host/serve: extract HTTP fanout from wayland.rs

The broadcast fanout, supervisor-facing listener bind, accept loop, and
per-viewer drain were all sitting inside host/wayland.rs even though
none of it is Wayland-specific. Move them to host/serve.rs so the X11
backend can share the same serving layer with a one-line constructor
call instead of copy-pasting (and drifting on) the fanout code.

No behavior change. Wayland's CaptureHandle now wraps a serve::Serve
instead of owning the listener/reader/server fields directly; gst
pipeline construction is unchanged. connect_to_capture moves alongside
Serve since it pairs with it.

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

src/host/serve.rs

@@ -0,0 +1,191 @@
+//! Display-server-agnostic serving layer: takes a capture child's stdout
+//! producing MPEG-TS bytes and fans them out to N concurrent HTTP viewers
+//! on a localhost port. One reader task pumps stdout chunks into a
+//! tokio::sync::broadcast channel; the accept loop spawns one drain task
+//! per accepted TCP connection. Slow consumers see Lagged and skip ahead;
+//! MPEG-TS resyncs at the next keyframe.
+//!
+//! Backends (host/wayland.rs, future host/x11.rs) build their own gst
+//! pipeline and hand the resulting ChildStdout to [`Serve::bind`].
+
+use anyhow::{Context, Result, bail};
+use std::sync::Arc;
+use std::time::Duration;
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+use tokio::net::{TcpListener, TcpStream};
+use tokio::process::ChildStdout;
+use tokio::sync::broadcast;
+use tokio::task::JoinHandle;
+use tokio::time::{Instant, sleep};
+
+/// Broadcast-channel capacity in chunks. Each chunk is up to 64 KiB from
+/// the capture child's stdout, so 16 chunks ≈ 1 MiB ≈ ~2 s of buffered
+/// jitter at typical bitrates. A viewer that falls behind by more than
+/// this gets Lagged and skips ahead — MPEG-TS recovers at the next
+/// keyframe.
+const FANOUT_CAPACITY: usize = 16;
+
+/// Size of each chunk read from the capture child's stdout.
+const READ_CHUNK: usize = 64 * 1024;
+
+/// Owns the localhost HTTP listener and the two long-running tasks that
+/// pump bytes from a capture child to all connected viewers.
+pub struct Serve {
+    port: u16,
+    reader: Option<JoinHandle<()>>,
+    server: Option<JoinHandle<()>>,
+}
+
+impl Serve {
+    /// Bind a localhost listener on a random port, set up the broadcast
+    /// fanout, and spawn the reader + accept-loop tasks. The provided
+    /// `stdout` is assumed to produce MPEG-TS bytes.
+    pub async fn bind(stdout: ChildStdout) -> Result<Self> {
+        let listener = TcpListener::bind("127.0.0.1:0")
+            .await
+            .context("could not bind local capture HTTP listener")?;
+        let port = listener.local_addr()?.port();
+
+        let (tx, _) = broadcast::channel::<Arc<Vec<u8>>>(FANOUT_CAPACITY);
+        let reader = tokio::spawn(pump_to_broadcast(stdout, tx.clone()));
+        let server = tokio::spawn(run_accept_loop(listener, tx));
+
+        Ok(Self {
+            port,
+            reader: Some(reader),
+            server: Some(server),
+        })
+    }
+
+    pub fn local_port(&self) -> u16 {
+        self.port
+    }
+
+    /// Abort the reader and accept-loop tasks. Backends typically call this
+    /// after killing their capture child so the reader sees stdout EOF and
+    /// exits on its own; the abort is a backstop.
+    pub async fn shutdown(mut self) {
+        if let Some(task) = self.reader.take() {
+            task.abort();
+        }
+        if let Some(task) = self.server.take() {
+            task.abort();
+        }
+    }
+}
+
+impl Drop for Serve {
+    fn drop(&mut self) {
