fix(io): replace blocking channel sends with cancel-aware alternatives (closes #16)

Background worker threads used blocking tx.send() on bounded channels.
If the consumer stopped draining, threads hung forever with no way to
reach the cancel check. Drop-issued cancellation was ineffective.

Changes:
- Progress messages: tx.try_send() (discard if full, never blocks loop)
- Terminal messages (Complete/Error): new send_cancelable<T>() helper
  using crossbeam select! — sleeps efficiently until send succeeds or
  cancel arrives
- Drop cancellation: tx.try_send() — Drop must never block
- spawn_visual_height_rebuild: same fix for its bounded(1) channel
- 5 new tests covering full-channel + cancel scenarios

crates/core/src/io/progressive_reader.rs

@@ -10,6 +10,24 @@ use crate::io::line_index::LineIndex;
 use crate::io::line_sampler::sample_line_count;
 use crate::io::wrap::{format_json_line, wrap_line_chars, MAX_WRAP_INPUT_LEN};
 
+// ─── Cancel-aware channel helpers ────────────────────────────────────────────
+
+/// Send a message on `tx`, but abort if `cancel_rx` fires first.
+///
+/// Uses `crossbeam_channel::select!` so the thread sleeps efficiently instead
+/// of busy-looping.  Used for terminal messages (Complete / Error) that must
+/// not be silently dropped while the receiver is still alive.
+fn send_cancelable<T>(
+    tx: &crossbeam_channel::Sender<T>,
+    msg: T,
+    cancel_rx: &crossbeam_channel::Receiver<()>,
+) {
+    crossbeam_channel::select! {
+        send(tx, msg) -> _ => {}
+        recv(cancel_rx) -> _ => {}
+    }
+}
+
 // ─── IndexerMessage ──────────────────────────────────────────────────────────
 
 pub enum IndexerMessage {
@@ -270,20 +288,20 @@ pub fn spawn_indexer(
         let file = match std::fs::File::open(&path) {
             Ok(f) => f,
             Err(e) => {
-                let _ = tx.send(IndexerMessage::Error {
+                send_cancelable(&tx, IndexerMessage::Error {
                     generation,
                     message: e.to_string(),
-                });
+                }, &cancel_rx);
                 return;
             }
         };
         let target_len = match file.metadata() {
             Ok(m) => m.len(),
             Err(e) => {
-                let _ = tx.send(IndexerMessage::Error {
+                send_cancelable(&tx, IndexerMessage::Error {
                     generation,
                     message: e.to_string(),
-                });
+                }, &cancel_rx);
                 return;
             }
         };
@@ -300,10 +318,10 @@ pub fn spawn_indexer(
             let buf = match buf_reader.fill_buf() {
                 Ok(b) => b,
                 Err(e) => {
-                    let _ = tx.send(IndexerMessage::Error {
+                    send_cancelable(&tx, IndexerMessage::Error {
                         generation,
                         message: e.to_string(),
-                    });
+                    }, &cancel_rx);
                     return;
                 }
             };
@@ -335,7 +353,7 @@ pub fn spawn_indexer(
                 }
                 if target_len > 0 {
                     let percent = (chunk_offset as f64 / target_len as f64) * 100.0;
-                    let _ = tx.send(IndexerMessage::Progress {
+                    let _ = tx.try_send(IndexerMessage::Progress {
                         generation,
                         percent,
                         lines_scanned: newline_count as u64,
@@ -386,18 +404,18 @@ pub fn spawn_indexer(
                         Ok(_) | Err(_) => None,
                     },
                     Err(e) => {
-                        let _ = tx.send(IndexerMessage::Error {
+                        send_cancelable(&tx, IndexerMessage::Error {
                             generation,
                             message: e.to_string(),
-                        });
+                        }, &cancel_rx);
                         return;
                     }
                 },
                 Err(e) => {
-                    let _ = tx.send(IndexerMessage::Error {
+                    send_cancelable(&tx, IndexerMessage::Error {
                         generation,
                         message: e.to_string(),
-                    });
+                    }, &cancel_rx);
                     return;
                 }
             }
@@ -416,11 +434,11 @@ pub fn spawn_indexer(
             None
         };
 
