fix(offline): hysteresis + cheap probe + streaming liveness override (#467)

Long sessions died when a single transient blip flipped the app to offline and
the queue then skipped every cached-but-not-downloaded track. Harden the
detector:

- Consecutive-failure hysteresis: declare offline only after 2 failed polls,
  not a single blip.
- Replace the HEAD https://1.1.1.1 probe with a concurrent race of cheap
  generate_204 endpoints, so a saturated link (long Hi-Res tracks) no longer
  times the probe out.
- Positive liveness override: if a segment downloaded within the last 45s we
  are online by definition, regardless of the probe.

cc

crates/qbz-audio/src/network_throttle.rs

@@ -73,6 +73,10 @@ struct ThrottleInner {
     /// Timestamp of the most recent audio underrun. `None` means we have
     /// not observed one this session.
     last_underrun: Option<Instant>,
+    /// Timestamp of the most recent successful segment download. `None`
+    /// until the first segment lands. Used as a positive liveness signal
+    /// for the offline detector (issue #467).
+    last_successful_download: Option<Instant>,
 }
 
 pub struct ThrottleState {
@@ -92,14 +96,18 @@ impl ThrottleState {
     /// Feed a fresh per-segment bandwidth measurement (MB/s). Called from
     /// the CMAF streaming loop every few segments.
     pub fn record_segment_bandwidth(&self, mbps: f64) {
-        if !mbps.is_finite() || mbps <= 0.0 {
-            return;
-        }
         if let Ok(mut inner) = self.inner.write() {
-            inner.bandwidth_ema_mbps = Some(match inner.bandwidth_ema_mbps {
-                Some(prev) => prev * (1.0 - BANDWIDTH_EMA_ALPHA) + mbps * BANDWIDTH_EMA_ALPHA,
-                None => mbps,
-            });
+            // Liveness signal (issue #467): reaching this path means a segment
+            // batch just downloaded — bytes are flowing, so we are online,
+            // independent of whether the throughput sample is usable for the
+            // EMA. Recorded before the validity guard on purpose.
+            inner.last_successful_download = Some(Instant::now());
+            if mbps.is_finite() && mbps > 0.0 {
+                inner.bandwidth_ema_mbps = Some(match inner.bandwidth_ema_mbps {
+                    Some(prev) => prev * (1.0 - BANDWIDTH_EMA_ALPHA) + mbps * BANDWIDTH_EMA_ALPHA,
+                    None => mbps,
+                });
+            }
         }
     }
 
@@ -116,6 +124,18 @@ impl ThrottleState {
         self.inner.read().ok().and_then(|i| i.bandwidth_ema_mbps)
     }
 
+    /// Seconds since the last successful segment download, or `None` if no
+    /// bytes have been pulled this session. Positive liveness signal for the
+    /// offline detector (issue #467): if segments are flowing we are online
+    /// by definition, regardless of what a connectivity probe reports.
+    pub fn seconds_since_download(&self) -> Option<u64> {
+        self.inner
+            .read()
+            .ok()
+            .and_then(|i| i.last_successful_download)
+            .map(|t| t.elapsed().as_secs())
+    }
+
     /// True when an underrun was recorded within `PANIC_WINDOW_SECS`.
     pub fn in_panic_mode(&self) -> bool {
         let inner = match self.inner.read() {

src-tauri/src/commands_v2/library.rs

@@ -1361,10 +1361,35 @@ pub async fn v2_get_offline_status(
         });
     }
 
-    // Check network connectivity
-    let has_network = crate::offline::check_network_connectivity().await;
+    // Fix E (issue #467): positive liveness override. If audio segments were
+    // flowing within the liveness window we are, by definition, online —
+    // regardless of what the connectivity probe says. Heavy Hi-Res tracks
+    // (long Dream Theater / Yes) saturate the link and can time the probe
+    // out; active streaming is the strongest possible "online" signal.
+    const LIVENESS_WINDOW_SECS: u64 = 45;
+    if let Some(secs) = qbz_audio::network_throttle::state().seconds_since_download() {
+        if secs <= LIVENESS_WINDOW_SECS {
+            crate::offline::reset_connectivity_failures();
+            return Ok(crate::offline::OfflineStatus {
+                is_offline: false,
+                reason: None,
+                manual_mode_enabled: false,
+            });
+        }
+    }
 
-    if !has_network {
+    // Check network connectivity (cheap concurrent probe) and apply
+    // consecutive-failure hysteresis (Fix A, issue #467): a single failed poll
+    // must not flip offline — only OFFLINE_FAILURE_THRESHOLD consecutive
+    // failures do.
+    let has_network = crate::offline::check_network_connectivity().await;
+    let failures = crate::offline::record_connectivity_result(has_network);
+
+    if !has_network && failures >= crate::offline::OFFLINE_FAILURE_THRESHOLD {
+        log::info!(
+            "[offline] Declaring offline after {} consecutive connectivity failures",
+            failures
+        );
         return Ok(crate::offline::OfflineStatus {
             is_offline: true,
             reason: Some(crate::offline::OfflineReason::NoNetwork),
@@ -1372,6 +1397,15 @@ pub async fn v2_get_offline_status(
         });
     }
 
