fix(setup): use consistent runtime paths and reject incomplete installs

tools/runtime_paths.py

@@ -0,0 +1,24 @@
+"""Shared host-side destinations for configured runtime installers."""
+import os
+from pathlib import Path
+
+
+def runtime_paths(config):
+    prefix = Path(os.environ.get("DPREFIX") or config["prefix"])
+    guest_home = Path(os.environ.get("RIOT_GUEST_HOME") or
+                      str(prefix / config["guest_home"].lstrip("/")))
+    if not prefix.is_absolute() or not guest_home.is_absolute():
+        raise ValueError("Runtime paths must be absolute host paths")
+    if ".." in guest_home.parts or not guest_home.is_relative_to(prefix) or guest_home == prefix:
+        raise ValueError("RIOT_GUEST_HOME must be inside DPREFIX")
+    if not guest_home.resolve().is_relative_to(prefix.resolve()) or guest_home.resolve() == prefix.resolve():
+        raise ValueError("RIOT_GUEST_HOME must resolve inside DPREFIX")
+    return prefix, guest_home
+
+
+if __name__ == '__main__':
+    import sys
+    try:
+        runtime_paths({'prefix': sys.argv[1], 'guest_home': '/'},)
+    except (ValueError, OSError) as error:
+        raise SystemExit(str(error))