1 files changed, 83 insertions, 2 deletions

diff --git a/src/transport/libp2p.rs b/src/transport/libp2p.rs
index ba97fb5..ebf50b4 100644
--- a/src/transport/libp2p.rs
+++ b/src/transport/libp2p.rs
@@ -243,6 +243,62 @@ struct Counts {
 }
 
 /// Counts a dial that failed with `error` (not one that wasn't needed).
+/// A dial error, shortly: for each address tried (without the peer ID the caller names
+/// already), the innermost cause, once. libp2p's own text nests each transport's errors
+/// in the next ("Multiple dial errors occurred: ..."), repeating the cause at each level.
+fn describe_dial_error(error: &::libp2p::swarm::DialError) -> String {
+    use ::libp2p::swarm::DialError;
+    let DialError::Transport(errors) = error else {
+        return error.to_string();
+    };
+    let described: Vec<String> = errors
+        .iter()
+        .map(|(address, error)| {
+            let mut address = address.clone();
+            if let Some(Protocol::P2p(_)) = address.iter().last() {
+                address.pop();
+            }
+            format!("{address}: {}", innermost_causes(error))
+        })
+        .collect();
+    if described.is_empty() {
+        return error.to_string();
+    }
+    described.join("; ")
+}
+
+/// The innermost of `error`'s causes, each distinct one once: those libp2p lists in a
+/// "Multiple dial errors occurred:" text (one per line, ` - <cause>: <its cause>...`),
+/// or the last of the source chain.
+fn innermost_causes(error: &(dyn std::error::Error + 'static)) -> String {
+    let mut innermost = error;
+    while let Some(source) = innermost.source() {
+        innermost = source;
+    }
+    let text = innermost.to_string();
+    let mut causes: Vec<&str> = Vec::new();
+    for line in text.lines() {
+        let Some(listed) = line.trim().strip_prefix("- ") else {
+            continue;
+        };
+        // A line ends in the next level's heading when that level lists more.
+        let cause = listed
+            .split(": ")
+            .filter(|part| !part.starts_with("Multiple dial errors"))
+            .last()
+            .unwrap_or(listed)
+            .trim();
+        if !cause.is_empty() && !causes.contains(&cause) {
+            causes.push(cause);
+        }
+    }
+    if causes.is_empty() {
+        let cause = text.rsplit(": ").next().unwrap_or(&text).trim();
+        return cause.to_owned();
+    }
+    causes.join(", ")
+}
+
 fn count_dial_failure(failures: &mut DialFailures, error: &::libp2p::swarm::DialError) {
     use ::libp2p::swarm::DialError;
     match error {
@@ -1081,14 +1137,14 @@ async fn run(
                 SwarmEvent::OutgoingConnectionError { connection_id, peer_id, error } => {
                     count_dial_failure(&mut lock(&book).counts.dial_failures, &error);
                     if let Some((_, done)) = identifying.remove(&connection_id) {
-                        let _ = done.send(Err(error.to_string()));
+                        let _ = done.send(Err(describe_dial_error(&error)));
                     }
                     if let Some(peer) = peer_id
                         && !swarm.is_connected(&peer)
                     {
                         if let Some(waiting) = connecting.remove(&peer) {
                             for done in waiting {
-                                let _ = done.send(Err(error.to_string()));
+                                let _ = done.send(Err(describe_dial_error(&error)));
                             }
                             find_on_dht(&mut swarm, peer);
                         }
@@ -1903,6 +1959,31 @@ mod tests {
         assert_eq!(book.prune(later, |_| false), 1); // `connected`, once it isn't.
     }
 
+    /// libp2p's nested dial errors come out as each address's cause, once.
+    #[test]
+    fn dial_errors_are_short() {
+        use ::libp2p::core::transport::TransportError;
+        let peer = peer();
+        let address: Multiaddr = format!("/ip6/fd00::1/tcp/9661/p2p/{peer}").parse().unwrap();
+        let nested = std::io::Error::other(
+            "Multiple dial errors occurred:\n - Network is unreachable (os error 101): Multiple dial errors occurred:\n - Network is unreachable (os error 101): Network is unreachable (os error 101)",
+        );
+        let error =
+            ::libp2p::swarm::DialError::Transport(vec![(address, TransportError::Other(nested))]);
+        assert_eq!(
+            describe_dial_error(&error),
+            "/ip6/fd00::1/tcp/9661: Network is unreachable (os error 101)"
+        );
+        let refused = ::libp2p::swarm::DialError::Transport(vec![(
+            "/ip4/10.0.0.1/tcp/1".parse().unwrap(),
+            TransportError::Other(std::io::Error::from(std::io::ErrorKind::ConnectionRefused)),
+        )]);
+        assert_eq!(
+            describe_dial_error(&refused),
+            "/ip4/10.0.0.1/tcp/1: connection refused"
+        );
+    }
+
     /// A relay doesn't dial the nodes it relays for through itself.
     #[test]
     fn no_circuits_through_ourselves() {