fix(backup): keep kernel objects out of a generation

-a implies -D, so a generation was written with --devices --specials and rsync recreated every unix socket and fifo it found in a volume. On the swarm manager that generation lives on an nfs-ganesha export, and ganesha accepts the socket on write but cannot serve it back: the remote pull's sender then fails with readdir/readlink_stat 'Invalid argument (22)' and exits 23, deterministically, for all twelve retries - 58 minutes per run.

Postfix's queue directory is the case that surfaced it, where public/ and private/ hold roughly fifty AF_UNIX sockets and nothing else. The class is wider: a discourse /shared with its in-container postgres socket, a checkmk OMD site with tmp/run/nagios.cmd, a container whose /tmp is a persisted volume. --no-D is type-based and closes all of them without anyone having to know which image binds a socket where.

Nothing restorable is lost. A socket inode is meaningless after a restore; postfix's master, checkmk's omd start and discourse's supervisor recreate theirs. The whole postfix queue survives - incoming, active, deferred, hold, maildrop - so accepted-but-undelivered mail stays in the backup, which excluding the volume outright would have dropped. Device nodes go too, and the only volume that could hold them is a nested docker data root, already carrying backup: false.

This is the writer side, whose source is a local docker volume. On the reader the same flag provably does nothing: rsync still stats the entry before -D decides, and getdents64 on the containing directory is outside its reach entirely.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/backup/test_volume.py

@@ -43,6 +43,9 @@ class TestBackupVolume(unittest.TestCase):
     def test_it_always_deletes_what_the_source_no_longer_has(self) -> None:
         self.assertIn("--delete", self.copy())
 
+    def test_it_carries_no_kernel_objects_into_a_generation(self) -> None:
+        self.assertIn("--no-D", self.copy())
+
     def test_it_creates_the_destination(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             dest = Path(tmp) / "gen" / "demo"