fix files cache rebuild from an archive without ctime

src/borg/cache.py

@@ -314,26 +314,30 @@ def _build_files_cache(self):
             # only put regular files' infos into the files cache:
             if stat.S_ISREG(item.mode):
                 path_hash = self.key.id_hash(safe_encode(item.path))
+                # an item does not necessarily have all of these timestamps: --noctime omits ctime,
+                # on Windows ctime is never archived (it is the file creation time there, see #8730)
+                # and very old archives only have mtime.
+                ctime_ns = item.get("ctime")
+                mtime_ns = item.get("mtime")
                 # keep track of the key(s) for the most recent timestamp(s):
-                ctime_ns = item.ctime
-                if self._newest_cmtime is None or ctime_ns > self._newest_cmtime:
-                    self._newest_cmtime = ctime_ns
-                    self._newest_path_hashes = {path_hash}
-                elif ctime_ns == self._newest_cmtime:
-                    self._newest_path_hashes.add(path_hash)
-                mtime_ns = item.mtime
-                if self._newest_cmtime is None or mtime_ns > self._newest_cmtime:
-                    self._newest_cmtime = mtime_ns
-                    self._newest_path_hashes = {path_hash}
-                elif mtime_ns == self._newest_cmtime:
-                    self._newest_path_hashes.add(path_hash)
-                # add the file to the in-memory files cache
+                for timestamp_ns in (ctime_ns, mtime_ns):
+                    if timestamp_ns is None:
+                        continue
+                    if self._newest_cmtime is None or timestamp_ns > self._newest_cmtime:
+                        self._newest_cmtime = timestamp_ns
+                        self._newest_path_hashes = {path_hash}
+                    elif timestamp_ns == self._newest_cmtime:
+                        self._newest_path_hashes.add(path_hash)
+                # Add the file to the in-memory files cache. A timestamp the archive does not have
+                # is cached as 0, so it can not compare equal to the timestamp seen in the file
+                # system: if that timestamp is part of the files cache mode, the file is considered
+                # changed and gets chunked again, which is the safe outcome.
                 entry = FileCacheEntry(
                     age=0,
                     inode=item.get("inode", 0),
                     size=item.size,
-                    ctime=int_to_timestamp(ctime_ns),
-                    mtime=int_to_timestamp(mtime_ns),
+                    ctime=int_to_timestamp(0 if ctime_ns is None else ctime_ns),
+                    mtime=int_to_timestamp(0 if mtime_ns is None else mtime_ns),
                     chunks=item.chunks,
                     digests=item.get("digests"),
                 )

src/borg/testsuite/archiver/create_cmd_test.py

@@ -7,6 +7,7 @@
 import socket
 import stat
 import subprocess
+from pathlib import Path
 
 import pytest
 from blake3 import blake3
@@ -2035,3 +2036,24 @@ def no_close_time_write(self, now, force=False, clear=False):
     with changedir("output"):
         cmd(archiver, "extract", "test")
     assert_dirs_equal("input", "output/input")
+
+
+def _remove_files_cache(archiver, archive_name):
+    """Remove the local files cache of an archive series, forcing a rebuild from the repository."""
+    from ...cache import files_cache_name
+    from ...helpers import get_cache_dir
+
+    repo_id = json.loads(cmd(archiver, "repo-info", "--json"))["repository"]["id"]
+    cache_file = Path(get_cache_dir(repo_id, create=False)) / files_cache_name(archive_name)
+    cache_file.unlink()
+
+
+def test_files_cache_rebuild_without_ctime(archivers, request):
+    """Rebuilding from an archive that has no ctime must work - --noctime, and always on Windows."""
+    archiver = request.getfixturevalue(archivers)
+    create_regular_file(archiver.input_path, "file1", size=1024 * 80)
+    cmd(archiver, "repo-create", RK_ENCRYPTION)
+    cmd(archiver, "create", "--noctime", "home", "input")
+    _remove_files_cache(archiver, "home")
+    output = cmd(archiver, "create", "--noctime", "--debug", "home", "input")
+    assert "Building files cache from" in output