roots: an epoch's root pinned, and named in every handshake

Review 2, N7, the care it raised: a joining node's empty document took
as the first epoch's root whatever dependency-less change came first,
from whichever peer it joined through; the first epoch's ID is all
zeros, so nothing pinned it. A remake's epoch has its ID, the snapshot
change's hash, which the rules hold its root to.

* Every node names its epoch's root in HELLO and HELLO_REPLY (the
  epoch's ID for a remake's, its genesis change for the first; wire
  version 45). A node whose document starts from another root of the
  same epoch refuses the handshake, logging it: another history under
  the swarm's ID, a forged root or a genesis started again.
* A joining node with an empty document takes as its root only the one
  it was given (--genesis-root, new), else the one its peers name, and
  none while they name different ones (access::screen::RootPin, kept
  in Screening across documents): one peer hands it another root only
  if it is all the joiner hears, or never, with --genesis-root. A root
  refused is listed with the refused, what builds on it with it.
* The genesis node logs the root (`The swarm's root`); GET
  /admin/epochs shows it (`root`).

A genesis started again under a remembered swarm ID used to merge with
the nodes of it still running; they now refuse it (another root), and
the docs say to join them with seeds instead.

The epoch's identity stays as it is (generation 0 all zeros): records
and epoch metadata name it so, and every comparison of epochs with
them.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/state/mod.rs b/src/state/mod.rs
index 441cdaf..d3f5859 100644
--- a/src/state/mod.rs
+++ b/src/state/mod.rs
@@ -161,6 +161,10 @@ pub(crate) struct AppState {
     /// may make within it (`access::challenge`).
     pub(crate) challenge_lifetime: Duration,
     pub(crate) key_logins_per_user: usize,
+    /// Its epoch's root, as peers name it and as it knows its own (`swarm::roots`), and
+    /// the one it was given (`--genesis-root`).
+    pub(crate) roots: Arc<std::sync::Mutex<crate::swarm::roots::Roots>>,
+    pub(crate) genesis_root: Option<crate::document::ChangeHash>,
     /// Whose signature on a strict remake's manifest this node takes (`access::strict`).
     pub(crate) strict_keys: Arc<Vec<crate::access::sshsig::PublicKey>>,
     /// Origins whose pages may call this node's open endpoints (`api::cors`).