don't mistake a malformed URL for a local path, #10215

[ThomasWaldmann]

Fixes the confusing behaviour reported in discussion #10215: borg -r rest://dcm/ repo-info failed with

Repository proto='file', user=None, pass=None, host=None, port=None, path='/home/borg/rest:/dcm' does not exist.

Cause

rest://dcm/ carries no repository path, so it does not match rest_re (which needs at least one
character after the separating slash). It then fell through to local_re, whose negative lookahead
excluded ssh://, file:// and the borgstore schemes, but not rest://. So the URL was taken
for a relative local path and prefixed with the cwd.

Change

All schemes borg parses itself (ssh, rest, file) are now excluded from local_re, in the
opaque scheme: form as well as in the scheme:// form. Nothing starting with those prefixes can
silently become a local path any more - such input is rejected instead. A local path that really
starts with such a prefix can still be given as ./rest:... or as an absolute path (there is a test
for that).

The reporter said they had tried numerous permutations without getting any useful feedback, so the
error now also names the accepted forms:

error: argument -r/--repo: Invalid location format: "rest://dcm/". Expected: rest://[user@]host[:port]/path/to/repo (path relative to the remote directory ssh logs into) or rest://[user@]host[:port]//path/to/repo (absolute path)

Docs

• quickstart.rst recommended rest://borg@remote_host (no path), which would have run into
  exactly this error.
• The repository URL reference called the single-slash form "path relative to the current directory"
  for the remote protocols, but it is relative to the remote login directory. Corrected for
  rest/ssh/sftp, and added a note that the path is required and what one vs. two slashes mean.

Tests

New tests cover the malformed rest:// forms, the hint per scheme, local paths starting with a
scheme name, and rest:// canonical-path round trips. Full test suite passes locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.29%. Comparing base (29022db) to head (132e0c3).
⚠️ Report is 12 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10216      +/-   ##
==========================================
- Coverage   87.29%   87.29%   -0.01%     
==========================================
  Files         102      102              
  Lines       18444    18451       +7     
  Branches     2834     2834              
==========================================
+ Hits        16101    16107       +6     
  Misses       1638     1638              
- Partials      705      706       +1     

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