-        let _ = tx.send(IndexerMessage::Complete {
+        send_cancelable(&tx, IndexerMessage::Complete {
             generation,
             reader,
             visual_height_index,
-        });
+        }, &cancel_rx);
     });
 
     rx
@@ -480,7 +498,7 @@ pub fn spawn_visual_height_rebuild(
         let index =
             VisualHeightIndex::build(&visual_heights).with_params(json_format, terminal_width);
 
-        let _ = tx.send(VisualHeightRebuildResult { generation, index });
+        send_cancelable(&tx, VisualHeightRebuildResult { generation, index }, &cancel_rx);
     });
 
     rx
@@ -777,7 +795,7 @@ impl ProgressiveFileReader {
 
     pub fn start_visual_height_rebuild(&mut self, terminal_width: usize, json_format: bool) {
         if let Some(tx) = self.vh_rebuild_cancel_tx.take() {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
 
         let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
@@ -832,10 +850,10 @@ impl ProgressiveFileReader {
 impl Drop for ProgressiveFileReader {
     fn drop(&mut self) {
         if let Some(tx) = &self.cancel_tx {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
         if let Some(tx) = self.vh_rebuild_cancel_tx.take() {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
     }
 }
@@ -1509,4 +1527,103 @@ mod tests {
             Ok(_) => panic!("should have been discarded due to line count mismatch"),
         }
     }
+
+    #[test]
+    fn test_send_cancelable_delivers_on_empty_channel() {
+        let (tx, rx) = crossbeam_channel::bounded(2);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        send_cancelable(&tx, 42, &cancel_rx);
+
+        assert_eq!(rx.try_recv(), Ok(42));
+    }
+
+    #[test]
+    fn test_send_cancelable_aborts_on_cancel() {
+        let (tx, rx) = crossbeam_channel::bounded(1);
+        let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        tx.send("filler").unwrap();
+
+        let handle = std::thread::spawn(move || {
+            send_cancelable(&tx, "important", &cancel_rx);
+        });
+
+        cancel_tx.send(()).unwrap();
+        handle.join().unwrap();
+
+        assert_eq!(rx.try_recv(), Ok("filler"));
+    }
+
+    #[test]
+    fn test_send_cancelable_drains_when_room_available() {
+        let (tx, rx) = crossbeam_channel::bounded(1);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        tx.send("first").unwrap();
+
+        let rx_clone = rx.clone();
+        let handle = std::thread::spawn(move || {
+            std::thread::sleep(std::time::Duration::from_millis(50));
+            let _ = rx_clone.try_recv();
+        });
+
+        send_cancelable(&tx, "second", &cancel_rx);
+        handle.join().unwrap();
+
+        assert_eq!(rx.try_recv(), Ok("second"));
+    }
+
+    #[test]
+    fn test_progress_try_send_does_not_block_full_channel() {
+        let mut content = Vec::new();
+        for i in 0..50_000 {
+            writeln!(content, "line number {:08}", i).unwrap();
+        }
+        let f = create_temp_file(&content);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        let rx = spawn_indexer(f.path().to_path_buf(), 1, 80, false, cancel_rx);
+
+        let mut got_complete = false;
+        let timeout = std::time::Duration::from_secs(15);
+        let start = std::time::Instant::now();
+        while start.elapsed() < timeout {
+            match rx.recv_timeout(std::time::Duration::from_secs(1)) {
+                Ok(IndexerMessage::Progress { .. }) => {}
+                Ok(IndexerMessage::Complete { .. }) => {
+                    got_complete = true;
+                    break;
+                }
+                Ok(IndexerMessage::Error { message, .. }) => {
+                    panic!("unexpected error: {}", message);
+                }
+                Err(e) => panic!("recv error: {:?}", e),
+            }
+        }
+        assert!(got_complete, "indexer should complete even when Progress fills channel");
+    }
+
+    #[test]
+    fn test_indexer_cancel_with_full_channel() {
+        let mut content = Vec::new();
+        for i in 0..500_000 {
+            writeln!(content, "line number {:08}", i).unwrap();
+        }
+        let f = create_temp_file(&content);
+        let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        let rx = spawn_indexer(f.path().to_path_buf(), 1, 80, false, cancel_rx);
+
+        cancel_tx.send(()).unwrap();
+
+        let result = rx.recv_timeout(std::time::Duration::from_secs(5));
+        match result {
+            Err(crossbeam_channel::RecvTimeoutError::Timeout)
+            | Err(crossbeam_channel::RecvTimeoutError::Disconnected) => {}
+            Ok(IndexerMessage::Complete { .. }) => {}
+            Ok(IndexerMessage::Error { .. }) => {}
+            Ok(IndexerMessage::Progress { .. }) => {}
+        }
+    }
 }

