ValueError: path is on mount crash when opening notebooks on UNC network shares (Windows)

[meitham]

Description

When opening a Jupyter notebook (or any file) stored on a Windows UNC network share (e.g. \\server\share\...) in JupyterLab with jupyter-lsp active, pylsp crashes on every textDocument/didOpen and textDocument/didChange notification. This makes language features (completions, diagnostics, etc.) unavailable for any file on a network drive.

Root Cause

In pylsp/_utils.py, find_parents() calls:

dirs = [root] + os.path.relpath(os.path.dirname(path), root).split(os.path.sep)

On Windows, os.path.relpath(path, start) raises ValueError when path and start are on different mounts e.g. when the workspace root is a UNC path (\ldnwifilep002\Department...) and the resolved document path lands on a local drive (C:), or vice versa.

The existing guard at line 90 using os.path.commonprefix() is insufficient it does a naïve character-by-character comparison and does not reliably detect all cross-mount cases.

Error Log

ERROR - pylsp_jsonrpc.endpoint - Failed to handle notification textDocument/didOpen
Traceback (most recent call last):
  File ".../pylsp/workspace.py", line 341, in source_roots
    _utils.find_parents(
  File ".../pylsp/_utils.py", line 97, in find_parents
    dirs = [root] + os.path.relpath(os.path.dirname(path), root).split(os.path.sep)
  File "<frozen ntpath>", line 791, in relpath
ValueError: path is on mount 'C:', start on mount '\\unc\share'

All subsequent textDocument/didChange notifications then raise KeyError because put_document (didOpen) never completed, leaving the document unregistered in the workspace.

Environment

• OS: Windows 11 Enterprise
• python-lsp-server: 1.12.2
• Python: 3.12
• Triggered by: opening a notebook on \unc\share... via
  JupyterLab + jupyter-lsp

Proposed Fix

Wrap the relpath call in a try/except ValueError and return early, matching the intent of the existing commonprefix guard:

try:
    dirs = [root] + os.path.relpath(os.path.dirname(path), root).split(os.path.sep)
except ValueError:
    # On Windows, relpath raises ValueError when path and root are on different
    # mounts (e.g. UNC share vs local drive letter). Nothing to search.
    log.warning("Path %r not in %r (different mounts)", path, root)
    return []

This is the minimal safe fix that mirrors the behaviour of the existing commonprefix guard and avoids any functional regression on non-Windows platforms.