Accept broken symlinks as path arguments of dandi validate

`dandi validate` already handles annexed files whose content has not been
fetched (the broken symlinks of a DataLad dataset) according to
`--missing-file-content`: `error`, `skip`, or `only-non-data`.  Those policies
were applied only when such a file was reached through its directory,
though: given directly on the command line, the file never got past argument
parsing, because `click.Path(exists=True)` follows the link and rejected it
with "does not exist".  Check the path with `lexists()` instead, so that

    dandi validate --missing-file-content=skip sub-01/sub-01.nwb

works the same as validating `sub-01/`, while paths that truly do not exist
are still rejected with the same error.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HFhMPs6vF5zyeHDvPTvRcA

Author: claude

dandi/cli/cmd_validate.py

@@ -7,7 +7,7 @@
 import os
 import re
 import sys
-from typing import IO, Union, cast
+from typing import IO, Any, Union, cast
 import warnings
 
 import click
@@ -35,6 +35,28 @@ class TruncationNotice:
     """Number of validation results omitted from this group."""
 
 
+class ExistingPath(click.Path):
+    """
+    Like ``click.Path(exists=True)``, but also accepting broken symbolic links,
+    such as the annexed files of a DataLad dataset whose content has not been
+    fetched.  ``click.Path(exists=True)`` follows the link and rejects those as
+    nonexistent, which prevented the ``--missing-file-content`` policies
+    (``error``, ``skip``, ``only-non-data``) from ever being applied to a file
+    given directly on the command line rather than via its directory.
+    """
+
+    def convert(
+        self, value: Any, param: click.Parameter | None, ctx: click.Context | None
+    ) -> Any:
+        if not os.path.lexists(value):
+            self.fail(
+                f"{self.name.title()} {os.fsdecode(value)!r} does not exist.",
+                param,
+                ctx,
+            )
+        return super().convert(value, param, ctx)
+
+
 STRUCTURED_FORMATS = ("json", "json_pp", "json_lines", "yaml")
 
 _EXT_TO_FORMAT = {
@@ -228,7 +250,7 @@ def validate_bids(
     multiple=True,
     default=(),
 )
-@click.argument("paths", nargs=-1, type=click.Path(exists=True, dir_okay=True))
+@click.argument("paths", nargs=-1, type=ExistingPath(dir_okay=True))
 @click.pass_context
 @devel_debug_option()
 @map_to_click_exceptions

dandi/cli/tests/test_cmd_validate.py

@@ -996,3 +996,25 @@ def test_validate_missing_file_content_no_broken_symlinks(tmp_path: Path) -> Non
     assert r_default.exit_code == r_skip.exit_code == 0
     assert "FILE_CONTENT_MISSING" not in r_default.output
     assert "FILE_CONTENT_MISSING" not in r_skip.output
+
+
+@pytest.mark.ai_generated
+def test_validate_broken_symlink_path_argument(tmp_path: Path) -> None:
+    """A broken symlink can be given directly as a path to validate.
+
+    The --missing-file-content policies then apply to it just as they do when
+    the containing directory is given.
+    """
+    ds = _make_dandiset_with_broken_symlinks(tmp_path)
+    link = ds / "sub-001" / "sub-001.nwb"
+    r = CliRunner().invoke(validate, [str(link)])
+    assert r.exit_code == 1
+    assert "does not exist" not in r.output
+    assert "FILE_CONTENT_MISSING" in r.output
+    r = CliRunner().invoke(validate, ["--missing-file-content", "skip", str(link)])
+    assert r.exit_code == 0
+    assert "FILE_CONTENT_MISSING_SKIPPED" in r.output
+    # Paths that truly do not exist are still rejected
+    r = CliRunner().invoke(validate, [str(ds / "sub-001" / "nonexistent.nwb")])
+    assert r.exit_code == 2
+    assert "does not exist" in r.output