screen: signatures required as the policy a change came under says

Review 3, R1: what a node received was vouched for only while its own
document's policy required signatures. A joiner's first sync, and
every node's message bringing `required`, went in by the rules alone:
a malicious first peer could plant changes under other members' actors
at their next seqs, their real ones then failing as duplicates for
good.

The signatures a message carries are always offered now, and the
screen always has them: a change waits for one where the policy at the
heads it is judged at requires it (rules::Reads::required, read once
per heads), whatever the receiving document holds. Changes written
under a policy not requiring signatures wait for none, as on every
other node; the check for forged changes runs always. Changes waiting
for a signature are judged again whenever signatures come.

The test of signatures required now takes the history into an empty
document; a new one takes changes written under a policy not requiring
them without any.

1 files changed, 23 insertions, 28 deletions

diff --git a/src/swarm/signatures.rs b/src/swarm/signatures.rs
index 9ea404e..111b3da 100644
--- a/src/swarm/signatures.rs
+++ b/src/swarm/signatures.rs
@@ -75,35 +75,30 @@ pub(crate) async fn received(state: &AppState, message: SignaturesMessage) {
         if replica.epoch != message.epoch {
             return 0;
         }
-        {
-            let (epoch, epoch_seq) = (replica.epoch, replica.epoch_seq);
-            let replica = &mut *replica;
-            replica.signatures.offer(&replica.doc, signatures);
-            // With signatures required, changes waiting for them judged again
-            // (`access::screen`).
-            if crate::access::policy::of(&replica.doc).required() {
-                let applied = replica.logged_signed(|doc, known, log| {
-                    let applied = crate::access::screen::apply_bytes(
-                        doc,
-                        epoch,
-                        epoch_seq,
-                        &[],
-                        &screening,
-                        Some(known),
-                        log,
-                    );
-                    (applied, true)
-                });
-                replica.refresh_heads();
-                return match applied {
-                    Ok(outcome) => outcome.applied,
-                    Err(error) => {
-                        tracing::warn!(error = %format!("{error:#}"), "Couldn't judge the changes waiting for signatures");
-                        0
-                    }
-                };
+        let (epoch, epoch_seq) = (replica.epoch, replica.epoch_seq);
+        let replica = &mut *replica;
+        replica.signatures.offer(&replica.doc, signatures);
+        // Changes waiting for them judged again (`access::screen`): those the policy they
+        // were written under requires signed.
+        let applied = replica.logged_signed(|doc, known, log| {
+            let applied = crate::access::screen::apply_bytes(
+                doc,
+                epoch,
+                epoch_seq,
+                &[],
+                &screening,
+                Some(known),
+                log,
+            );
+            (applied, true)
+        });
+        replica.refresh_heads();
+        match applied {
+            Ok(outcome) => outcome.applied,
+            Err(error) => {
+                tracing::warn!(error = %format!("{error:#}"), "Couldn't judge the changes waiting for signatures");
+                0
             }
-            0
         }
     })
     .await