fix(portal): replace blocking send with try_send in PW process callback

PipeWire .process callback called frame_tx.send() on a bounded(3)
channel. If the encoder stalled, this blocked the PipeWire data loop,
delaying buffer recycling and potentially causing XRUNs.

Replace with try_send + AtomicU64 drop counter. Frames are silently
dropped when the channel is full (preferred for screen capture: latest
frame wins). A warning is logged every 30 dropped frames.

Fixes #2 from BUGS.md.

src/cap_portal.rs

@@ -1,4 +1,5 @@
 use std::os::fd::{AsRawFd, FromRawFd, OwnedFd};
+use std::sync::atomic::{AtomicU64, Ordering};
 use std::thread::{self, JoinHandle};
 
 use anyhow::Result;
@@ -33,6 +34,7 @@ pub struct CapPortal {
 
 struct PwThreadCtx {
     frame_tx: Sender<PwEvent>,
+    dropped: AtomicU64,
     shutdown_read: OwnedFd,
     pw_fd: OwnedFd,
     node_id: u32,
@@ -69,6 +71,7 @@ impl CapPortal {
 
         let ctx = PwThreadCtx {
             frame_tx,
+            dropped: AtomicU64::new(0),
             shutdown_read: unsafe { OwnedFd::from_raw_fd(efd) },
             pw_fd,
             node_id,
@@ -178,6 +181,7 @@ fn pipewire_thread(ctx: PwThreadCtx) {
 
     let PwThreadCtx {
         frame_tx,
+        dropped,
         shutdown_read,
         pw_fd,
         node_id,
@@ -269,6 +273,7 @@ fn pipewire_thread(ctx: PwThreadCtx) {
         .process({
             let format_info = format_info.clone();
             let frame_tx = frame_tx.clone();
+            let dropped = dropped;
             move |stream, _| {
                 let raw_buf = unsafe { stream.dequeue_raw_buffer() };
                 if raw_buf.is_null() {
@@ -341,7 +346,14 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                     pts,
                 };
 
-                let _ = frame_tx.send(PwEvent::Frame(frame));
+                if let Err(crossbeam_channel::TrySendError::Full(_)) =
+                    frame_tx.try_send(PwEvent::Frame(frame))
+                {
+                    let prev = dropped.fetch_add(1, Ordering::Relaxed);
+                    if prev > 0 && prev % 30 == 0 {
+                        tracing::warn!("dropped {prev} frames total: encoder backlog");
+                    }
+                }
                 unsafe { stream.queue_raw_buffer(raw_buf) };
             }
         })