core: survive a failed net-stack rebuild instead of silently dying

The four live-rebuild sites (deferred rebuild on Join/Leave, idle
SetNetworkMode, idle RegenerateIdentity) all did
`net.shutdown().await` then `build_net_stack(...).await?` — a build
failure propagated out of run_core_loop, which its supervisor only
logs. Every subsequent command went nowhere: window alive, app dead,
user told nothing. (The initial startup build already reported.)

New replace_net_stack() helper: tear down the old stack, build for the
requested posture, and on failure fall back to the posture the old
stack was actually running (tracked in the new `net_mode` local; when
the postures are equal the fallback is a plain retry — e.g. identity
regeneration, where reverting the already-persisted key would be
wrong). If the fallback lands, the UI is told the change didn't stick
and `network_mode` reverts so state stays honest and the change stays
re-attemptable. If both builds fail the UI gets a fatal 'Networking
lost … restart' error before the loop exits — informed, not a zombie.

Retry policy isolated in rebuild_with_fallback(), generic over the
builder: 4 new unit tests cover first-try success, fall-back, plain
retry, and double failure without binding sockets.

Fixes finding 2 of the 2026-07-16 full-codebase review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/mod.rs

@@ -888,6 +888,103 @@ async fn build_net_stack(
     })
 }
 
+/// Retry policy for a live net-stack replacement, generic over the builder so
+/// it is unit-testable without binding sockets: build for `requested`; if that
+/// fails, build for `live` (the posture the old stack was actually running) so
+/// a bad posture change degrades to the previous posture instead of leaving no
+/// stack at all. When `requested == live` the second attempt is a plain retry.
+///
+/// `Ok((stack, mode, primary_err))` — a stack is up on `mode`; `primary_err`
+/// is `Some` when the first attempt failed. `Err((primary, fallback))` — both
+/// attempts failed and networking is gone.
+async fn rebuild_with_fallback<T, E, F, Fut>(
+    mut build: F,
+    requested: NetworkMode,
+    live: NetworkMode,
+) -> Result<(T, NetworkMode, Option<E>), (E, E)>
+where
+    F: FnMut(NetworkMode) -> Fut,
+    Fut: std::future::Future<Output = Result<T, E>>,
+{
+    match build(requested).await {
+        Ok(stack) => Ok((stack, requested, None)),
+        Err(primary) => match build(live).await {
+            Ok(stack) => Ok((stack, live, Some(primary))),
+            Err(fallback) => Err((primary, fallback)),
+        },
+    }
+}
+
+/// Tear down `old` and stand up a replacement stack for `requested_mode`.
+///
+/// A build failure here is rare (only the local socket bind can fail; the
+/// relay handshake is backgrounded), but it used to propagate straight out of
+/// `run_core_loop` with no `UiEvent`, silently killing every future command —
+/// the app looked alive and did nothing. Instead, fall back to `live_mode`
+/// via `rebuild_with_fallback`, tell the UI when the requested change did not
+/// stick, and return the mode the new stack actually runs so the caller can
+/// keep its state honest. `Err` only when both builds fail: networking is
+/// gone (already reported to the UI as fatal) and the caller should exit.
+#[allow(clippy::too_many_arguments)]
+async fn replace_net_stack(
+    old: NetStack,
+    what: &str,
+    secret_key: &SecretKey,
+    requested_mode: NetworkMode,
+    live_mode: NetworkMode,
+    friends_handler: &crate::presence_net::Handler,
+    publish: bool,
+    ui_tx: &mpsc::Sender<UiEvent>,
+) -> Result<(NetStack, NetworkMode), anyhow::Error> {
+    let lookup = old.memory_lookup.clone();
+    old.shutdown().await;
+    let outcome = rebuild_with_fallback(
+        |mode| {
+            build_net_stack(
+                secret_key.clone(),
+                mode,
+                lookup.clone(),
+                friends_handler.clone(),
+                publish,
+            )
+        },
+        requested_mode,
+        live_mode,
+    )
+    .await;
+    match outcome {
+        Ok((stack, mode, None)) => Ok((stack, mode)),
+        Ok((stack, mode, Some(primary))) => {
+            if mode == requested_mode {
+                // Same-posture retry succeeded — everything the user asked for
+                // is in effect, so log it rather than raising a UI error.
+                crate::log_msg(&format!(
+                    "{what}: net stack build failed once ({primary:#}); retry succeeded"
+                ));
+            } else {
+                let _ = ui_tx
+                    .send(UiEvent::Error(format!(
+                        "{what} failed ({primary:#}); staying on the previous \
+                         network mode for this session"
+                    )))
+                    .await;
+            }
+            Ok((stack, mode))
+        }
+        Err((primary, fallback)) => {
+            let _ = ui_tx
+                .send(UiEvent::Error(format!(
+                    "Networking lost: {primary:#} (recovery attempt also failed: \
+                     {fallback:#}). Restart PeerSpeak to reconnect."
+                )))
+                .await;
+            Err(anyhow::anyhow!(
+                "net stack rebuild failed: {primary:#}; fallback: {fallback:#}"
+            ))
+        }
+    }
+}
+
 /// Maximum number of *automatic* chat-attachment fetches in flight at once.
 ///
 /// Auto-fetch (inline image preview) is triggered by an untrusted peer's chat
@@ -1335,6 +1432,10 @@ async fn run_core_loop(
     // rebuilt on the next Leave (or before the next Join), preserving the old
     // "applies on next join" semantics while keeping the endpoint up while idle.
     let mut net_rebuild_pending = false;
+    // The posture the live stack was actually built with. Trails `network_mode`
+    // while a rebuild is pending, and is the fallback posture when a rebuild
+    // fails (see `replace_net_stack`).
+    let mut net_mode = network_mode;
 
