feat(launch): punktfunk/1 launch integration — client picks a title, host runs it

Plan step 4 (plumbing + host behavior). A client can ask the host to launch a
library title on connect; the host resolves it against ITS OWN library and runs it
in the session — the client sends only the store-qualified id, never a command, so a
remote peer can't inject one.

- Protocol (quic.rs): `Hello.launch: Option<String>` (the GameEntry id). Appended
  after `name`; when launch is present but name absent, a zero-length name placeholder
  keeps the offset deterministic — so a Hello with neither field stays byte-identical
  to the bitrate-era 26-byte form (test-asserted). Old peers ignore it; new hosts
  decode None from old clients. Round-trip + back-compat + truncation tests.
- Host: `library::launch_command(id)` resolves id → command via the host's own library —
  `steam_appid` → `steam steam://rungameid/<appid>` (appid validated as digits, the only
  client-influenced part), `command` → the host-stored command verbatim (trusted, never
  from the client). m3.rs sets PUNKTFUNK_GAMESCOPE_APP from it before bringup, exactly
  as the GameStream /launch path does (one session at a time). Unit-tested incl. an
  injection-attempt guard. Takes effect on the bare-spawn gamescope path; a no-op on a
  shared desktop / attach-to-existing session.
- C ABI: `punktfunk_connect_ex4` adds `launch_id` (NULL = none); `_ex3` now delegates to
  it. Threaded through NativeClient::connect → WorkerArgs → Hello.
- client-rs gains `--launch ID` (headless testing); client-linux passes None (no picker
  yet). Header regenerated.

Next: the Apple library grid passes the picked id via punktfunk_connect_ex4.

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

crates/punktfunk-client-linux/src/app.rs

@@ -286,6 +286,7 @@ fn speed_test(app: Rc<App>, req: ConnectRequest) {
                 CompositorPref::Auto,
                 GamepadPref::Auto,
                 0,
+                None, // launch: speed-test probe connect, no game
                 pin,
                 Some(identity),
                 std::time::Duration::from_secs(15),

crates/punktfunk-client-linux/src/session.rs

@@ -90,6 +90,7 @@ fn pump(
         params.compositor,
         params.gamepad,
         params.bitrate_kbps,
+        None, // launch: the Linux client has no library picker yet
         params.pin,
         Some(params.identity),
         Duration::from_secs(15),

crates/punktfunk-client-rs/src/main.rs

@@ -76,6 +76,9 @@ struct Args {
     gamepad: GamepadPref,
     /// `--bitrate KBPS` — request this encoder bitrate (kilobits/s); 0 = host default.
     bitrate_kbps: u32,
+    /// `--launch ID` — ask the host to launch a library title in this session (a store-qualified
+    /// id from the host's `GET /api/v1/library`, e.g. `steam:570`). Host resolves it; `None` = none.
+    launch: Option<String>,
     /// `--speed-test KBPS:MS` — after the stream starts, ask the host for a `MS`-millisecond
     /// bandwidth probe burst at `KBPS`, then report measured throughput + loss.
     speed_test: Option<(u32, u32)>,
@@ -194,6 +197,7 @@ fn parse_args() -> Args {
         compositor,
         gamepad,
         bitrate_kbps: get("--bitrate").and_then(|s| s.parse().ok()).unwrap_or(0),
+        launch: get("--launch").map(str::to_string),
         speed_test: get("--speed-test").and_then(|s| {
             let (kbps, ms) = s.split_once(':')?;
             Some((kbps.parse().ok()?, ms.parse().ok()?))
@@ -374,6 +378,8 @@ async fn session(args: Args) -> Result<()> {
             // `--name` (also the pairing label) — shown in the host's pending-approval list when
             // this client knocks on a pairing-required host.
             name: Some(args.name.clone()),
+            // `--launch ID` — host resolves it against its own library and runs it this session.
+            launch: args.launch.clone(),
         }
         .encode(),
     )

