+        ),
+    )
+    if subdir:
+        rtdir: Path = _rt_dir / subdir
+
     if not rtdir.is_dir():
         rtdir.mkdir()
+
     return rtdir
 
 

uv.lock

@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.11"
 
 [[package]]
@@ -236,6 +236,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/9e/c3/059298687310d527a58bb01f3b1965787ee3b40dce76752eda8b44e9a2c5/pexpect-4.9.0-py2.py3-none-any.whl", hash = "sha256:7236d1e080e4936be2dc3e326cec0af72acf9212a7e1d060210e70a47e253523", size = 63772, upload-time = "2023-11-25T06:56:14.81Z" },
 ]
 
+[[package]]
+name = "platformdirs"
+version = "4.4.0"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/23/e8/21db9c9987b0e728855bd57bff6984f67952bea55d6f75e055c46b5383e8/platformdirs-4.4.0.tar.gz", hash = "sha256:ca753cf4d81dc309bc67b0ea38fd15dc97bc30ce419a7f58d13eb3bf14c4febf", size = 21634, upload-time = "2025-08-26T14:32:04.268Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/40/4b/2028861e724d3bd36227adfa20d3fd24c3fc6d52032f4a93c133be5d17ce/platformdirs-4.4.0-py3-none-any.whl", hash = "sha256:abd01743f24e5287cd7a5db3752faf1a2d65353f38ec26d98e25a6db65958c85", size = 18654, upload-time = "2025-08-26T14:32:02.735Z" },
+]
+
 [[package]]
 name = "pluggy"
 version = "1.5.0"
@@ -387,6 +396,7 @@ dependencies = [
 dev = [
     { name = "greenback" },
     { name = "pexpect" },
+    { name = "platformdirs" },
     { name = "prompt-toolkit" },
     { name = "psutil" },
     { name = "pyperclip" },
@@ -412,6 +422,7 @@ requires-dist = [
 dev = [
     { name = "greenback", specifier = ">=1.2.1,<2" },
     { name = "pexpect", specifier = ">=4.9.0,<5" },
+    { name = "platformdirs", specifier = ">=4.4.0" },
     { name = "prompt-toolkit", specifier = ">=3.0.50" },
     { name = "psutil", specifier = ">=7.0.0" },
     { name = "pyperclip", specifier = ">=1.9.0" },