remake: a strict remake of a strict swarm isn't followed

Review 3, R3: 1186 kept honest nodes from proposing or voting for a
strict remake of a swarm strict already, but a majority of members,
each with keys of its own, could still commit one, and switch and
catchup followed it without asking what kind of remake made the
epoch: the registry, policy and bans of a strict swarm changing by a
member majority, the honest minority following.

A node now refuses to switch to such an epoch (its document's policy
strict, the remake's kind strict), and catchup refuses to rejoin one
whose chain of epochs holds a strict remake after the swarm was strict
(as the old document is, or once a remake since made it so). access.md
says so.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/remake/catchup.rs b/src/remake/catchup.rs
index 4df5c90..6ef8121 100644
--- a/src/remake/catchup.rs
+++ b/src/remake/catchup.rs
@@ -528,6 +528,22 @@ fn prepare(
         .filter(|next| next.previous == previous);
     let gap = target.generation - previous.generation;
 
+    // A strict remake of a swarm strict already isn't followed: there its registry,
+    // policy and bans change by its users' signatures, not by a majority of members
+    // (`vote::strict_taken`). Strict as our document is, or once a remake since made it so.
+    let mut strict = crate::access::policy::of(old).strict();
+    for generation in previous.generation + 1..=target.generation {
+        if let Ok(epoch) = snapshot::read_epoch_metadata(copy, generation)
+            && epoch.kind == "strict"
+        {
+            if strict {
+                return Prepared::Failed(format!(
+                    "epoch generation {generation} was made by a strict remake of a swarm strict already: not followed"
+                ));
+            }
+            strict = true;
+        }
+    }
     // Writes made in our epoch go through every remake since, in order.
     let translator = (previous.generation + 1..=target.generation).try_fold(
         Translator::default(),