crates/punktfunk-core/src/abi.rs

@@ -796,6 +796,53 @@ pub unsafe extern "C" fn punktfunk_connect_ex3(
     client_cert_pem: *const std::os::raw::c_char,
     client_key_pem: *const std::os::raw::c_char,
     timeout_ms: u32,
+) -> *mut PunktfunkConnection {
+    // Delegate to the launch-aware variant with no game requested (the host's default session).
+    unsafe {
+        punktfunk_connect_ex4(
+            host,
+            port,
+            width,
+            height,
+            refresh_hz,
+            compositor,
+            gamepad,
+            bitrate_kbps,
+            std::ptr::null(),
+            pin_sha256,
+            observed_sha256_out,
+            client_cert_pem,
+            client_key_pem,
+            timeout_ms,
+        )
+    }
+}
+
+/// Like [`punktfunk_connect_ex3`], but additionally asks the host to launch a library title in
+/// this session. `launch_id` is a store-qualified [`crate::library`-style] id as returned by the
+/// host's `GET /api/v1/library` (`steam:<appid>` / `custom:<id>`); the host resolves it against
+/// its OWN library and runs the matching recipe — the client never sends a raw command. `NULL`
+/// (or an empty / unknown id) ⇒ the host's default session, no game launched.
+///
+/// # Safety
+/// Same as [`punktfunk_connect`]; `launch_id`, when non-NULL, must be a NUL-terminated C string.
+#[cfg(feature = "quic")]
+#[no_mangle]
+pub unsafe extern "C" fn punktfunk_connect_ex4(
+    host: *const std::os::raw::c_char,
+    port: u16,
+    width: u32,
+    height: u32,
+    refresh_hz: u32,
+    compositor: u32,
+    gamepad: u32,
+    bitrate_kbps: u32,
+    launch_id: *const std::os::raw::c_char,
+    pin_sha256: *const u8,
+    observed_sha256_out: *mut u8,
+    client_cert_pem: *const std::os::raw::c_char,
+    client_key_pem: *const std::os::raw::c_char,
+    timeout_ms: u32,
 ) -> *mut PunktfunkConnection {
     let r = std::panic::catch_unwind(AssertUnwindSafe(|| {
         if host.is_null() {
@@ -805,6 +852,11 @@ pub unsafe extern "C" fn punktfunk_connect_ex3(
             Ok(s) => s,
             Err(_) => return std::ptr::null_mut(),
         };
+        // A bad-UTF-8 launch id is non-fatal — treat it as "no game" rather than failing connect.
+        let launch = match unsafe { opt_cstr(launch_id) } {
+            Ok(Some(s)) if !s.is_empty() => Some(s.to_string()),
+            _ => None,
+        };
         let mode = crate::config::Mode {
             width,
             height,
@@ -839,6 +891,7 @@ pub unsafe extern "C" fn punktfunk_connect_ex3(
             pref,
             gamepad,
             bitrate_kbps,
+            launch,
             pin,
             identity,
             std::time::Duration::from_millis(timeout_ms as u64),

crates/punktfunk-core/src/client.rs

@@ -172,6 +172,7 @@ impl NativeClient {
         compositor: CompositorPref,
         gamepad: GamepadPref,
         bitrate_kbps: u32,
+        launch: Option<String>,
         pin: Option<[u8; 32]>,
         identity: Option<(String, String)>,
         timeout: Duration,
@@ -214,6 +215,7 @@ impl NativeClient {
                     compositor,
                     gamepad,
                     bitrate_kbps,
+                    launch,
                     pin,
                     identity,
                     frame_tx,
@@ -526,6 +528,7 @@ struct WorkerArgs {
     compositor: CompositorPref,
     gamepad: GamepadPref,
     bitrate_kbps: u32,
+    launch: Option<String>,
     pin: Option<[u8; 32]>,
     identity: Option<(String, String)>,
     frame_tx: SyncSender<Frame>,
@@ -552,6 +555,7 @@ async fn worker_main(args: WorkerArgs) {
         compositor,
         gamepad,
         bitrate_kbps,
+        launch,
         pin,
         identity,
         frame_tx,
@@ -608,6 +612,8 @@ async fn worker_main(args: WorkerArgs) {
                 // No device name yet: the connect ABI has no name parameter (pairing does). The
                 // host falls back to a fingerprint-derived label in its pending-approval list.
                 name: None,
+                // Library id to launch this session, if the embedder asked for one.
+                launch: launch.clone(),
             }
             .encode(),
         )

crates/punktfunk-core/src/quic.rs

@@ -62,12 +62,24 @@ pub struct Hello {
     /// Appended to the wire form as `len u8 || UTF-8` (≤ [`HELLO_NAME_MAX`] bytes) — omitted by
     /// older clients (decodes to `None`; the host falls back to a fingerprint-derived label).
     pub name: Option<String>,
+    /// Library entry the client wants this session to launch (the store-qualified `GameEntry.id`,
+    /// e.g. `steam:570` / `custom:abc123`). The host resolves it against ITS OWN library and runs
+    /// the matching launch recipe in the session — the client never sends a raw command, so a
+    /// remote peer can't inject one. `None` = no game requested (the host's default session).
+    /// Appended after `name` as `len u8 || UTF-8` (≤ [`HELLO_LAUNCH_MAX`] bytes); when present but
+    /// `name` is absent, a zero-length name placeholder precedes it so the offset stays
+    /// deterministic. Omitted by older clients (decodes to `None`).
+    pub launch: Option<String>,
 }
 
