hive-priv read requests with BufReader::lines, which cannot surface
SCM_RIGHTS: ancillary data is attached to one specific recvmsg call, so
a buffered line reader takes the bytes and silently drops the
descriptor. Replace it with a recvmsg loop.
The pairing is deliberately trivial. hive-sock-client connects per
request, so a connection carries one line and at most one descriptor;
a second descriptor arriving before its line is a protocol error rather
than something to queue. check_fd_agreement rejects both mismatches --
an fd-taking op that got none, and a descriptor sent to an op that
takes none -- and dropping the OwnedFd on that path closes it.
recv_with_fds claims every descriptor the kernel attaches, including
ones this protocol never expects, because an fd we fail to claim leaks
for the life of the process. MSG_CMSG_CLOEXEC keeps a received
descriptor out of every btrfs and nixos-container child. The control
buffer is only cmsghdr-aligned, so descriptors are copied out
byte-wise instead of read through a more strictly aligned pointer.
SendAgentSnapshotToFd is SendAgentSnapshotToFile without the staging
file: same validation and -p parent handling, stdout wired to the
passed descriptor. It exists so hive-c0re can connect to a peer hive's
snapshot store, write the header itself, and hand over the connected
socket -- leaving this helper with no address, no protocol, and nobody
in the data path once the send starts.
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