Add darling-lol --set-prefix to keep the prefix on another drive

The prefix holds the installed clients and game (about 40 GB). The Nix
launcher always exported DPREFIX=${DPREFIX:-~/.darling}, so the prefix saved
in build/config.json was never used and the only way to move it was an
environment variable in every launch context.

- The launcher uses DPREFIX if set, then the saved prefix, then ~/.darling.
- darling-lol --set-prefix PATH runs scripts/nix-prefix.py in the namespace,
  whose root owns the prefix files, so cp -a keeps their owners and xattrs.
  It renames the prefix when possible; across filesystems it copies to
  PATH.partial and removes the old prefix and its overlay work directory
  only after the copy completes. A PATH that already holds a Darling prefix
  is used as it is. It refuses non-empty directories that are not prefixes,
  nested paths and paths with commas or colons (overlayfs separators), then
  saves the path with configure.py --prefix.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/nix-prefix.py

@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+"""darling-lol --set-prefix PATH: choose where the guest prefix, and with it the
+installed games, is stored.
+
+An existing prefix is moved to PATH. Across filesystems it is copied first and
+the old copy is removed only once the new one is complete. If PATH already holds
+a Darling prefix, the launcher switches to it and leaves both prefixes alone.
+"""
+import errno
+import os
+from pathlib import Path
+import shutil
+import subprocess
+import sys
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def has_files(path):
+    return path.is_dir() and any(path.iterdir())
+
+
+def is_prefix(path):
+    # Darling creates this mount point in every prefix it starts.
+    return (path / 'Volumes/SystemRoot').is_dir()
+
+
+def move(old, new):
+    new.parent.mkdir(parents=True, exist_ok=True)
+    if new.is_dir():
+        new.rmdir()
+    try:
+        os.rename(old, new)
+        return
+    except OSError as error:
+        if error.errno != errno.EXDEV:
+            raise
+    partial = new.with_name(new.name + '.partial')
+    if partial.exists():
+        shutil.rmtree(partial)
+    print(f'Copying {old} to {new}; this can take several minutes.', flush=True)
+    try:
+        subprocess.run(['cp', '-a', '--', str(old), str(partial)], check=True)
+    except BaseException:
+        shutil.rmtree(partial, ignore_errors=True)
+        raise
+    os.rename(partial, new)
+    print(f'Removing {old}.', flush=True)
+    shutil.rmtree(old)
+
+
+def main():
+    old = Path(os.environ['DPREFIX']).resolve()
+    new = Path(sys.argv[1]).expanduser()
+    if not new.is_absolute():
+        raise SystemExit('The prefix must be an absolute path.')
+    new = new.resolve()
+    # Darling passes the prefix to overlayfs, which splits its options on these.
+    if any(character in str(new) for character in ',:\n'):
+        raise SystemExit('The prefix path must not contain commas, colons or newlines.')
+    if new != old:
+        if new.is_relative_to(old) or old.is_relative_to(new):
+            raise SystemExit(f'{new} and {old} must not contain each other.')
+        if has_files(new):
+            if not is_prefix(new):
+                raise SystemExit(f'{new} is not empty and is not a Darling prefix; '
+                                 'choose a new or empty directory.')
+            print(f'Using the existing prefix in {new}; {old} is left as it is.')
+        elif has_files(old):
+            move(old, new)
+            # Darling's overlay work directory; it creates a new one next to the prefix.
+            shutil.rmtree(old.with_name(old.name + '.workdir'), ignore_errors=True)
+    subprocess.run([sys.executable, str(ROOT / 'scripts/configure.py'), '--prefix', str(new)],
+                   cwd=ROOT, stdout=subprocess.DEVNULL, check=True)
+    print(f'The prefix is now {new}. The next darling-lol launch sets up the runtime there if needed.')
+    return 0
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())