feat(gui): list connected viewers and let the host kick them

Track viewers by endpoint id instead of a bare count. The JSON event
stream gains viewer_joined / viewer_left (each carrying the id),
replacing viewer_count; active/max still ride along so the count
display is unchanged.

The host screen now renders one row per connected viewer with a Kick
button. Clicking it sends `kick <id>` to the headless child over a new
stdin command channel, which the host turns into a per-viewer
CancellationToken cancel; the existing teardown path then emits the
leave, so a kick and a self-disconnect look identical downstream.

The stdin channel only runs under --output json (the GUI shell-out) and
on a detached OS thread, so a read parked on stdin can't hold up the
host's Ctrl+C shutdown.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/common/output.rs

@@ -22,7 +22,11 @@ pub fn set_json(enabled: bool) {
     JSON_ENABLED.store(enabled, Ordering::Relaxed);
 }
 
-fn json_enabled() -> bool {
+/// Whether the JSON event stream is on — i.e. we're being driven by a
+/// machine front-end (the `--gui` shell-out) rather than a human terminal.
+/// Gates features that only make sense under that front-end, like the
+/// stdin command channel the host reads `kick` requests from.
+pub fn json_enabled() -> bool {
     JSON_ENABLED.load(Ordering::Relaxed)
 }
 
@@ -42,8 +46,12 @@ pub enum Event<'a> {
         max_viewers: u32,
         max_viewers_source: &'a str,
     },
-    /// Active viewer count changed.
-    ViewerCount { active: u32, max: u32 },
+    /// A viewer joined. `id` is the viewer's endpoint id; `active` is the new
+    /// total after the join.
+    ViewerJoined { id: &'a str, active: u32, max: u32 },
+    /// A viewer left — disconnected on their own or kicked by the host. `id`
+    /// is the viewer's endpoint id; `active` is the new total after.
+    ViewerLeft { id: &'a str, active: u32, max: u32 },
     /// Capture pipeline lifecycle (spawned on first viewer, torn down on last).
     Capture { state: CaptureState },
     /// A viewer was turned away (host full, or capture spawn failed).

src/gui/child.rs

@@ -6,8 +6,8 @@
 //! egui app drains each frame. stderr is captured into a small ring so a
 //! failed launch (e.g. a missing gst plugin) can be surfaced in the window.
 
-use std::io::{BufRead, BufReader};
-use std::process::{Child, Command, Stdio};
+use std::io::{BufRead, BufReader, Write};
+use std::process::{Child, ChildStdin, Command, Stdio};
 use std::sync::mpsc::Receiver;
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
@@ -35,7 +35,13 @@ pub enum ChildEvent {
         max_viewers: u32,
         max_viewers_source: String,
     },
-    ViewerCount {
+    ViewerJoined {
+        id: String,
+        active: u32,
+        max: u32,
+    },
+    ViewerLeft {
+        id: String,
         active: u32,
         max: u32,
     },
@@ -63,6 +69,9 @@ pub struct ChildProc {
     child: Child,
     pub rx: Receiver<ChildEvent>,
     stderr_tail: Arc<Mutex<Vec<String>>>,
+    /// Write end of the child's stdin, for the line-based command channel
+    /// (see [`ChildProc::send_command`]). `None` once it's been closed.
+    stdin: Option<ChildStdin>,
 }
 
 impl ChildProc {
@@ -72,11 +81,14 @@ impl ChildProc {
         let exe = std::env::current_exe()?;
         let mut child = Command::new(exe)
             .args(args)
-            .stdin(Stdio::null())
+            // Piped so we can send line commands (e.g. `kick <id>`); the host
+            // only reads it when driven this way (`--output json`).
+            .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
             .spawn()?;
 
+        let stdin = child.stdin.take();
         let (tx, rx) = std::sync::mpsc::channel();
         let stdout = child.stdout.take().expect("stdout piped");
         std::thread::spawn(move || {
@@ -114,9 +126,23 @@ impl ChildProc {
             child,
             rx,
             stderr_tail,
+            stdin,
         })
     }
 