     // When Discoverable is on, the instant it auto-reverts to Normal (W7 P6 time-box).
     // `None` = not Discoverable, no pending revert. Set on SetPresenceMode(Discoverable),
@@ -1552,17 +1653,24 @@ async fn run_core_loop(
                 // active, rebuild the persistent stack now — after the old session is
                 // gone, before the new one binds — so this join uses the new posture.
                 if net_rebuild_pending {
-                    let lookup = net.memory_lookup.clone();
-                    net.shutdown().await;
                     let publish = presence_mode.lock().unwrap().publishes_to_discovery();
-                    net = build_net_stack(
+                    let (stack, live) = replace_net_stack(
-                        secret_key.clone(),
+                        net,
+                        "Applying deferred network settings",
+                        &secret_key,
                         network_mode,
-                        lookup,
+                        net_mode,
-                        friends_handler.clone(),
+                        &friends_handler,
                         publish,
+                        &ui_tx,
                     )
                     .await?;
+                    net = stack;
+                    net_mode = live;
+                    // If the new posture failed and we fell back, keep the mode
+                    // state honest (and re-attemptable) rather than pretending
+                    // the change applied. The join proceeds on the live stack.
+                    network_mode = live;
                     net_rebuild_pending = false;
                 }
 
@@ -2594,17 +2702,21 @@ async fn run_core_loop(
                 // Apply any network-mode / identity change that was deferred while we
                 // were in the call (rebuild while idle keeps the endpoint reachable).
                 if net_rebuild_pending {
-                    let lookup = net.memory_lookup.clone();
-                    net.shutdown().await;
                     let publish = presence_mode.lock().unwrap().publishes_to_discovery();
-                    net = build_net_stack(
+                    let (stack, live) = replace_net_stack(
-                        secret_key.clone(),
+                        net,
+                        "Applying deferred network settings",
+                        &secret_key,
                         network_mode,
-                        lookup,
+                        net_mode,
-                        friends_handler.clone(),
+                        &friends_handler,
                         publish,
+                        &ui_tx,
                     )
                     .await?;
+                    net = stack;
+                    net_mode = live;
+                    network_mode = live;
                     net_rebuild_pending = false;
                 }
             }
@@ -2750,17 +2862,21 @@ async fn run_core_loop(
                     // idle; if a call is active, defer to the next Leave/Join so the
                     // live call isn't disrupted (preserves "applies on next join").
                     if active_session.is_none() {
-                        let lookup = net.memory_lookup.clone();
-                        net.shutdown().await;
                         let publish = presence_mode.lock().unwrap().publishes_to_discovery();
-                        net = build_net_stack(
+                        let (stack, live) = replace_net_stack(
-                            secret_key.clone(),
+                            net,
+                            "Network mode change",
+                            &secret_key,
                             network_mode,
-                            lookup,
+                            net_mode,
-                            friends_handler.clone(),
+                            &friends_handler,
                             publish,
+                            &ui_tx,
                         )
                         .await?;
+                        net = stack;
+                        net_mode = live;
+                        network_mode = live;
                     } else {
                         net_rebuild_pending = true;
                     }
@@ -2798,17 +2914,22 @@ async fn run_core_loop(
                 // key unchanged, so a rebuild would be pointless churn).
                 if regenerated {
                     if active_session.is_none() {
-                        let lookup = net.memory_lookup.clone();
-                        net.shutdown().await;
                         let publish = presence_mode.lock().unwrap().publishes_to_discovery();
-                        net = build_net_stack(
+                        // Same mode both attempts — the fallback is a plain
-                            secret_key.clone(),
+                        // retry under the (already persisted) new key.
+                        let (stack, live) = replace_net_stack(
+                            net,
+                            "Endpoint restart after identity change",
+                            &secret_key,
                             network_mode,
-                            lookup,
+                            net_mode,
-                            friends_handler.clone(),
+                            &friends_handler,
                             publish,
+                            &ui_tx,
                         )
                         .await?;
+                        net = stack;
+                        net_mode = live;
                     } else {
                         net_rebuild_pending = true;
                     }
@@ -3322,10 +3443,10 @@ fn replace_viewer_index<T>(viewers: &[(String, T)], ticket: &str) -> Option<usiz
 mod tests {
     use super::{
         KnownPeers, MAX_OPUS_PAYLOAD, MAX_RETAINED_PEERS, MIC_LEVEL_REPORT_SAMPLES, MicLevelMeter,
-        PLAYBACK_HANDOFF_QUEUE_FRAMES, PeerSpeakTicket, admit_retained, apply_peer_volume,
+        NetworkMode, PLAYBACK_HANDOFF_QUEUE_FRAMES, PeerSpeakTicket, admit_retained,
-        apply_volume, audio_datagram_len_ok, coalesce_insert, coalesce_pop, frame_level,
+        apply_peer_volume, apply_volume, audio_datagram_len_ok, coalesce_insert, coalesce_pop,
-        mix_frames, mix_stereo_frames, next_game_change, replace_viewer_index, send_playback_frame,
+        frame_level, mix_frames, mix_stereo_frames, next_game_change, rebuild_with_fallback,
-        should_auto_fetch, stereo_to_mono,
+        replace_viewer_index, send_playback_frame, should_auto_fetch, stereo_to_mono,
     };
     use crate::core::messages::{CoalesceKey, CoreCommand, coalesce_key};
     use std::collections::{HashMap, HashSet};
@@ -3349,6 +3470,101 @@ mod tests {
         assert_eq!(replace_viewer_index::<u8>(&[], "ticket-A"), None);
     }
 
