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fix: preserve Happy Eyeballs resolver preference #878

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Merged
ryanfowler merged 1 commit into from
Jul 18, 2026
Merged

fix: preserve Happy Eyeballs resolver preference #878

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60 changes: 53 additions & 7 deletions src/net.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -610,8 +610,11 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
if !addrs.is_empty() {
append_unique_socket_addrs(&mut resolved_addrs, &addrs);
ipv4_addrs = addrs;
if first_positive_family.is_none() && !ipv6_done {
let ipv4_was_first = first_positive_family.is_none();
if ipv4_was_first {
first_positive_family = Some(AddressFamily::Ipv4);
}
if ipv4_was_first && !ipv6_done {
held_ipv4_until_resolution_delay = true;
resolution_delay.as_mut().reset(tokio::time::Instant::now() + HAPPY_EYEBALLS_RESOLUTION_DELAY);
} else {
Expand All @@ -620,6 +623,7 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
&mut scheduled_addrs,
&ipv4_addrs,
&ipv6_addrs,
first_positive_family.expect("a family produced addresses"),
);
}
}
Expand All @@ -642,6 +646,7 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
&mut scheduled_addrs,
&ipv4_addrs,
&ipv6_addrs,
first_positive_family.expect("a family produced addresses"),
);
held_ipv4_until_resolution_delay = false;
} else if held_ipv4_until_resolution_delay {
Expand All @@ -650,6 +655,7 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
&mut scheduled_addrs,
&ipv4_addrs,
&ipv6_addrs,
first_positive_family.expect("IPv4 produced addresses"),
);
held_ipv4_until_resolution_delay = false;
}
Expand All @@ -662,6 +668,7 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
&mut scheduled_addrs,
&ipv4_addrs,
&ipv6_addrs,
first_positive_family.expect("IPv4 produced addresses"),
);
held_ipv4_until_resolution_delay = false;
}
Expand All @@ -674,6 +681,7 @@ pub(crate) async fn connect_host_happy_eyeballs_traced(
&mut scheduled_addrs,
&ipv4_addrs,
&ipv6_addrs,
first_positive_family.expect("IPv4 produced addresses"),
);
held_ipv4_until_resolution_delay = false;
}
Expand Down Expand Up @@ -725,13 +733,11 @@ fn enqueue_interleaved(
scheduled_addrs: &mut Vec<SocketAddr>,
ipv4_addrs: &[SocketAddr],
ipv6_addrs: &[SocketAddr],
preferred_family: AddressFamily,
) {
let ordered = if ipv6_addrs.is_empty() {
ipv4_addrs.to_vec()
} else if ipv4_addrs.is_empty() {
ipv6_addrs.to_vec()
} else {
interleave_socket_addr_families(ipv6_addrs, ipv4_addrs)
let ordered = match preferred_family {
AddressFamily::Ipv4 => interleave_socket_addr_families(ipv4_addrs, ipv6_addrs),
AddressFamily::Ipv6 => interleave_socket_addr_families(ipv6_addrs, ipv4_addrs),
};
for addr in ordered {
if !scheduled_addrs.contains(&addr) {
Expand Down Expand Up @@ -1480,6 +1486,46 @@ mod tests {
assert_eq!(addr.scope_id(), 42);
}

#[tokio::test]
async fn happy_eyeballs_keeps_ipv4_preferred_when_ipv6_resolves_during_grace_period() {
let ipv4 = SocketAddr::new("127.0.0.1".parse().unwrap(), 443);
let ipv6 = SocketAddr::new("::1".parse().unwrap(), 443);
let mut pending = VecDeque::new();
let mut scheduled = Vec::new();

// Model A resolving first and AAAA resolving during the 50 ms grace period.
enqueue_interleaved(
&mut pending,
&mut scheduled,
&[ipv4],
&[ipv6],
AddressFamily::Ipv4,
);
assert_eq!(pending.front(), Some(&ipv4));

let result = tokio::time::timeout(
Duration::from_millis(100),
race_staggered(
pending.into(),
HAPPY_EYEBALLS_FALLBACK_DELAY,
"lookup returned no addresses",
"test connect",
|addr| async move {
if addr.is_ipv4() {
Ok("ipv4")
} else {
std::future::pending::<Result<&str, FetchError>>().await
}
},
),
)
.await
.expect("IPv4 should start before the 300 ms fallback delay")
.unwrap();

assert_eq!(result, "ipv4");
}

#[tokio::test]
async fn happy_eyeballs_starts_fallback_immediately_after_preferred_failure() {
let addrs = vec![
Expand Down
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