 /// Longest device name carried in a [`Hello`] (bytes of UTF-8; longer names are truncated on
 /// encode, rejected on decode — a one-byte length prefix caps it at 255 anyway).
 pub const HELLO_NAME_MAX: usize = 64;
 
+/// Longest library id carried in a [`Hello::launch`] (bytes of UTF-8). Ids are short
+/// (`steam:<appid>` / `custom:<12 hex>`); the cap just bounds an attacker-controlled field.
+pub const HELLO_LAUNCH_MAX: usize = 128;
+
 /// `host → client`: the complete session offer.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Welcome {
@@ -473,6 +485,19 @@ pub mod pake {
     }
 }
 
+/// Truncate `s` to at most `max` bytes on a UTF-8 char boundary (so a multi-byte char straddling
+/// the cap is dropped whole, never split). Shared by Hello's length-prefixed name/launch fields.
+fn truncate_to(s: &str, max: usize) -> &str {
+    if s.len() <= max {
+        return s;
+    }
+    let mut cut = max;
+    while !s.is_char_boundary(cut) {
+        cut -= 1;
+    }
+    &s[..cut]
+}
+
 impl Hello {
     pub fn encode(&self) -> Vec<u8> {
         let mut b = Vec::with_capacity(22);
@@ -484,21 +509,23 @@ impl Hello {
         b.push(self.compositor.to_u8()); // appended at offset 20 — older hosts read [0..20] and skip it
         b.push(self.gamepad.to_u8()); // appended at offset 21 — same back-compat discipline
         b.extend_from_slice(&self.bitrate_kbps.to_le_bytes()); // appended at offset 22..26
-        if let Some(name) = &self.name {
-            // Appended at offset 26: len u8 || UTF-8. This is the LAST trailing field — `None`
-            // emits nothing (so a no-name Hello is byte-identical to the bitrate-era form), which
-            // means a *future* field can't simply follow `name` at a fixed offset; it would need
-            // its own presence flag. Truncate to a char boundary within HELLO_NAME_MAX.
-            let mut n = name.as_str();
-            while n.len() > HELLO_NAME_MAX {
-                let mut cut = HELLO_NAME_MAX;
-                while !n.is_char_boundary(cut) {
-                    cut -= 1;
-                }
-                n = &n[..cut];
+                                                               // name at offset 26: len u8 || UTF-8. Omitted when `None` *and* there is no later field —
+                                                               // so a Hello with neither name nor launch stays byte-identical to the bitrate-era form
+                                                               // (26 bytes). When `launch` is present we must still emit name's length byte (0 for None)
+                                                               // so `launch` lands at a deterministic offset.
+        match (&self.name, &self.launch) {
+            (None, None) => {}
+            (name, _) => {
+                let n = truncate_to(name.as_deref().unwrap_or(""), HELLO_NAME_MAX);
+                b.push(n.len() as u8);
+                b.extend_from_slice(n.as_bytes());
             }
-            b.push(n.len() as u8);
-            b.extend_from_slice(n.as_bytes());
+        }
+        // launch after name: len u8 || UTF-8. Last trailing field.
+        if let Some(launch) = &self.launch {
+            let l = truncate_to(launch, HELLO_LAUNCH_MAX);
+            b.push(l.len() as u8);
+            b.extend_from_slice(l.as_bytes());
         }
         b
     }
@@ -540,6 +567,19 @@ impl Hello {
                     .and_then(|s| std::str::from_utf8(s).ok())
                     .map(String::from)
             }),
+            // Optional trailing launch id, positioned right after name's `len u8 || UTF-8` block.
+            // The raw name-length byte (even when oversized/zero) determines where launch starts,
+            // so a corrupt name never panics — it just pushes the launch offset out of range → None.
+            launch: b.get(26).and_then(|&name_len| {
+                let off = 27 + name_len as usize; // start of launch's length byte
+                let len = *b.get(off)? as usize;
+                if len == 0 || len > HELLO_LAUNCH_MAX {
+                    return None;
+                }
+                b.get(off + 1..off + 1 + len)
+                    .and_then(|s| std::str::from_utf8(s).ok())
+                    .map(String::from)
+            }),
         })
     }
 }
@@ -1495,6 +1535,7 @@ mod tests {
             gamepad: GamepadPref::DualSense,
             bitrate_kbps: 25_000,
             name: Some("Test Device".into()),
+            launch: Some("steam:570".into()),
         };
         assert_eq!(Hello::decode(&h.encode()).unwrap(), h);
         let s = Start {
@@ -1560,6 +1601,7 @@ mod tests {
             gamepad: GamepadPref::DualSense,
             bitrate_kbps: 80_000,
             name: None,
+            launch: None,
         };
         let enc = h.encode();
         assert_eq!(enc.len(), 26);
@@ -1629,6 +1671,7 @@ mod tests {
             gamepad: GamepadPref::Auto,
             bitrate_kbps: 0,
             name: Some("Enrico's MacBook".into()),
+            launch: None,
         };
         let enc = base.encode();
         assert_eq!(
@@ -1661,6 +1704,60 @@ mod tests {
         assert_eq!(Hello::decode(&bad_utf8).unwrap().name, None);
     }
 
