fix: windows drive letters remap to unc paths

[maxnbk]

Closes #2045 and #1438 .
Partial but not complete fix for #1909 (Intentional, I feel that the work that was occurring in the gitbash PR would ultimately resolve this behavior?) .

Explanations:

os.path.realpath on Python 3.8+ Windows silently expands mapped drive letters to their underlying UNC paths. Because canonical_path calls realpath, FileSystemPackageRepository was storing a UNC self.location even when the caller supplied a drive-letter path. This caused make_resource_handle, which does a raw string comparison, to raise ResourceError on every drive-letter handle against a UNC-stored repository.

The fix:

On Windows, canonical_path now uses os.path.abspath instead of os.path.realpath. This preserves path style (drive-letter stays drive-letter, UNC stays UNC) and restores the pre-3.8 normalization behavior without a network-resolution side-effect. A make_resource_handle override is also added to the filesystem plugin to handle residual case or separator mismatches via canonical_path.

Extension of work:

A resolve_links_on_windows config flag (default False) has been added for sites that need some form of symlink/junction resolution on Windows. When enabled, a purpose-built _windows_realpath walks components of the path using os.readlink, resolving real symlinks without actually touching the drive root, in order to avoid the UNC-expansion side-effect entirely. Long-path support (\\?\ prefix handling) is included, with the relevant test skipped on hosts that do not have LongPathsEnabled=1 in the registry. Note: I'm aware that we need to move off of the winreg module, but since we have it for now, might as well use it.

Testing:

This was tested against an actual Drive-letter-map-with-matching-UNC-network-share .

Disclosure:

This PR was AI-assisted with Claude Code, Sonnet 4.6, primarily for diagnostics, logic-tracing, documentation and edge-case verification.

[codecov]

Codecov Report

❌ Patch coverage is 80.85106% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.69%. Comparing base (d415b96) to head (1681466).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/utils/filesystem.py	82.05%	3 Missing and 4 partials ⚠️
src/rez/rezconfig.py	0.00%	1 Missing ⚠️
src/rezplugins/package_repository/filesystem.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2104      +/-   ##
==========================================
+ Coverage   60.65%   60.69%   +0.04%     
==========================================
  Files         164      164              
  Lines       20584    20630      +46     
  Branches     3579     3593      +14     
==========================================
+ Hits        12485    12522      +37     
- Misses       7224     7228       +4     
- Partials      875      880       +5     

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