Add --removal-debounce for "remove" FS events, with a higher default

This should fix one pain point of mine when writing docs, running
`cargo watch -x doc` and running `penguin serve target/doc`. Cargo first
wipes the output directory clean and seem to only then start building
the docs. There is quite a bit of time between the wipe and writing all
the new files. This used to make Penguin already reload the sessions,
resulting in 404 pages. Now that "remove" events are treated as less
important, this should be fixed. I think it's generally a valid
assumption that a quick reload after only a deletion event is rarely
the intended behavior.

app/src/args.rs

@@ -85,7 +85,7 @@ pub(crate) enum Command {
     Reload,
 }
 
-#[derive(Debug, StructOpt)]
+#[derive(Debug, Clone, StructOpt)]
 pub(crate) struct ServeOptions {
     /// Mount a directory on an URI path: '--mount <uri>:<path>'.
     ///
@@ -126,6 +126,21 @@ pub(crate) struct ServeOptions {
         parse(try_from_str = parse_duration)
     )]
     pub(crate) debounce_duration: Duration,
+
+    /// The debounce duration (in ms) for when a file in a watched path was
+    /// removed.
+    ///
+    /// Like `--debounce`, but for "remove" file system events. This is treated
+    /// separately as usually reloading quickly on deletion is not useful: the
+    /// reload would result in a 404 page. And in many situation (e.g. cargo
+    /// doc) the watched directory is first wiped by a build process and then
+    /// populated again after a while. So this default is much higher.
+    #[structopt(
+        long = "--removal-debounce",
+        default_value = "3000",
+        parse(try_from_str = parse_duration)
+    )]
+    pub(crate) removal_debounce_duration: Duration,
 }
 
 fn parse_mount(s: &str) -> Result<Mount, &'static str> {

app/src/server.rs

@@ -2,6 +2,7 @@ use std::{env, ops::Deref, path::Path, thread, time::Duration};
 
 use anyhow::{Context, Result};
 use log::{debug, info, trace, LevelFilter};
+use notify::Op;
 use penguin::{Config, Controller, Mount, ProxyTarget, Server};
 
 use crate::args::{Args, DEFAULT_PORT, ServeOptions};
@@ -44,7 +45,7 @@ pub(crate) async fn run(
     };
 
     if !watched_paths.is_empty() {
-        watch(controller, options.debounce_duration, &watched_paths)?;
+        watch(controller, options, &watched_paths)?;
     }
 
     // Nice output of what is being done
@@ -89,7 +90,7 @@ pub(crate) async fn run(
 
 fn watch<'a>(
     controller: Controller,
-    debounce_duration: Duration,
+    options: &ServeOptions,
     paths: &[&Path],
 ) -> Result<()> {
     use std::sync::mpsc::{channel, RecvTimeoutError};
@@ -114,13 +115,24 @@ fn watch<'a>(
 
     // We create a new thread that will react to incoming events and trigger a
     // page reload.
+    let options = options.clone();
     thread::spawn(move || {
         // Move it to the thread to avoid dropping it early.
         let _watcher = watcher;
+        let debounce_duration_of = |event: &RawEvent| {
+            if event.op.as_ref().is_ok_and(|&op| op == Op::REMOVE) {
+                options.removal_debounce_duration
+            } else {
+                options.debounce_duration
+            }
+        };
 
         while let Ok(event) = rx.recv() {
+            let mut debounce_duration = debounce_duration_of(&event);
+
             debug!(
-                "Received watch-event for '{}'. Debouncing now for {:?}.",
+                "Received watch-event '{:?}' for '{}'. Debouncing now for {:?}.",
+                event.op,
                 pretty_path(&event),
                 debounce_duration,
             );
@@ -129,7 +141,21 @@ fn watch<'a>(
             // `debounce_duration`.
             loop {
                 match rx.recv_timeout(debounce_duration) {
-                    Ok(event) => trace!("Debounce interrupted by '{}'", pretty_path(&event)),
+                    Ok(event) => {
+                        trace!(
+                            "Debounce interrupted by '{:?}' of '{}'",
+                            event.op,
+                            pretty_path(&event),
+                        );
+
+                        // We reset the waiting duration to the minimum of both
+                        // events' durations. So if any non-remove event is
+                        // involved, the shorter duration is used.
+                        debounce_duration = std::cmp::min(
+                            debounce_duration_of(&event),
+                            debounce_duration,
+                        );
+                    },
                     Err(RecvTimeoutError::Timeout) => break,
                     Err(RecvTimeoutError::Disconnected) => return,
                 }