+    /// Send one newline-terminated command to the child over its stdin (the
+    /// host parses these as `kick <endpoint-id>`). Best-effort: a closed pipe
+    /// (child already gone) just drops the command.
+    pub fn send_command(&mut self, cmd: &str) {
+        let Some(stdin) = self.stdin.as_mut() else {
+            return;
+        };
+        if let Err(e) = writeln!(stdin, "{cmd}") {
+            tracing::warn!("failed to send command to host child: {e}");
+            self.stdin = None; // pipe is dead; stop trying
+        }
+    }
+
     /// Whether the child is still running.
     pub fn is_alive(&mut self) -> bool {
         matches!(self.child.try_wait(), Ok(None))
@@ -205,10 +231,14 @@ mod tests {
     }
 
     #[test]
-    fn viewer_count_round_trips() {
+    fn viewer_join_leave_round_trip() {
         assert!(matches!(
-            parse(Event::ViewerCount { active: 2, max: 4 }),
-            ChildEvent::ViewerCount { active: 2, max: 4 }
+            parse(Event::ViewerJoined { id: "nodeXYZ", active: 2, max: 4 }),
+            ChildEvent::ViewerJoined { id, active: 2, max: 4 } if id == "nodeXYZ"
+        ));
+        assert!(matches!(
+            parse(Event::ViewerLeft { id: "nodeXYZ", active: 1, max: 4 }),
+            ChildEvent::ViewerLeft { id, active: 1, max: 4 } if id == "nodeXYZ"
         ));
     }
 

src/gui/mod.rs

@@ -168,6 +168,9 @@ struct HostState {
     copied: bool,
     last_refusal: Option<String>,
     error: Option<String>,
+    /// Endpoint ids of the currently-connected viewers, in arrival order.
+    /// Drives the per-viewer list and its Kick buttons.
+    viewers: Vec<String>,
 }
 
 impl Default for HostState {
@@ -186,6 +189,7 @@ impl Default for HostState {
             copied: false,
             last_refusal: None,
             error: None,
+            viewers: Vec::new(),
         }
     }
 }
@@ -362,6 +366,23 @@ impl PixelPassApp {
         ui.add_space(6.0);
         ui.label(format!("Viewers: {} / {}", self.host.active, self.host.max));
 
+        // Per-viewer list with a Kick button each. Collect the click first so
+        // we're not borrowing self.host.viewers while we reach for the child.
+        let mut kick: Option<String> = None;
+        for id in &self.host.viewers {
+            ui.horizontal(|ui| {
+                ui.label(format!("• endpoint {}", short_id(id)));
+                if ui.small_button("Kick").clicked() {
+                    kick = Some(id.clone());
+                }
+            });
+        }
+        if let Some(id) = kick
+            && let Some(p) = &mut self.host.proc
+        {
+            p.send_command(&format!("kick {id}"));
+        }
+
         if let Some(info) = &self.host.info {
             ui.label(
                 egui::RichText::new(format!("{} · {}", info.display, info.capture))
@@ -452,6 +473,7 @@ impl PixelPassApp {
         self.host.max = 0;
         self.host.capturing = false;
         self.host.copied = false;
+        self.host.viewers.clear();
 
         let mut args = vec![
             "--host".to_string(),
@@ -483,6 +505,7 @@ impl PixelPassApp {
         self.host.capturing = false;
         self.host.ticket = None;
         self.host.copied = false;
+        self.host.viewers.clear();
     }
 
     /// Drain the host child's event channel into state, and detect an
@@ -543,9 +566,17 @@ impl PixelPassApp {
                     cap_source: max_viewers_source,
                 });
             }
-            ChildEvent::ViewerCount { active, max } => {
+            ChildEvent::ViewerJoined { id, active, max } => {
                 self.host.active = active;
                 self.host.max = max;
+                if !self.host.viewers.contains(&id) {
+                    self.host.viewers.push(id);
+                }
+            }
+            ChildEvent::ViewerLeft { id, active, max } => {
+                self.host.active = active;
+                self.host.max = max;
+                self.host.viewers.retain(|v| v != &id);
             }
             ChildEvent::Capture { state } => {
                 self.host.capturing = matches!(state, child::CaptureState::Started);

src/host/mod.rs

@@ -10,6 +10,7 @@ use anyhow::{Result, bail};
 use iroh::endpoint::{Connection, presets};
 use iroh::{Endpoint, EndpointAddr};
 use iroh_tickets::endpoint::EndpointTicket;
+use std::collections::HashMap;
 use std::time::Duration;
 use tokio::sync::{mpsc, oneshot};
 use tokio_util::sync::CancellationToken;
@@ -23,15 +24,28 @@ use crate::common::{
 use self::pipeline::CaptureHandle;
 use self::quality::EffectiveQuality;
 
