fix files cache rebuild from an archive without ctime

[ThomasWaldmann]

Split out of #10292, where the Windows CI run turned this up. It is an independent pre-existing bug.

The bug

_build_files_cache() reads item.ctime unconditionally, but an archive item does not necessarily
have a ctime. stat_attrs() stores it only conditionally:

# borg can work with archives only having mtime (very old borg archives do not have
# atime/ctime). [...]
attrs["mtime"] = safe_ns(st.st_mtime_ns)
...
if not self.noctime and not is_win32:
    # win32: st_ctime is the file creation time, that is archived as birthtime, see #8730.
    attrs["ctime"] = safe_ns(st.st_ctime_ns)

So rebuilding the files cache from a previous archive crashes:

src/borg/cache.py:347: in _build_files_cache
    ctime_ns = item.ctime
AttributeError: attribute ctime not found. Did you mean: 'atime'?

This happens always on Windows (ctime is never archived there) and with --noctime on every
platform. It is reachable whenever the local files cache is missing while a previous archive exists
— a fresh machine, or a lost/cleared cache directory — i.e. exactly the situation the repo-side
rebuild exists for.

Reproducer on Linux/macOS:

borg create --noctime home input
rm <cache dir>/<repo id>/files.*
borg create --noctime home input   # AttributeError: attribute ctime not found

It went unnoticed because nothing exercised the repo-side rebuild path on Windows CI until #10292
added a test that removes the local files cache.

The fix

Both timestamps are read with item.get(). A timestamp the archive does not have is cached as 0,
which cannot compare equal to the timestamp seen in the file system — so if that timestamp is part
of the files cache mode, the file is considered changed and gets chunked again, which is the safe
outcome. The tracking of the newest ctime/mtime skips missing values instead of comparing them; the
two copy-pasted blocks for the two timestamps collapsed into a loop.

Tests

test_files_cache_rebuild_without_ctime uses --noctime rather than relying on Windows CI, so it
covers this on every platform. Verified it fails without the fix on both archiver and
remote_archiver.

It comes with a _remove_files_cache() test helper (removes the local files cache of a series to
force the rebuild), which #10292 then also uses.

Full test suite: 2928 passed, 981 skipped. ruff check clean.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.61%. Comparing base (7004e87) to head (0a31ddc).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/cache.py	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10299      +/-   ##
==========================================
+ Coverage   87.55%   87.61%   +0.06%     
==========================================
  Files         103      103              
  Lines       18676    18674       -2     
  Branches     2872     2872              
==========================================
+ Hits        16352    16362      +10     
+ Misses       1622     1611      -11     
+ Partials      702      701       -1     

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