+    if !has_network {
+        log::info!(
+            "[offline] Connectivity probe failed ({}/{}); staying online (hysteresis)",
+            failures,
+            crate::offline::OFFLINE_FAILURE_THRESHOLD
+        );
+    }
+
+    // Online, or a transient failure still under the threshold: stay online.
     Ok(crate::offline::OfflineStatus {
         is_offline: false,
         reason: None,

src-tauri/src/offline/mod.rs

@@ -633,75 +633,84 @@ impl OfflineState {
     }
 }
 
-/// Counter for alternating between neutral endpoint and Qobuz checks.
-/// Using atomic for thread safety across async calls.
-static CHECK_COUNTER: std::sync::atomic::AtomicU32 = std::sync::atomic::AtomicU32::new(0);
+/// Consecutive failed connectivity polls. The offline flag is only raised
+/// once this reaches `OFFLINE_FAILURE_THRESHOLD`, so a single transient blip
+/// (Wi-Fi micro-drop, DNS hiccup, or a saturated link losing the probe race
+/// during a big Hi-Res download) can no longer flip the app offline. See
+/// issue #467.
+static CONSECUTIVE_FAILURES: std::sync::atomic::AtomicU32 = std::sync::atomic::AtomicU32::new(0);
 
-/// Check network connectivity using a hybrid strategy:
-/// - 9 out of 10 checks go to a neutral endpoint (1.1.1.1) to verify basic internet
-/// - 1 out of 10 checks go to Qobuz to verify service availability
+/// Number of consecutive failed polls (~30 s apart) required before the app
+/// is declared offline. 2 ≈ 60 s of confirmed loss — lax enough to ride out
+/// blips, still reacts to a real outage within about a minute.
+pub const OFFLINE_FAILURE_THRESHOLD: u32 = 2;
+
+/// Record a connectivity-poll outcome and return the running consecutive
+/// failure count. Any success resets the counter to 0.
+pub fn record_connectivity_result(success: bool) -> u32 {
+    use std::sync::atomic::Ordering;
+    if success {
+        CONSECUTIVE_FAILURES.store(0, Ordering::Relaxed);
+        0
+    } else {
+        CONSECUTIVE_FAILURES.fetch_add(1, Ordering::Relaxed) + 1
+    }
+}
+
+/// Reset the consecutive-failure counter. Called on a strong positive signal
+/// (active streaming liveness, manual-mode change) so the next real loss
+/// starts counting from zero.
+pub fn reset_connectivity_failures() {
+    CONSECUTIVE_FAILURES.store(0, std::sync::atomic::Ordering::Relaxed);
+}
+
+/// Lightweight "generate_204"-style connectivity probes. A success is a
+/// near-empty 204/200 response, so the probe barely competes for bandwidth
+/// even while a large Hi-Res download is saturating the link — the exact
+/// condition (long Dream Theater / Yes tracks) that timed the old
+/// `HEAD https://1.1.1.1` probe out and produced a false offline (issue #467).
+const PROBE_ENDPOINTS: &[&str] = &[
+    "https://connectivitycheck.gstatic.com/generate_204",
+    "https://www.gstatic.com/generate_204",
+    "https://cloudflare.com/cdn-cgi/trace",
+];
+
+/// Check basic internet connectivity with a cheap, concurrent probe.
 ///
-/// This reduces load on Qobuz API and avoids false positives from rate limiting
-/// when the app is making many concurrent API calls.
+/// Races the lightweight endpoints above and returns `true` as soon as any
+/// one succeeds. This is intentionally *not* a Qobuz-service check: a slow or
+/// rate-limited Qobuz must never read as "offline" (issue #467). Actual Qobuz
+/// API failures are handled with retries/backoff in the fetch path instead.
 pub async fn check_network_connectivity() -> bool {
-    let counter = CHECK_COUNTER.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
-    let check_qobuz = counter % 10 == 0;
-
-    let client = reqwest::Client::builder()
-        .timeout(Duration::from_secs(10))
-        .build();
-
-    let client = match client {
+    let client = match reqwest::Client::builder()
+        .timeout(Duration::from_secs(8))
+        .build()
+    {
         Ok(c) => c,
         Err(_) => return false,
     };
 
-    let endpoint = if check_qobuz {
-        "https://www.qobuz.com"
-    } else {
-        // Cloudflare DNS - highly reliable, low latency
-        "https://1.1.1.1"
-    };
-
-    // Try up to 2 times before declaring offline
-    for attempt in 1..=2 {
-        match client.head(endpoint).send().await {
-            Ok(response) => {
-                if response.status().is_success() || response.status().is_redirection() {
-                    return true;
+    let mut set = tokio::task::JoinSet::new();
+    for endpoint in PROBE_ENDPOINTS {
+        let client = client.clone();
+        let endpoint = (*endpoint).to_string();
+        set.spawn(async move {
+            match client.get(&endpoint).send().await {
+                Ok(response) => {
+                    response.status().is_success() || response.status().is_redirection()
                 }
+                Err(_) => false,
             }
-            Err(e) => {
-                log::warn!(
-                    "Network check attempt {} to {} failed: {}",
-                    attempt,
-                    endpoint,
-                    e
-                );
-            }
-        }
+        });
+    }
 
-        // Wait 1 second before retry
-        if attempt < 2 {
-            tokio::time::sleep(Duration::from_secs(1)).await;
+    while let Some(joined) = set.join_next().await {
+        if matches!(joined, Ok(true)) {
+            set.abort_all();
+            return true;
         }
     }
 
-    // If neutral endpoint failed, try Qobuz as fallback (maybe it's a DNS issue)
-    if !check_qobuz {
-        log::info!("Neutral endpoint failed, trying Qobuz as fallback...");
-        match client.head("https://www.qobuz.com").send().await {
-            Ok(response) => {
-                if response.status().is_success() || response.status().is_redirection() {
-                    return true;
-                }
-            }
-            Err(e) => {
-                log::warn!("Qobuz fallback check also failed: {}", e);
-            }
-        }
-    }
-
-    log::info!("Network connectivity check failed after all attempts");
+    log::info!("[offline] Connectivity probe failed on all endpoints");
     false
 }