Ensure that cache directory is writable

Fixes: ansible/ansible-lint#4497

src/ansible_compat/prerun.py

@@ -1,6 +1,8 @@
 """Utilities for configuring ansible runtime environment."""
 
+import hashlib
 import os
+import tempfile
 from pathlib import Path
 
 
@@ -13,17 +15,37 @@ def get_cache_dir(project_dir: Path, *, isolated: bool = True) -> Path:
 
     Returns:
         Cache directory path.
+
+    Raises:
+        RuntimeError: if cache directory is not writable.
     """
+    cache_dir = Path(os.environ.get("ANSIBLE_HOME", "~/.ansible")).expanduser()
+
     if "VIRTUAL_ENV" in os.environ:
-        cache_dir = Path(os.environ["VIRTUAL_ENV"]) / ".ansible"
+        path = Path(os.environ["VIRTUAL_ENV"])
+        if not path.exists():
+            msg = f"VIRTUAL_ENV={os.environ['VIRTUAL_ENV']} does not exist."
+            raise RuntimeError(msg)
+        cache_dir = path.resolve() / ".ansible"
     elif isolated:
-        cache_dir = project_dir / ".ansible"
-    else:
-        cache_dir = Path(os.environ.get("ANSIBLE_HOME", "~/.ansible")).expanduser()
+        if not project_dir.exists() or not os.access(project_dir, os.W_OK):
+            # As "project_dir" can also be "/" and user might not be able
+            # to write to it, we use a temporary directory as fallback.
+            checksum = hashlib.sha256(
+                project_dir.as_posix().encode("utf-8"),
+            ).hexdigest()[:4]
+
+            cache_dir = Path(tempfile.gettempdir()) / f".ansible-{checksum}"
+            cache_dir.mkdir(parents=True, exist_ok=True)
+        else:
+            cache_dir = project_dir.resolve() / ".ansible"
 
     # Ensure basic folder structure exists so `ansible-galaxy list` does not
     # fail with: None of the provided paths were usable. Please specify a valid path with
+    if not os.access(cache_dir, os.W_OK):
+        msg = f"Cache directory {cache_dir} is not writable."
+        raise RuntimeError(msg)
+
     for name in ("roles", "collections"):  # pragma: no cover
         (cache_dir / name).mkdir(parents=True, exist_ok=True)
-
     return cache_dir