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, 7 insertions, 3 deletions

diff --git a/src/swarm/bootstrap.rs b/src/swarm/bootstrap.rs
index 98e273a..4384edb 100644
--- a/src/swarm/bootstrap.rs
+++ b/src/swarm/bootstrap.rs
@@ -70,13 +70,14 @@ pub(crate) async fn bootstrap(state: &AppState, seeds: &[String], start: Start)
         return Ok(());
     }
     if start == Start::Genesis {
-        // A remembered swarm starts again under its ID: nodes of it still running then
-        // merge with this one instead of ending up in another swarm.
+        // A remembered swarm starts again under its ID, from a root of its own: nodes of
+        // it still running hold another, and refuse it (`swarm::roots`); joining them
+        // takes seeds instead.
         let swarm_id = match state.swarm_id().await {
             Some(remembered) => {
                 warn!(
                     swarm_id = %remembered,
-                    "--genesis with a remembered swarm: starting it again, empty; nodes of it still running merge with this one"
+                    "--genesis with a remembered swarm: starting it again, empty, from a new root; nodes of it still running refuse it (another root): give seeds to join them instead"
                 );
                 remembered
             }
@@ -131,6 +132,9 @@ pub(crate) async fn bootstrap(state: &AppState, seeds: &[String], start: Start)
             .context(
                 "Recording the swarm ID, its neighbours, its topology and dissemination, its first users",
             )?;
+        if let Some(root) = crate::swarm::roots::own(state).await {
+            info!(root = %root, "The swarm's root: a node joining may be given it (--genesis-root), to take no other");
+        }
         state.initial_sync_done.store(true, Ordering::Release);
         state.mark_ready(swarm_id).await?;
         // Remembered members, if any, to find those still running.