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, 6 insertions, 1 deletions

diff --git a/src/remake/api.rs b/src/remake/api.rs
index 1da0539..a767706 100644
--- a/src/remake/api.rs
+++ b/src/remake/api.rs
@@ -881,9 +881,14 @@ pub(crate) async fn strict_manifest<H: Host>(
 /// `GET /admin/epochs`: the local epoch, the previous epochs this node still keeps, and
 /// an ongoing catch-up with a peer's newer epoch (`null` if none).
 pub(crate) async fn epochs<H: Host>(State(host): State<H>) -> Response {
-    let current = EpochRecord::from(host.current_epoch());
+    let epoch = host.current_epoch();
+    let current = EpochRecord::from(epoch);
+    let root = host
+        .read_doc(move |doc| crate::swarm::roots::of(doc, epoch).map(|root| root.to_string()))
+        .await;
     Json(json!({
         "current": current,
+        "root": root,
         "archived": host.remake().archived_epochs(),
         "catchUp": host.remake().catch_up_report(),
         "quarantine": host.remake().list_quarantine(|entry| entry.id.clone()),