From de560ee34ba0297194cae6ac9a1966e3ef614945 Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Fri, 23 Oct 2026 22:25:41 +0200
Subject: bump number of heads in keepalive messages

Instead of arbitrarily limiting the number to 16, allow filling
keepalive messages as long as they don't cause mycelium to chunk it.
---
 src/transport/mycelium.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

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

diff --git a/src/transport/mycelium.rs b/src/transport/mycelium.rs
index 9d1fb90..2683f82 100644
--- a/src/transport/mycelium.rs
+++ b/src/transport/mycelium.rs
@@ -973,3 +973,45 @@ pub enum TransportError {
         status: reqwest::StatusCode,
     },
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Mycelium splits messages into chunks of this size.
+    const MYCELIUM_CHUNK: usize = 1300;
+
+    #[test]
+    fn largest_keepalive_fits_in_one_chunk() {
+        let keepalive = Keepalive {
+            heads: HeadsSummary::Heads(
+                (0..MAX_LISTED_HEADS as u8)
+                    .map(|n| ChangeHash([n; 32]))
+                    .collect(),
+            ),
+            epoch: Epoch {
+                generation: u64::MAX,
+                id: [0xff; 32],
+            },
+            ready: true,
+            syncs_with_you: true,
+        };
+        let payload = keepalive_to_bytes(&keepalive).unwrap();
+        // Mycelium's message data: a length byte, the topic, then our payload.
+        let size = 1 + TOPIC_PONG.len().max(TOPIC_PING.len()) + payload.len();
+        assert!(
+            size + 50 <= MYCELIUM_CHUNK,
+            "keepalive with {MAX_LISTED_HEADS} heads takes {size} bytes"
+        );
+        assert_eq!(bytes_to_keepalive(&payload).unwrap().heads, keepalive.heads);
+    }
+
+    #[test]
+    fn listed_heads_equal_their_digest() {
+        let heads: Vec<_> = (0..3u8).map(|n| ChangeHash([n; 32])).collect();
+        let listed = HeadsSummary::of(heads.clone());
+        let digest = HeadsSummary::Digest(listed.digest());
+        assert_eq!(listed, digest);
+        assert_ne!(listed, HeadsSummary::of(heads[..2].to_vec()));
+    }
+}
-- 
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