-/// Messages from per-viewer tasks to the capture supervisor.
+/// Messages from per-viewer tasks (and the GUI command channel) to the
+/// capture supervisor.
+// The shared `Viewer` suffix is the point — these are all viewer lifecycle
+// messages — so keep the descriptive names.
+#[allow(clippy::enum_variant_names)]
 enum SupervisorMsg {
-    /// A new viewer wants in. Supervisor replies with the local capture
-    /// HTTP port to connect to, or an error string if the host is full or
-    /// capture spawn failed.
-    AddViewer(oneshot::Sender<Result<u16, String>>),
+    /// A new viewer wants in. Supervisor replies with the local capture HTTP
+    /// port to connect to, or an error string if the host is full or capture
+    /// spawn failed. `cancel` is the viewer's own token — the supervisor keeps
+    /// it so a later `KickViewer` can tear this viewer's stream down.
+    AddViewer {
+        id: String,
+        cancel: CancellationToken,
+        reply: oneshot::Sender<Result<u16, String>>,
+    },
     /// A viewer's session ended. Supervisor decrements the count and tears
     /// down capture if it just hit zero.
-    RemoveViewer,
+    RemoveViewer { id: String },
+    /// Host asked (via the GUI command channel) to disconnect a viewer by
+    /// endpoint id. Cancels that viewer's token; the normal teardown path then
+    /// emits the `ViewerLeft`.
+    KickViewer { id: String },
 }
 
 pub async fn run(opts: HostOpts) -> Result<()> {
@@ -113,6 +127,15 @@ pub async fn run(opts: HostOpts) -> Result<()> {
         sup_rx,
     ));
 
+    // Command channel for the GUI front-end: read `kick <endpoint-id>` lines
+    // off stdin. Only when machine-driven (`--output json`) — a human host has
+    // nothing to type here, and we don't want to swallow terminal input. Runs
+    // on a plain OS thread (not a tokio task) so a read parked on stdin can't
+    // hold up runtime shutdown on Ctrl+C; the thread dies with the process.
+    if output::json_enabled() {
+        spawn_kick_listener(sup_tx.clone());
+    }
+
     accept_loop(&endpoint, sup_tx.clone(), cancel.clone()).await;
 
     drop(sup_tx);
@@ -159,9 +182,18 @@ async fn handle_peer(
     cancel: CancellationToken,
 ) {
     let remote = conn.remote_id();
+    let id = remote.to_string();
+    // This viewer's own kill switch: the supervisor holds a clone so a `kick`
+    // can cancel it, and the stream select! below watches it.
+    let peer_cancel = CancellationToken::new();
 
     let (reply_tx, reply_rx) = oneshot::channel();
-    if sup_tx.send(SupervisorMsg::AddViewer(reply_tx)).await.is_err() {
+    let add = SupervisorMsg::AddViewer {
+        id: id.clone(),
+        cancel: peer_cancel.clone(),
+        reply: reply_tx,
+    };
+    if sup_tx.send(add).await.is_err() {
         tracing::warn!(%remote, "supervisor channel closed; dropping peer");
         return;
     }
@@ -182,7 +214,7 @@ async fn handle_peer(
         Ok(s) => s,
         Err(e) => {
             tracing::warn!(%remote, "accept_bi failed: {e:#}");
-            let _ = sup_tx.send(SupervisorMsg::RemoveViewer).await;
+            let _ = sup_tx.send(SupervisorMsg::RemoveViewer { id }).await;
             return;
         }
     };
@@ -193,7 +225,7 @@ async fn handle_peer(
         Ok(t) => t,
         Err(e) => {
             tracing::warn!(%remote, "connect_to_capture failed: {e:#}");
-            let _ = sup_tx.send(SupervisorMsg::RemoveViewer).await;
+            let _ = sup_tx.send(SupervisorMsg::RemoveViewer { id }).await;
             return;
         }
     };
@@ -207,10 +239,34 @@ async fn handle_peer(
         _ = cancel.cancelled() => {
             tracing::info!(%remote, "cancellation during stream");
         }
+        _ = peer_cancel.cancelled() => {
+            tracing::info!(%remote, "kicked by host");
+        }
     }
 
