Address review findings on the persistent dummy-key sidecar:
- Publish atomically: write a private same-directory temp file
(<store>.dummykey.tmp.<pid>, 0600), fsync, then link(2) into place;
fsync the containing directory and drop the temp name. A concurrent
starter can no longer observe a zero/partial sidecar and fail closed.
On EEXIST adopt the winner's sidecar; otherwise warn and use a
transient ephemeral key.
- Harden the read path (initial and EEXIST-adopt) with
O_RDONLY|O_NOFOLLOW|O_NONBLOCK|O_CLOEXEC: reject planted symlinks
(ELOOP fails closed) and never block on a planted FIFO.
- Require the exact owner-only mode (st_mode & 07777) == 0600 and make
the rejection message truthful.
- Report a clear "short write" instead of a stale strerror(errno) when
write() returns 0.
- Document the artifact and its creation-failure caveat (FIFO store
path, read-only filesystem, missing directory) in README.md and
RSYNC_COMPAT.md.
- Tests: known-key sidecar adoption (dummy salt KAT + reload), symlink
rejection, and the exact-0600 rule (0400 now rejected).
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