+    // --- rebuild_with_fallback: the retry policy behind replace_net_stack ---
+    // The builder is injected, so these cover the policy without sockets. The
+    // closure does its bookkeeping synchronously and returns a ready future.
+
+    #[tokio::test]
+    async fn rebuild_keeps_requested_posture_on_first_success() {
+        let calls = std::cell::RefCell::new(Vec::new());
+        let out = rebuild_with_fallback(
+            |mode| {
+                calls.borrow_mut().push(mode);
+                std::future::ready(Ok::<u8, String>(7))
+            },
+            NetworkMode::DirectOnly,
+            NetworkMode::N0Full,
+        )
+        .await;
+        assert_eq!(out, Ok((7, NetworkMode::DirectOnly, None)));
+        // No second build: the live posture is only a fallback.
+        assert_eq!(*calls.borrow(), vec![NetworkMode::DirectOnly]);
+    }
+
+    #[tokio::test]
+    async fn rebuild_falls_back_to_the_live_posture_when_the_requested_one_fails() {
+        let calls = std::cell::RefCell::new(Vec::new());
+        let out = rebuild_with_fallback(
+            |mode| {
+                calls.borrow_mut().push(mode);
+                std::future::ready(if mode == NetworkMode::DirectOnly {
+                    Err("bind failed".to_string())
+                } else {
+                    Ok(7u8)
+                })
+            },
+            NetworkMode::DirectOnly,
+            NetworkMode::N0Full,
+        )
+        .await;
+        // A stack is up on the OLD posture and the caller learns both that it
+        // fell back (mode) and why (the primary error) — no silent zombie.
+        assert_eq!(
+            out,
+            Ok((7, NetworkMode::N0Full, Some("bind failed".to_string())))
+        );
+        assert_eq!(
+            *calls.borrow(),
+            vec![NetworkMode::DirectOnly, NetworkMode::N0Full]
+        );
+    }
+
+    #[tokio::test]
+    async fn rebuild_reports_both_errors_when_networking_is_gone() {
+        let out = rebuild_with_fallback(
+            |_| std::future::ready(Err::<u8, String>("bind failed".to_string())),
+            NetworkMode::DirectOnly,
+            NetworkMode::N0Full,
+        )
+        .await;
+        assert_eq!(
+            out,
+            Err(("bind failed".to_string(), "bind failed".to_string()))
+        );
+    }
+
+    #[tokio::test]
+    async fn rebuild_with_equal_postures_is_a_plain_retry() {
+        // RegenerateIdentity rebuilds under the same mode: the fallback is a
+        // second attempt with identical parameters, not a posture change.
+        let calls = std::cell::Cell::new(0u8);
+        let out = rebuild_with_fallback(
+            |mode| {
+                calls.set(calls.get() + 1);
+                assert_eq!(mode, NetworkMode::RelayNoDiscovery);
+                std::future::ready(if calls.get() == 1 {
+                    Err("transient".to_string())
+                } else {
+                    Ok(7u8)
+                })
+            },
+            NetworkMode::RelayNoDiscovery,
+            NetworkMode::RelayNoDiscovery,
+        )
+        .await;
+        // Succeeded on the requested posture, so the caller treats the change
+        // as applied (the Some(err) is logged, not surfaced as a UI error).
+        assert_eq!(
+            out,
+            Ok((
+                7,
+                NetworkMode::RelayNoDiscovery,
+                Some("transient".to_string())
+            ))
+        );
+        assert_eq!(calls.get(), 2);
+    }
+
     #[test]
     fn admit_retained_rejects_only_new_ids_at_the_cap() {
         // Below the cap, a brand-new identity is retained.