     eprintln!("[pixelpass] viewer disconnected: {remote}");
-    let _ = sup_tx.send(SupervisorMsg::RemoveViewer).await;
+    let _ = sup_tx.send(SupervisorMsg::RemoveViewer { id }).await;
+}
+
+/// Read `kick <endpoint-id>` lines off stdin and forward them to the
+/// supervisor. Runs on a detached OS thread (see the call site for why). Ends
+/// when stdin hits EOF (the GUI closed the pipe) or the supervisor is gone.
+fn spawn_kick_listener(sup_tx: mpsc::Sender<SupervisorMsg>) {
+    use std::io::BufRead;
+    std::thread::spawn(move || {
+        let stdin = std::io::stdin();
+        for line in stdin.lock().lines().map_while(Result::ok) {
+            let Some(id) = line.trim().strip_prefix("kick ") else {
+                continue;
+            };
+            let msg = SupervisorMsg::KickViewer { id: id.trim().to_string() };
+            // blocking_send is valid here: this is a plain thread, not inside
+            // the tokio runtime. An Err means the supervisor closed — stop.
+            if sup_tx.blocking_send(msg).is_err() {
+                break;
+            }
+        }
+    });
 }
 
 /// Owns the single shared CaptureHandle and the active viewer count. Spawns
@@ -225,11 +281,15 @@ async fn supervise(
     mut rx: mpsc::Receiver<SupervisorMsg>,
 ) {
     let mut handle: Option<CaptureHandle> = None;
-    let mut count: u32 = 0;
+    // Active viewers, keyed by endpoint id, holding each one's kill switch.
+    // The count is just `viewers.len()`. (A given endpoint connecting twice is
+    // a non-case here: each viewer process uses a fresh ephemeral identity.)
+    let mut viewers: HashMap<String, CancellationToken> = HashMap::new();
 
     while let Some(msg) = rx.recv().await {
         match msg {
-            SupervisorMsg::AddViewer(reply) => {
+            SupervisorMsg::AddViewer { id, cancel, reply } => {
+                let count = viewers.len() as u32;
                 if count >= max_viewers {
                     let reason =
                         format!("host is full ({count} of {max_viewers} viewers connected)");
@@ -255,16 +315,22 @@ async fn supervise(
                 }
 
                 let port = handle.as_ref().expect("handle was just set").local_port();
-                count += 1;
+                viewers.insert(id.clone(), cancel);
+                let active = viewers.len() as u32;
                 let _ = reply.send(Ok(port));
-                output::emit(output::Event::ViewerCount { active: count, max: max_viewers });
-                tracing::info!(active = count, cap = max_viewers, "viewer joined");
+                output::emit(output::Event::ViewerJoined { id: &id, active, max: max_viewers });
+                tracing::info!(active, cap = max_viewers, "viewer joined");
             }
-            SupervisorMsg::RemoveViewer => {
-                count = count.saturating_sub(1);
-                output::emit(output::Event::ViewerCount { active: count, max: max_viewers });
-                tracing::info!(active = count, cap = max_viewers, "viewer left");
-                if count == 0
+            SupervisorMsg::RemoveViewer { id } => {
+                // A given viewer task only ever sends RemoveViewer once, but the
+                // map remove is the source of truth either way.
+                if viewers.remove(&id).is_none() {
+                    continue;
+                }
+                let active = viewers.len() as u32;
+                output::emit(output::Event::ViewerLeft { id: &id, active, max: max_viewers });
+                tracing::info!(active, cap = max_viewers, "viewer left");
+                if active == 0
                     && let Some(h) = handle.take()
                 {
                     tracing::info!("last viewer left — tearing down capture");
@@ -274,6 +340,17 @@ async fn supervise(
                     });
                 }
             }
+            SupervisorMsg::KickViewer { id } => {
+                match viewers.get(&id) {
+                    // Cancel the viewer's token; its handle_peer select! wakes,
+                    // sends RemoveViewer, and the leave is emitted there.
+                    Some(cancel) => {
+                        tracing::info!(%id, "kicking viewer");
+                        cancel.cancel();
+                    }
+                    None => tracing::debug!(%id, "kick for unknown/already-gone viewer"),
+                }
+            }
         }
     }