+    #[test]
+    fn hello_launch_roundtrip_and_back_compat() {
+        let base = Hello {
+            abi_version: 2,
+            mode: Mode {
+                width: 1920,
+                height: 1080,
+                refresh_hz: 60,
+            },
+            compositor: CompositorPref::Auto,
+            gamepad: GamepadPref::Auto,
+            bitrate_kbps: 0,
+            name: None,
+            launch: None,
+        };
+        // launch alone (no name): a zero-length name placeholder keeps the offset deterministic.
+        let with_launch = Hello {
+            launch: Some("steam:570".into()),
+            ..base.clone()
+        };
+        assert_eq!(Hello::decode(&with_launch.encode()).unwrap(), with_launch);
+        // launch + name together.
+        let both = Hello {
+            name: Some("Enrico's Mac".into()),
+            launch: Some("custom:abc123".into()),
+            ..base.clone()
+        };
+        assert_eq!(Hello::decode(&both.encode()).unwrap(), both);
+        // name but no launch (a name-era client): launch decodes None.
+        let name_only = Hello {
+            name: Some("Enrico's Mac".into()),
+            ..base.clone()
+        };
+        assert_eq!(Hello::decode(&name_only.encode()).unwrap().launch, None);
+        // Neither field → still the 26-byte bitrate-era form (no launch placeholder emitted).
+        assert_eq!(base.encode().len(), 26);
+        assert_eq!(Hello::decode(&base.encode()).unwrap().launch, None);
+        // A bitrate-era (26-byte) peer reading a launch-bearing Hello ignores it.
+        assert_eq!(
+            Hello::decode(&with_launch.encode()[..26]).unwrap().launch,
+            None
+        );
+        // Over-long ids truncate on a char boundary within HELLO_LAUNCH_MAX.
+        let long = Hello {
+            launch: Some(format!("{}ü", "x".repeat(HELLO_LAUNCH_MAX - 1))),
+            ..base.clone()
+        };
+        let dec = Hello::decode(&long.encode())
+            .unwrap()
+            .launch
+            .expect("present");
+        assert!(dec.len() <= HELLO_LAUNCH_MAX && dec.starts_with('x'));
+    }
+
     #[test]
     fn reconfigure_roundtrip() {
         let rq = Reconfigure {
@@ -1784,6 +1881,7 @@ mod tests {
                 gamepad: GamepadPref::Auto,
                 bitrate_kbps: 0,
                 name: None,
+                launch: None,
             }
             .encode();
             assert!(PairRequest::decode(&h).is_err(), "abi {abi} parsed as pair");

crates/punktfunk-host/src/library.rs

@@ -353,6 +353,35 @@ pub fn delete_custom(id: &str) -> Result<bool> {
 // Unified library
 // ---------------------------------------------------------------------------------------
 