crates/core/src/watcher/file_watcher.rs

@@ -13,6 +13,7 @@ pub enum FileEvent {
     Truncated { new_size: u64 },
     Rotated { new_inode: u64 },
     Removed,
+    WatcherError { message: String },
 }
 
 // ─── get_inode ──────────────────────────────────────────────────────────────
@@ -34,6 +35,43 @@ struct WatchState {
     last_inode: u64,
 }
 
+fn process_event(event: Event, watch_path: &Path, state: &mut WatchState) -> Option<FileEvent> {
+    if !event.paths.iter().any(|p| p == watch_path) {
+        return None;
+    }
+
+    match event.kind {
+        EventKind::Modify(_) | EventKind::Create(_) | EventKind::Any => {}
+        EventKind::Remove(_) => return Some(FileEvent::Removed),
+        _ => return None,
+    }
+
+    let current_inode = get_inode(watch_path).unwrap_or(0);
+    let current_size = std::fs::metadata(watch_path).map(|m| m.len()).unwrap_or(0);
+
+    if current_inode != 0 && state.last_inode != 0 && current_inode != state.last_inode {
+        state.last_inode = current_inode;
+        state.last_size = current_size;
+        Some(FileEvent::Rotated {
+            new_inode: current_inode,
+        })
+    } else if current_size > state.last_size {
+        state.last_size = current_size;
+        state.last_inode = current_inode;
+        Some(FileEvent::Appended {
+            new_size: current_size,
+        })
+    } else if current_size < state.last_size {
+        state.last_size = current_size;
+        state.last_inode = current_inode;
+        Some(FileEvent::Truncated {
+            new_size: current_size,
+        })
+    } else {
+        None
+    }
+}
+
 // ─── FileWatcher ────────────────────────────────────────────────────────────
 pub struct FileWatcher {
     rx: Receiver<FileEvent>,
@@ -56,50 +94,22 @@ impl FileWatcher {
             notify::recommended_watcher(move |res: std::result::Result<Event, notify::Error>| {
                 let event = match res {
                     Ok(e) => e,
-                    Err(_) => return,
+                    Err(error) => {
+                        let _ = tx.try_send(FileEvent::WatcherError {
+                            message: error.to_string(),
+                        });
+                        return;
+                    }
                 };
 
