From 4bbadf6fdc4733b4c3c5eeb575508b2f9334ebdb Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Mon, 26 Oct 2026 19:29:59 +0100
Subject: keepalive sends head samples alongside digest

When there are more than 32 heads that can be send in a single keepalive
message, the digest is accompanied by 32 heads, a different window each
time, so after a few keepalives every head has been listed.
---
 src/transport/wire.rs | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

(limited to 'src/transport/wire.rs')

diff --git a/src/transport/wire.rs b/src/transport/wire.rs
index 11d36cf..3c80b11 100644
--- a/src/transport/wire.rs
+++ b/src/transport/wire.rs
@@ -14,7 +14,7 @@ use super::{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 = 1;
+pub(crate) const VERSION: u8 = 2;
 
 /// What the second byte of a payload says.
 fn tag(kind: MessageKind) -> u8 {
@@ -200,7 +200,10 @@ pub(crate) mod codec {
                     heads.iter().map(|head| Bytes::new(&head.0)).collect(),
                     Bytes::new(&[]),
                 ),
-                HeadsSummary::Digest(digest) => (Vec::new(), Bytes::new(digest)),
+                HeadsSummary::Digest { digest, sample } => (
+                    sample.iter().map(|head| Bytes::new(&head.0)).collect(),
+                    Bytes::new(digest),
+                ),
             };
             (listed, digest).serialize(s)
         }
@@ -209,21 +212,24 @@ pub(crate) mod codec {
             d: D,
         ) -> Result<HeadsSummary, D::Error> {
             let (listed, digest): (Vec<ByteBuf>, ByteBuf) = Deserialize::deserialize(d)?;
-            if !digest.is_empty() {
-                return Ok(HeadsSummary::Digest(fixed(&digest, "a heads digest")?));
-            }
             if listed.len() > MAX_LISTED_HEADS {
                 return Err(D::Error::custom(format!(
                     "more than {MAX_LISTED_HEADS} heads listed"
                 )));
             }
-            listed
+            let listed = listed
                 .iter()
                 .map(|head| -> Result<ChangeHash, D::Error> {
                     Ok(ChangeHash(fixed(head, "a head")?))
                 })
-                .collect::<Result<Vec<_>, _>>()
-                .map(HeadsSummary::Heads)
+                .collect::<Result<Vec<_>, _>>()?;
+            if digest.is_empty() {
+                return Ok(HeadsSummary::Heads(listed));
+            }
+            Ok(HeadsSummary::Digest {
+                digest: fixed(&digest, "a heads digest")?,
+                sample: listed,
+            })
         }
     }
 
@@ -317,7 +323,10 @@ mod tests {
     #[test]
     fn keepalives_keep_their_kind_and_heads() {
         let keepalive = Keepalive {
-            heads: HeadsSummary::Digest([8; 32]),
+            heads: HeadsSummary::Digest {
+                digest: [8; 32],
+                sample: vec![ChangeHash([9; 32])],
+            },
             epoch: Epoch::GENESIS,
             epoch_seq: 0,
             ready: true,
@@ -328,6 +337,7 @@ mod tests {
             panic!("not a PONG");
         };
         assert_eq!(decoded.heads, keepalive.heads);
+        assert_eq!(decoded.heads.listed(), [ChangeHash([9; 32])]);
     }
 
     #[test]
@@ -401,7 +411,7 @@ mod tests {
         });
         assert_eq!(
             hex(&encode(&hello).unwrap()),
-            format!("010097a5737761726d{epoch_hex}01c403010203c3{}07", run("04"))
+            format!("020097a5737761726d{epoch_hex}01c403010203c3{}07", run("04"))
         );
 
         let reply = ParsedMessage::HelloReply(HelloReply {
@@ -418,7 +428,7 @@ mod tests {
         assert_eq!(
             hex(&encode(&reply).unwrap()),
             format!(
-                "010199a5737761726d91a170{epoch_hex}01c400c2{}05{}",
+                "020199a5737761726d91a170{epoch_hex}01c400c2{}05{}",
                 run("04"),
                 run("06")
             )
@@ -433,7 +443,7 @@ mod tests {
         });
         assert_eq!(
             hex(&encode(&ping).unwrap()),
-            format!("0103959290c400{epoch_hex}01c3c2")
+            format!("0203959290c400{epoch_hex}01c3c2")
         );
     }
 }
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