+/// Resolve a store-qualified library id (as sent by a client in `Hello::launch`) to the shell
+/// command the host should run for it — looked up in the host's OWN library so a client can only
+/// pick an existing title, never inject a command. `None` = unknown id, no launch recipe, or a
+/// malformed Steam appid.
+///
+/// - `steam_appid` → `steam steam://rungameid/<appid>` (appid validated as digits, so the only
+///   client-controlled part of the command is a number).
+/// - `command` → the stored command verbatim. This string comes from the host's own custom store
+///   (added by the host operator via the admin UI), never from the client, so it is trusted.
+pub fn launch_command(id: &str) -> Option<String> {
+    let spec = all_games().into_iter().find(|g| g.id == id)?.launch?;
+    command_for(&spec)
+}
+
+/// Map a resolved [`LaunchSpec`] to its shell command (pure — the unit-testable core of
+/// [`launch_command`], split out so the appid-validation can be tested without a Steam install).
+fn command_for(spec: &LaunchSpec) -> Option<String> {
+    match spec.kind.as_str() {
+        "steam_appid" => {
+            // Only digits — the appid is the sole client-influenced part of the command.
+            (!spec.value.is_empty() && spec.value.bytes().all(|b| b.is_ascii_digit()))
+                .then(|| format!("steam steam://rungameid/{}", spec.value))
+        }
+        // Trusted: the command comes from the host's own custom store, never the client.
+        "command" => (!spec.value.trim().is_empty()).then(|| spec.value.clone()),
+        _ => None,
+    }
+}
+
 /// The full library: every store's titles merged + the custom entries, sorted by title.
 pub fn all_games() -> Vec<GameEntry> {
     let mut games = SteamProvider.list();
@@ -420,6 +449,45 @@ mod tests {
         assert!(art.header.unwrap().ends_with("/570/header.jpg"));
     }
 
+    #[test]
+    fn launch_command_resolves_and_guards() {
+        let steam = LaunchSpec {
+            kind: "steam_appid".into(),
+            value: "570".into(),
+        };
+        assert_eq!(
+            command_for(&steam).as_deref(),
+            Some("steam steam://rungameid/570")
+        );
+        // A non-numeric "appid" (e.g. a client trying to inject) is rejected, never interpolated.
+        let evil = LaunchSpec {
+            kind: "steam_appid".into(),
+            value: "570; rm -rf ~".into(),
+        };
+        assert_eq!(command_for(&evil), None);
+        // Custom commands (from the host's own store) pass through verbatim.
+        let custom = LaunchSpec {
+            kind: "command".into(),
+            value: "dolphin-emu --batch".into(),
+        };
+        assert_eq!(command_for(&custom).as_deref(), Some("dolphin-emu --batch"));
+        // Empty / unknown kinds → no command.
+        assert_eq!(
+            command_for(&LaunchSpec {
+                kind: "command".into(),
+                value: "  ".into()
+            }),
+            None
+        );
+        assert_eq!(
+            command_for(&LaunchSpec {
+                kind: "wat".into(),
+                value: "x".into()
+            }),
+            None
+        );
+    }
+
     #[test]
     fn custom_entry_maps_to_game_entry() {
         let g: GameEntry = CustomEntry {

crates/punktfunk-host/src/m3.rs

@@ -520,6 +520,24 @@ async fn serve_session(
             M3Source::Synthetic => None,
         };
 
+        // Resolve a requested library launch (the client sends only the store-qualified id;
+        // we look it up in OUR library so a client can't inject a command). Set the gamescope
+        // backend's app env var, exactly as the GameStream /launch path does — safe per-session
+        // (one session at a time). Only the bare-spawn gamescope path reads it; on a shared
+        // desktop (kwin/mutter/wlroots) or an attach-to-existing session it's a harmless no-op.
+        if let Some(id) = hello.launch.as_deref() {
+            match crate::library::launch_command(id) {
+                Some(cmd) => {
+                    tracing::info!(launch_id = id, command = %cmd, "launching library title");
+                    std::env::set_var("PUNKTFUNK_GAMESCOPE_APP", &cmd);
+                }
+                None => tracing::warn!(
+                    launch_id = id,
+                    "client requested a launch id not in this host's library — ignoring"
+                ),
+            }
+        }
+
         // Resolve the client's gamepad-backend preference (pure env/cfg check — no probing
         // needed; the actual pads are created lazily by the input thread).
         let gamepad = resolve_gamepad(hello.gamepad);
@@ -2389,6 +2407,7 @@ mod tests {
                 CompositorPref::Auto,
                 GamepadPref::Auto,
                 0,
+                None, // launch
                 None,
                 Some((cert.clone(), key.clone())),
                 timeout
@@ -2419,6 +2438,7 @@ mod tests {
             CompositorPref::Auto,
             GamepadPref::Auto,
             0,
+            None, // launch
             None,
             Some((cert, key)),
             timeout,
@@ -2478,6 +2498,7 @@ mod tests {
                 CompositorPref::Auto,
                 GamepadPref::Auto,
                 0,
+                None, // launch
                 None,
                 None,
                 timeout
@@ -2503,6 +2524,7 @@ mod tests {
             CompositorPref::Auto,
             GamepadPref::Auto,
             0,
+            None, // launch
             Some(host_fp),
             Some((cert.clone(), key.clone())),
             timeout,