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, 10 insertions, 1 deletions

diff --git a/src/node.rs b/src/node.rs
index ddb9b7e..84a5c01 100644
--- a/src/node.rs
+++ b/src/node.rs
@@ -78,6 +78,8 @@ pub(crate) struct NodeConfig {
     /// `--challenge-lifetime`, `--key-logins-per-user`.
     pub(crate) challenge_lifetime: Duration,
     pub(crate) key_logins_per_user: usize,
+    /// `--genesis-root`.
+    pub(crate) genesis_root: Option<crate::document::ChangeHash>,
     /// `--strict-keys`.
     pub(crate) strict_keys: Vec<crate::access::sshsig::PublicKey>,
     /// `--cors-origins`.
@@ -167,7 +169,14 @@ impl Node {
             first_users: Arc::new(config.first_users),
             node_key: config.node_key,
             sign_every: config.sign_every,
-            screening: Default::default(),
+            screening: Arc::new(std::sync::Mutex::new(
+                crate::access::screen::Screening::pinned(crate::swarm::roots::pin(
+                    config.genesis_root,
+                    &Default::default(),
+                )),
+            )),
+            roots: Default::default(),
+            genesis_root: config.genesis_root,
             challenges: Default::default(),
             token_lifetime: config.token_lifetime,
             session_lifetime: config.session_lifetime,