feat(host): concurrent punktfunk/1 sessions (bounded by --max-concurrent)

The accept loop no longer awaits each session inline — it spawns each onto a
JoinSet, bounded by a semaphore (--max-concurrent, default 4: a NVENC session
bound; overflow clients wait in QUIC's accept backlog until a slot frees). The
QUIC handshake stays in the accept loop so a failed handshake (e.g. a pin
mismatch where the client aborts) doesn't consume a session slot or block
accepting the next client; the slow part (control handshake, pairing, the
capture/encode pipeline) runs in the spawned task.

Each session already had its own virtual output + NVENC encoder; the
host-lifetime input/audio/mic services stay shared — the natural "multiple
devices viewing/controlling the same desktop" semantic on kwin/mutter/wlroots.
gamescope's independent-desktops (per-session input/audio) isolation is a
follow-up. New M3Options.max_concurrent + the `--max-concurrent` CLI flag.

Validated live (GNOME box): two clients connected at once -> two independent
Mutter virtual outputs (720p60 + 1080p60) streaming simultaneously (39 MB +
48 MB). All 61 host tests green (the c_abi/pairing tests exercise the new loop +
the failed-handshake-doesn't-count semantics).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/m3.rs

@@ -53,6 +53,12 @@ pub struct M3Options {
     pub frames: u32,
     /// Exit after this many sessions (0 = serve forever).
     pub max_sessions: u32,
+    /// Maximum sessions streaming **at once** (a NVENC/GPU bound); further clients wait in the
+    /// accept queue until a slot frees. Concurrent sessions each get their own virtual output +
+    /// encoder but share the host-lifetime input/audio/mic services — i.e. multiple devices viewing
+    /// (and controlling) the *same* desktop on the shared-desktop backends (kwin/mutter/wlroots).
+    /// `0` = unlimited (bounded only by the GPU). Default a conservative few.
+    pub max_concurrent: usize,
     /// Only serve clients whose certificate fingerprint is in the paired set. Implies
     /// `allow_pairing` (a host that requires pairing must accept ceremonies to admit
     /// anyone).
@@ -127,6 +133,11 @@ pub(crate) struct NativeServe {
 /// Options for the native host when the unified `serve --native` runs it: real virtual capture,
 /// persistent (no session/duration cut), pairing armed on demand via the management API (the
 /// shared [`NativePairing`] starts disarmed).
+/// Default cap on simultaneously-streaming sessions (each holds an NVENC session; high-res
+/// split-encode holds two). Conservative — consumer NVENC historically capped concurrent sessions;
+/// overflow clients wait in the accept queue. Override with `--max-concurrent`.
+pub(crate) const DEFAULT_MAX_CONCURRENT: usize = 4;
+
 pub(crate) fn native_serve_opts(cfg: &NativeServe) -> M3Options {
     M3Options {
         port: cfg.port,
@@ -134,6 +145,7 @@ pub(crate) fn native_serve_opts(cfg: &NativeServe) -> M3Options {
         seconds: 7 * 24 * 3600, // per-session cap; large enough not to cut a live stream
         frames: 0,
         max_sessions: 0,
+        max_concurrent: DEFAULT_MAX_CONCURRENT,
         require_pairing: cfg.require_pairing,
         allow_pairing: false,
         pairing_pin: None,
@@ -202,45 +214,84 @@ pub(crate) async fn serve(opts: M3Options, np: Arc<NativePairing>) -> Result<()>
             "PAIRING ARMED — enter this PIN on the client to pair: {pin}"
         );
     }
-    let last_pairing = std::sync::Mutex::new(None::<std::time::Instant>);
+    let last_pairing = Arc::new(std::sync::Mutex::new(None::<std::time::Instant>));
+    let opts = Arc::new(opts);
+
+    // Concurrency: serve up to `max_concurrent` sessions at once. Each gets its own virtual output +
+    // NVENC encoder; they share the host-lifetime input/audio/mic services — i.e. multiple devices
+    // viewing (and controlling) the SAME desktop on the shared-desktop backends. A permit is taken
+    // before accepting, so overflow clients wait in QUIC's accept backlog until a slot frees;
+    // `max_concurrent == 0` means unlimited (GPU-bounded). The heavy handshake + pipeline run inside
+    // the spawned task, so a slow client never blocks the accept loop.
+    let permits = match opts.max_concurrent {
+        0 => tokio::sync::Semaphore::MAX_PERMITS,
+        n => n,
+    };
+    let sem = Arc::new(tokio::sync::Semaphore::new(permits));
+    let mut sessions = tokio::task::JoinSet::new();
+    let max_sessions = opts.max_sessions;
+    let mut accepted = 0u32;
+    tracing::info!(
+        max_concurrent = opts.max_concurrent,
+        "accepting sessions (concurrent)"
+    );
 
