From c2468964b11bd3606f03375c370fd181a1723113 Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Sat, 24 Oct 2026 04:20:21 +0200
Subject: find reachable neighbours

---
 src/state.rs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

(limited to 'src/state.rs')

diff --git a/src/state.rs b/src/state.rs
index 64fe6a4..7574e35 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -137,8 +137,18 @@ pub(crate) struct PeerState {
 
     /// How many heads its most recent keepalive listed (None if it sent a digest).
     pub(crate) reported_heads: Option<usize>,
+
+    /// HELLOs sent in the current handshake attempt without hearing anything back.
+    pub(crate) hello_attempts: u32,
+    /// Since when we consider the peer unreachable (three unanswered HELLOs, or
+    /// dormant). Cleared by any message from it; ignored after `dormant_probe`, so the
+    /// peer becomes a neighbour candidate again and gets tried once more.
+    pub(crate) unreachable_since: Option<Instant>,
 }
 
+/// Unanswered HELLOs after which a peer counts as unreachable (1 + 2 + 4 s apart).
+const UNREACHABLE_AFTER_HELLOS: u32 = 3;
+
 /// One Automerge sync with a peer, from a reset to the next. Everything a reset must
 /// forget lives here, so a reset replaces it whole and can't miss a field.
 #[derive(Clone)]
@@ -284,6 +294,8 @@ impl PeerState {
             reports_ready: None,
             syncs_with_us: None,
             reported_heads: None,
+            hello_attempts: 0,
+            unreachable_since: None,
         }
     }
 
@@ -309,6 +321,8 @@ impl PeerState {
     /// Records inbound activity. Returns true if the peer was dormant.
     pub(crate) fn mark_seen(&mut self, now: Instant) -> bool {
         self.last_seen = Some(now);
+        self.hello_attempts = 0;
+        self.unreachable_since = None;
 
         if matches!(self.liveness, Liveness::Dormant { .. }) {
             self.liveness = Liveness::Active;
@@ -321,6 +335,21 @@ impl PeerState {
         false
     }
 
+    /// A HELLO went out (or failed to). After the third unanswered one, the peer counts
+    /// as unreachable.
+    pub(crate) fn hello_sent(&mut self, now: Instant) {
+        self.hello_attempts += 1;
+        if self.hello_attempts >= UNREACHABLE_AFTER_HELLOS {
+            self.unreachable_since.get_or_insert(now);
+        }
+    }
+
+    /// Whether to leave the peer out when choosing neighbours (`swarm::topology`).
+    pub(crate) fn is_unreachable(&self, timing: &Timing, now: Instant) -> bool {
+        self.unreachable_since
+            .is_some_and(|since| now.saturating_duration_since(since) < timing.dormant_probe)
+    }
+
     /// No more HELLO retries (the peer isn't our neighbour any more). A HELLO in flight
     /// finishes; a later handshake starts over.
     pub(crate) fn stop_hello_retries(&mut self) {
@@ -336,6 +365,7 @@ impl PeerState {
         }
 
         self.liveness = Liveness::Dormant { since: now };
+        self.unreachable_since.get_or_insert(now);
         self.reset_sync();
 
         if self.hello_state != HelloState::Sending {
-- 
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