Native: panic in `Yamux::index_mut` from `single_stream_connection_task` (iOS, smoldot-light)

[lrubasze]

Seen via Sentry in a native (Rust, smoldot-light) integration on iOS: BREVITY-MORPHEUS-4C - 2 events, 2 users, substatus regressed.

Crash

SIGABRT on thread smoldot-light-2, ~1 minute after app start:

smoldot_light::network_service::tasks::single_stream_connection_task::{{closure}}
smoldot::libp2p::connection::yamux::Yamux<T>::index_mut
core::panicking::panic

Yamux::index_mut (lib/src/libp2p/connection/yamux.rs:1736) panics when the SubstreamId is not in the substream map:

impl<TNow, TSub> ops::IndexMut<SubstreamId> for Yamux<TNow, TSub> {
    fn index_mut(&mut self, substream_id: SubstreamId) -> &mut TSub {
        ...
            .unwrap_or_else(|| panic!())

Analysis

• The concrete caller is inlined away; candidates are the self.inner.yamux[substream_id] accessors in lib/src/libp2p/connection/established/single_stream.rs (~16 sites).
• SPECULATED: a coordinator message (request start, notification write, close, ...) is processed for a substream that has already died and been removed from the yamux state - a race between substream death and coordinator messages. Related past fix: Properly process dead substreams #2869.

Environment

• iOS 26.2, iPhone17,1, app io.parity.brevity@1.0.0+39 (Flutter, native smoldot-light)
• smoldot version pinned by the app: unknown, needs confirmation from the app team

Next steps

• Confirm smoldot(-light) version used by the reporting app
• Identify which accessor fires (debug log or better symbolication)

[sentry]

Sentry Issue: BREVITY-MORPHEUS-4C

[lrubasze]

Related Brevity Sentry issues :

• BREVITY-MORPHEUS-4S — same panic, from a macOS debug build (2 events, 2026-07-23). Symbolication resolves the frames that were inlined away in the iOS release build:

  inject_coordinator_message (single_stream.rs:488)
  → reject_in_notifications_substream
  → Yamux<T>::index_mut (yamux.rs:1736)
  

This answers the two open questions in the issue:

• Version: the app pins smoldot-light 1.3.1 / smoldot 2.1.0 (crates.io).
• Accessor: the reject path — reject_in_notifications_substream indexes a substream that is already gone, consistent with a duplicate RejectInNotifications racing a remote reset.

For completeness, other smoldot panics from the same app map to already-closed issues: BREVITY-MORPHEUS-41 (Subscription::unpin_block, #3286) and fresh dev-machine events matching #3313.