-    let mut served = 0u32;
     loop {
-        let incoming = ep
-            .accept()
+        let permit = sem
+            .clone()
+            .acquire_owned()
             .await
-            .ok_or_else(|| anyhow!("endpoint closed"))?;
+            .expect("session semaphore is never closed");
+        let incoming = match ep.accept().await {
+            Some(i) => i,
+            None => break, // endpoint closed
+        };
+        // Complete the QUIC handshake in the accept loop (it's ~1 RTT): a failed handshake (e.g. a
+        // pin mismatch — the client aborts) must NOT consume a session slot, mirroring the old
+        // serial loop. The slow part (control handshake, pairing, the capture/encode pipeline) runs
+        // in the spawned task, so a slow client still never blocks accepting the next one.
         let conn = match incoming.await {
             Ok(c) => c,
             Err(e) => {
                 tracing::warn!(error = %e, "QUIC accept failed");
-                continue;
+                continue; // `permit` drops here → slot freed; not counted toward max_sessions
             }
         };
         let peer = conn.remote_address();
         tracing::info!(%peer, "punktfunk/1 client connected");
-        if let Err(e) = serve_session(
-            conn,
-            &opts,
-            &audio_cap,
-            injector.sender(),
-            mic_service.sender(),
-            &fingerprint,
-            &np,
-            &last_pairing,
-        )
-        .await
-        {
-            tracing::warn!(%peer, error = %format!("{e:#}"), "session ended with error");
-        } else {
-            tracing::info!(%peer, "session complete");
-        }
-        served += 1;
-        if opts.max_sessions != 0 && served >= opts.max_sessions {
+        let opts = opts.clone();
+        let audio_cap = audio_cap.clone();
+        let np = np.clone();
+        let last_pairing = last_pairing.clone();
+        let inj_tx = injector.sender();
+        let mic_tx = mic_service.sender();
+        sessions.spawn(async move {
+            let _permit = permit; // held for the session's lifetime; frees a slot on completion
+            match serve_session(
+                conn,
+                &opts,
+                &audio_cap,
+                inj_tx,
+                mic_tx,
+                &fingerprint,
+                &np,
+                &last_pairing,
+            )
+            .await
+            {
+                Ok(()) => tracing::info!(%peer, "session complete"),
+                Err(e) => {
+                    tracing::warn!(%peer, error = %format!("{e:#}"), "session ended with error")
+                }
+            }
+        });
+        accepted += 1;
+        if max_sessions != 0 && accepted >= max_sessions {
             break;
         }
-        tracing::info!("ready for the next client");
     }
+    // Stop accepting; let the in-flight sessions finish (max_sessions reached or endpoint closed).
+    while sessions.join_next().await.is_some() {}
     ep.wait_idle().await;
     Ok(())
 }
@@ -2014,6 +2065,7 @@ mod tests {
                 seconds: 0,
                 frames: 25,
                 max_sessions: 3,
+                max_concurrent: 1,
                 require_pairing: false,
                 allow_pairing: false,
                 pairing_pin: None,
@@ -2169,6 +2221,7 @@ mod tests {
                 seconds: 0,
                 frames: 25,
                 max_sessions: 4,
+                max_concurrent: 1,
                 require_pairing: true,
                 allow_pairing: false,
                 pairing_pin: Some("4321".into()),

crates/punktfunk-host/src/main.rs

@@ -158,6 +158,9 @@ fn real_main() -> Result<()> {
                 max_sessions: get("--max-sessions")
                     .and_then(|s| s.parse().ok())
                     .unwrap_or(0),
+                max_concurrent: get("--max-concurrent")
+                    .and_then(|s| s.parse().ok())
+                    .unwrap_or(m3::DEFAULT_MAX_CONCURRENT),
                 require_pairing: args.iter().any(|a| a == "--require-pairing"),
                 allow_pairing: args.iter().any(|a| a == "--allow-pairing"),
                 pairing_pin: None,
@@ -417,6 +420,8 @@ M3-HOST OPTIONS:
     --seconds <N>                per-session stream duration, virtual source (default: 30)
     --frames <N>                 per-session frame count, synthetic source (default: 300)
     --max-sessions <N>           exit after N sessions; 0 = serve forever (default: 0)
+    --max-concurrent <N>         stream at most N sessions at once (NVENC bound); overflow waits
+                                 in the accept queue; 0 = unlimited (default: 4)
     --allow-pairing              accept PIN pairing ceremonies (arm pairing mode)
     --require-pairing            only serve PIN-paired clients (implies --allow-pairing;
                                  the host logs a 4-digit PIN when a client starts pairing)

docs-site/content/docs/status.md

@@ -29,6 +29,14 @@ All three appliances advertise over mDNS (`_punktfunk._udp`) and require PIN pai
 ## Progress log
 
 ### 2026-06-12
+- **Concurrent sessions** — the host no longer serves one client at a time. The accept loop spawns
+  each session (`JoinSet`), bounded by `--max-concurrent` (default 4, a NVENC bound; overflow waits
+  in the accept queue). Each session keeps its own virtual output + encoder; they share the
+  host-lifetime input/audio/mic services — i.e. **multiple devices viewing/controlling the same
+  desktop** on kwin/mutter/wlroots. Validated live on the GNOME box: two clients connected at once
+  → **two independent Mutter virtual outputs (1280×720 + 1920×1080) streaming simultaneously**
+  (39 MB + 48 MB). gamescope's *independent-desktops* (multi-user) isolation — per-session
+  input/audio — is a follow-up.
 - **Apple client latency HUD** — `PunktfunkConnection.clockOffsetNs` (from the C-ABI getter) +
   `LatencyMeter` surface a skew-corrected **capture→client-receipt** p50/p95 in the macOS HUD: the
   first cross-machine latency the real Apple client reports. (Stage-1 `AVSampleBufferDisplayLayer`
@@ -59,5 +67,7 @@ All three appliances advertise over mDNS (`_punktfunk._udp`) and require PIN pai
 See the [Roadmap](/docs/roadmap) for the ordered list. Near-term:
 - **True glass-to-glass**: Apple client present-stamp (decode→present) + host render→capture term.
 - **Apple stage-2 presenter** (`VTDecompressionSession` → `CAMetalLayer`).
-- **Mandatory PIN pairing + delegated pairing approval**; concurrent sessions.
+- **Mandatory PIN pairing + delegated pairing approval** (an already-paired device approves a new one).
+- **gamescope multi-user isolation** — per-session input/audio so concurrent sessions are independent
+  desktops (the shared-desktop multi-view case landed).
 - **bazzite** kept up to date (currently offline; one rebuild behind).