2 files changed, 79 insertions, 7 deletions

diff --git a/src/transport/mod.rs b/src/transport/mod.rs
index 3fd4490..32be29b 100644
--- a/src/transport/mod.rs
+++ b/src/transport/mod.rs
@@ -389,12 +389,14 @@ impl ParsedMessage {
     }
 
     /// The sender's epoch and `SyncEnvelope::epoch_seq`, as every message carries them
-    /// but WHO, ME, PING and PONG (which the node never sees).
+    /// but WHO, ME, PING and PONG (which the node never sees), and SYNC, which carries a
+    /// tag of it only (`EpochTag`).
     pub(crate) fn epoch_stamp(&self) -> Option<(Epoch, u32)> {
         match self {
             ParsedMessage::Hello(hello) => Some((hello.epoch, hello.epoch_seq)),
             ParsedMessage::HelloReply(reply) => Some((reply.epoch, reply.epoch_seq)),
-            ParsedMessage::Sync(envelope) => Some((envelope.epoch, envelope.epoch_seq)),
+            // Only a tag (`EpochTag`): resolved by the receiver, which knows the epochs.
+            ParsedMessage::Sync(_) => None,
             ParsedMessage::Status(status) | ParsedMessage::StatusReply(status) => {
                 Some((status.epoch, status.epoch_seq))
             }
@@ -656,9 +658,9 @@ impl std::fmt::Debug for HelloReply {
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub(crate) struct SyncEnvelope {
-    /// Epoch of the document the message was generated from.
-    #[serde(with = "crate::transport::wire::codec::epoch")]
-    pub(crate) epoch: Epoch,
+    /// Epoch of the document the message was generated from, by a tag (`EpochTag`): the
+    /// receiver knows the epoch it names, as ours or as the sender's.
+    pub(crate) epoch: EpochTag,
     /// How many times the sender changed epoch in its current run, when it sent this
     /// (`Replica::epoch_seq`). Messages may arrive late and out of order: the receiver
     /// ignores an epoch older than the newest it has seen in the same session.
@@ -682,6 +684,46 @@ pub(crate) struct SyncEnvelope {
     pub(crate) sent: crate::clock::UnixMillis,
 }
 
+/// An epoch, as sync messages name it: its generation and the first bytes of its ID,
+/// about a third of the whole. Sync messages go often, and only between peers on one
+/// epoch, which each knows from the other's handshake and keepalives: the tag tells which
+/// of the epochs the receiver knows it is, and one it knows of no other is dropped (an
+/// epoch of the same generation agreeing in 8 bytes, but not the rest, would take
+/// competing remakes that also collide there).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub(crate) struct EpochTag {
+    pub(crate) generation: u64,
+    #[serde(with = "crate::transport::wire::codec::bytes8")]
+    pub(crate) prefix: [u8; 8],
+}
+
+impl EpochTag {
+    pub(crate) fn of(epoch: Epoch) -> Self {
+        let mut prefix = [0; 8];
+        prefix.copy_from_slice(&epoch.id[..8]);
+        Self {
+            generation: epoch.generation,
+            prefix,
+        }
+    }
+
+    /// Whether this tags `epoch`.
+    pub(crate) fn names(&self, epoch: &Epoch) -> bool {
+        *self == Self::of(*epoch)
+    }
+
+    /// The first of `known` it names, if any.
+    pub(crate) fn resolve(&self, known: impl IntoIterator<Item = Epoch>) -> Option<Epoch> {
+        known.into_iter().find(|epoch| self.names(epoch))
+    }
+}
+
+impl std::fmt::Display for EpochTag {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}:{}…", self.generation, Hex(&self.prefix))
+    }
+}
+
 /// What a sync envelope carries.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub(crate) enum SyncBody {
@@ -1042,4 +1084,21 @@ mod tests {
         }
         assert_eq!(seen.len(), heads.len());
     }
+
+    /// A tag names its epoch, not one of another generation or another ID, and resolves
+    /// to the epoch it names among those known.
+    #[test]
+    fn epoch_tags_name_their_epoch() {
+        let epoch = |generation, byte| Epoch {
+            generation,
+            id: [byte; 32],
+        };
+        let ours = epoch(3, 1);
+        let tag = EpochTag::of(ours);
+        assert!(tag.names(&ours));
+        assert!(!tag.names(&epoch(4, 1)), "another generation");
+        assert!(!tag.names(&epoch(3, 2)), "another remake of it");
+        assert_eq!(tag.resolve([epoch(3, 2), ours]), Some(ours));
+        assert_eq!(tag.resolve([epoch(2, 1)]), None);
+    }
 }
diff --git a/src/transport/wire.rs b/src/transport/wire.rs
index 3f7736d..d1ac14d 100644
--- a/src/transport/wire.rs
+++ b/src/transport/wire.rs
@@ -14,7 +14,7 @@ use super::{EncodedMessage, MessageKind, ParsedMessage};
 
 /// The wire format's version, the first byte of every payload. Bump it with every change
 /// to a message type or a codec (`golden_bytes` fails then).
-pub(crate) const VERSION: u8 = 29;
+pub(crate) const VERSION: u8 = 30;
 
 /// What the second byte of a payload says.
 fn tag(kind: MessageKind) -> u8 {
@@ -166,6 +166,19 @@ pub(crate) mod codec {
         }
     }
 
+    /// 8 bytes (`EpochTag::prefix`).
+    pub(crate) mod bytes8 {
+        use super::*;
+
+        pub(crate) fn serialize<S: Serializer>(value: &[u8; 8], s: S) -> Result<S::Ok, S::Error> {
+            Bytes::new(value).serialize(s)
+        }
+
+        pub(crate) fn deserialize<'de, D: Deserializer<'de>>(d: D) -> Result<[u8; 8], D::Error> {
+            fixed(&ByteBuf::deserialize(d)?, "an epoch tag")
+        }
+    }
+
     /// A 64-byte signature.
     pub(crate) mod bytes64 {
         use super::*;
@@ -401,7 +414,7 @@ pub(crate) fn samples() -> Vec<ParsedMessage> {
             sender_run_id: [6; 16],
         }),
         ParsedMessage::Sync(SyncEnvelope {
-            epoch,
+            epoch: crate::transport::EpochTag::of(epoch),
             epoch_seq: 1,
             body: SyncBody::Delta(DeltaSpec {
                 from: Vec::new(),