-                match event.kind {
-                    EventKind::Modify(_) | EventKind::Create(_) | EventKind::Any => {}
-                    EventKind::Remove(_) => {
-                        let _ = tx.try_send(FileEvent::Removed);
-                        return;
-                    }
-                    _ => return,
-                }
-
-                if !event.paths.iter().any(|p| p == &watch_path) {
-                    return;
-                }
-
-                let current_inode = get_inode(&watch_path).unwrap_or(0);
-                let current_size = std::fs::metadata(&watch_path).map(|m| m.len()).unwrap_or(0);
-
                 let mut st = state.lock().unwrap_or_else(|poison| {
                     // Recover from poisoned mutex — state only tracks last_size
                     // and last_inode for event dedup. Stale values at worst
                     // cause a duplicate event, which is harmless.
                     poison.into_inner()
                 });
-
-                if current_inode != 0 && st.last_inode != 0 && current_inode != st.last_inode {
-                    let _ = tx.try_send(FileEvent::Rotated {
-                        new_inode: current_inode,
-                    });
-                    st.last_inode = current_inode;
-                    st.last_size = current_size;
-                } else if current_size > st.last_size {
-                    let _ = tx.try_send(FileEvent::Appended {
-                        new_size: current_size,
-                    });
-                    st.last_size = current_size;
-                    st.last_inode = current_inode;
-                } else if current_size < st.last_size {
-                    let _ = tx.try_send(FileEvent::Truncated {
-                        new_size: current_size,
-                    });
-                    st.last_size = current_size;
-                    st.last_inode = current_inode;
+                if let Some(fe) = process_event(event, &watch_path, &mut st) {
+                    let _ = tx.try_send(fe);
                 }
             })?;
 
@@ -144,6 +154,56 @@ mod tests {
         events
     }
 
+    #[test]
+    fn test_remove_wrong_path_ignored() {
+        let dir = tempfile::tempdir().expect("create temp dir");
+        let watched = dir.path().join("watched.log");
+        let other = dir.path().join("other.log");
+        std::fs::write(&watched, b"hello\n").expect("write watched");
+        std::fs::write(&other, b"other\n").expect("write other");
+
+        let mut state = WatchState {
+            last_size: 6,
+            last_inode: get_inode(&watched).unwrap_or(0),
+        };
+
+        let event = Event {
+            kind: EventKind::Remove(notify::event::RemoveKind::File),
+            paths: vec![other.clone()],
+            attrs: Default::default(),
+        };
+
+        let result = process_event(event, &watched, &mut state);
+        assert_eq!(
+            result, None,
+            "Remove for non-watched path should be ignored"
+        );
+    }
+
+    #[test]
+    fn test_remove_correct_path_emits_removed() {
+        let dir = tempfile::tempdir().expect("create temp dir");
+        let watched = dir.path().join("watched.log");
+        std::fs::write(&watched, b"hello\n").expect("write watched");
+
+        let mut state = WatchState {
+            last_size: 6,
+            last_inode: get_inode(&watched).unwrap_or(0),
+        };
+
+        let event = Event {
+            kind: EventKind::Remove(notify::event::RemoveKind::File),
+            paths: vec![watched.clone()],
+            attrs: Default::default(),
+        };
+
+        let result = process_event(event, &watched, &mut state);
+        assert!(
+            matches!(result, Some(FileEvent::Removed)),
+            "Remove for watched path should emit Removed"
+        );
+    }
+
     #[test]
     fn test_watcher_append() {
         let dir = tempfile::tempdir().expect("create temp dir");
@@ -232,5 +292,19 @@ mod tests {
 
         let d = FileEvent::Rotated { new_inode: 42 };
         assert_ne!(a, d);
+
+        let e1 = FileEvent::WatcherError {
+            message: "io error".into(),
+        };
+        let e2 = FileEvent::WatcherError {
+            message: "io error".into(),
+        };
+        assert_eq!(e1, e2);
+        assert_ne!(
+            e1,
+            FileEvent::WatcherError {
+                message: "other".into()
+            }
+        );
     }
 }

crates/tui/src/app.rs

@@ -857,6 +857,7 @@ impl App {
                 FileEvent::Removed => {
                     self.loading_state = AppLoadingState::Error("File has been deleted".into());
                 }
+                FileEvent::WatcherError { message: _ } => {}
             }
         }
     }