The push side modelled a store per peer hive: a --peer argument, a
swarm.peers.<domain>.snapshotStorePort option, and a swarm_peers module
whose entire job was answering "which peer". A swarm has exactly one
store, so none of that had anything to select between.
The receiver already proved it. It keys destination directories by
agent, not by sending hive, precisely so an agent that migrates keeps
one unbroken incremental chain -- which only makes sense if every hive
pushes to the same place. Per-hive stores would split the chain in two,
the case that keying exists to prevent.
So the destination moves to services.hyperhive.swarm.snapshotStore,
rendered into HYPERHIVE_SNAPSHOT_STORE, and swarm_peers is deleted
rather than adapted. address has no default because it is a
deployment fact this host cannot derive; port defaults because it is a
convention both ends read from the same option docs. An unset or empty
address fails naming the option instead of connecting somewhere
arbitrary, and a test asserts the message suggests no value.
Adds the caller the fd-passing machinery existed for: hivectl agent
<name> subvol snapshot push --peer <hive> resolves the peer, connects
to its snapshot store, writes the agent header, and hands the connected
socket to hive-priv, which runs btrfs send straight into it.
The split keeps the root helper ignorant. Everything that involves
knowing where a peer is, what the wire protocol looks like, and which
hive to trust happens in the unprivileged daemon; hive-priv only ever
receives an already-open descriptor. Once btrfs send starts, neither
process is in the data path, so a multi-gigabyte transfer costs no
per-byte work and survives a hive-c0re restart.
call_with_fd takes the descriptor by value and closes it as soon as the
kernel has it. A socket stays open until every copy closes, so holding
one back would leave the receiver waiting for an EOF that never comes:
btrfs receive blocks and this side reports success for a transfer the
peer never committed. Ownership makes that unrepresentable.
The peer's store port is a new swarm.peers.<domain>.snapshotStorePort
option rather than a constant matching the module default. A pushing
hive cannot read the receiver's configuration, so assuming 51821 would
push at a port nobody promised to listen on; absent, the push fails
naming the option. swarm_peers parses the mesh address the host module
has always rendered into HYPERHIVE_PEERS but nothing read.