+        if let Some(task) = self.reader.as_ref() {
+            task.abort();
+        }
+        if let Some(task) = self.server.as_ref() {
+            task.abort();
+        }
+    }
+}
+
+/// Connect to the local capture HTTP listener, retrying until it's up or
+/// we time out. Returns the connected socket — the bridge layer pipes
+/// QUIC↔this socket once it's open.
+pub async fn connect_to_capture(port: u16, max_wait: Duration) -> Result<TcpStream> {
+    let deadline = Instant::now() + max_wait;
+    loop {
+        match TcpStream::connect(("127.0.0.1", port)).await {
+            Ok(stream) => return Ok(stream),
+            Err(_) if Instant::now() < deadline => {
+                sleep(Duration::from_millis(50)).await;
+            }
+            Err(e) => bail!("capture HTTP listener never came up on 127.0.0.1:{port}: {e}"),
+        }
+    }
+}
+
+/// Read the capture child's stdout in chunks and broadcast each to all
+/// current subscribers. `broadcast::send` returns Err when there are no
+/// receivers; we ignore it so the capture child isn't backpressured
+/// waiting for a viewer.
+async fn pump_to_broadcast(mut stdout: ChildStdout, tx: broadcast::Sender<Arc<Vec<u8>>>) {
+    let mut buf = vec![0u8; READ_CHUNK];
+    loop {
+        match stdout.read(&mut buf).await {
+            Ok(0) => {
+                tracing::info!("capture stdout EOF — fanout reader exiting");
+                return;
+            }
+            Ok(n) => {
+                let chunk = Arc::new(buf[..n].to_vec());
+                let _ = tx.send(chunk);
+            }
+            Err(e) => {
+                tracing::warn!("capture stdout read error: {e}");
+                return;
+            }
+        }
+    }
+}
+
+async fn run_accept_loop(listener: TcpListener, tx: broadcast::Sender<Arc<Vec<u8>>>) {
+    loop {
+        let sock = match listener.accept().await {
+            Ok((s, _)) => s,
+            Err(e) => {
+                tracing::warn!("capture HTTP accept failed: {e}");
+                return;
+            }
+        };
+        let rx = tx.subscribe();
+        tokio::spawn(serve_one_viewer(sock, rx));
+    }
+}
+
+async fn serve_one_viewer(mut sock: TcpStream, mut rx: broadcast::Receiver<Arc<Vec<u8>>>) {
+    if !drain_http_request(&mut sock).await {
+        return;
+    }
+
+    const RESPONSE: &[u8] = b"HTTP/1.1 200 OK\r\n\
+        Content-Type: video/mp2t\r\n\
+        Cache-Control: no-cache, no-store\r\n\
+        Connection: close\r\n\
+        \r\n";
+    if sock.write_all(RESPONSE).await.is_err() {
+        return;
+    }
+
+    loop {
+        match rx.recv().await {
+            Ok(chunk) => {
+                if sock.write_all(&chunk).await.is_err() {
+                    return;
+                }
+            }
+            Err(broadcast::error::RecvError::Lagged(skipped)) => {
+                tracing::warn!(
+                    skipped,
+                    "viewer fanout lagged — MPEG-TS will resync at next keyframe"
+                );
+                continue;
+            }
+            Err(broadcast::error::RecvError::Closed) => return,
+        }
+    }
+}
+
+async fn drain_http_request(sock: &mut TcpStream) -> bool {
+    let mut buf = [0u8; 1024];
+    let mut total = Vec::with_capacity(512);
+    loop {
+        match sock.read(&mut buf).await {
+            Ok(0) => return false,
+            Ok(n) => total.extend_from_slice(&buf[..n]),
+            Err(_) => return false,
+        }
+        if total.windows(4).any(|w| w == b"\r\n\r\n") {
+            return true;
+        }
+        if total.len() > 16 * 1024 {
+            return false;
+        }
+    }
+}