perf(yt/update/updater): Acknowledge, that `yt_dlp` has a sync API

The previous code was written with the assumption, that `yt_dlp` had an
async API (which means, that calls to it should never block).
Unfortunately, the API is sync and all calls into it block. Therefore,
all the parallelism in the previous code accounted for nothing; The
actual update ran completely sequentially.

The new version uses a local thread pool to avoid blocking the tokio
runtime and as such achieves higher speed.

Unfortunately, because the rustpython implementation is way slower
than cpython, the whole update takes longer. But that is a problem for
another day.

3 files changed, 116 insertions, 103 deletions

diff --git a/crates/yt/Cargo.toml b/crates/yt/Cargo.toml
index 17d4016..6803e68 100644
--- a/crates/yt/Cargo.toml
+++ b/crates/yt/Cargo.toml
@@ -50,6 +50,7 @@ yt_dlp.workspace = true
 termsize.workspace = true
 uu_fmt.workspace = true
 notify = { version = "8.0.0", default-features = false }
+tokio-util = { version = "0.7.15", features = ["rt"] }
 
 [[bin]]
 name = "yt"
diff --git a/crates/yt/src/update/mod.rs b/crates/yt/src/update/mod.rs
index 07674de..a25c233 100644
--- a/crates/yt/src/update/mod.rs
+++ b/crates/yt/src/update/mod.rs
@@ -13,7 +13,7 @@ use std::{str::FromStr, time::Duration};
 
 use anyhow::{Context, Ok, Result};
 use chrono::{DateTime, Utc};
-use log::{info, warn};
+use log::warn;
 use url::Url;
 use yt_dlp::{InfoJson, json_cast, json_get};
 
@@ -39,12 +39,12 @@ pub async fn update(
 ) -> Result<()> {
     let subscriptions = subscriptions::get(app).await?;
 
-    let urls: Vec<_> = if subscription_names_to_update.is_empty() {
-        subscriptions.0.values().collect()
+    let subs: Vec<Subscription> = if subscription_names_to_update.is_empty() {
+        subscriptions.0.into_values().collect()
     } else {
         subscriptions
             .0
-            .values()
+            .into_values()
             .filter(|sub| subscription_names_to_update.contains(&sub.name))
             .collect()
     };
@@ -53,10 +53,8 @@ pub async fn update(
     // should not contain duplicates.
     let hashes = get_all_hashes(app).await?;
 
-    {
-        let mut updater = Updater::new(max_backlog, &hashes);
-        updater.update(app, &urls).await?;
-    }
+    let updater = Updater::new(max_backlog, hashes);
+    updater.update(app, subs).await?;
 
     Ok(())
 }
@@ -182,9 +180,9 @@ pub fn video_entry_to_video(entry: &InfoJson, sub: Option<&Subscription>) -> Res
     Ok(video)
 }
 
-async fn process_subscription(app: &App, sub: &Subscription, entry: InfoJson) -> Result<()> {
-    let video =
-        video_entry_to_video(&entry, Some(sub)).context("Failed to parse search entry as Video")?;
+async fn process_subscription(app: &App, sub: Subscription, entry: InfoJson) -> Result<()> {
+    let video = video_entry_to_video(&entry, Some(&sub))
+        .context("Failed to parse search entry as Video")?;
 
     add_video(app, video.clone())
         .await
diff --git a/crates/yt/src/update/updater.rs b/crates/yt/src/update/updater.rs
index b314172..934b84b 100644
--- a/crates/yt/src/update/updater.rs
+++ b/crates/yt/src/update/updater.rs
@@ -8,17 +8,18 @@
 // You should have received a copy of the License along with this program.
 // If not, see <https://www.gnu.org/licenses/gpl-3.0.txt>.
 
-use std::io::{Write, stderr};
+use std::{
+    io::{Write, stderr},
+    sync::atomic::AtomicUsize,
+};
 
 use anyhow::{Context, Result};
 use blake3::Hash;
-use futures::{
-    StreamExt, TryStreamExt,
-    stream::{self},
-};
+use futures::{StreamExt, future::join_all, stream};
 use log::{Level, debug, error, log_enabled};
 use serde_json::json;
-use yt_dlp::{InfoJson, YoutubeDL, YoutubeDLOptions, json_cast, json_get, process_ie_result};
+use tokio_util::task::LocalPoolHandle;
+use yt_dlp::{InfoJson, YoutubeDLOptions, json_cast, json_get, process_ie_result};
 
 use crate::{
     ansi_escape_codes::{clear_whole_line, move_to_col},
@@ -28,44 +29,41 @@ use crate::{
 
 use super::process_subscription;
 
-pub(super) struct Updater<'a> {
+pub(super) struct Updater {
     max_backlog: usize,
-    hashes: &'a [Hash],
+    hashes: Vec<Hash>,
+    pool: LocalPoolHandle,
 }
 
-impl<'a> Updater<'a> {
-    pub(super) fn new(max_backlog: usize, hashes: &'a [Hash]) -> Self {
+impl Updater {
+    pub(super) fn new(max_backlog: usize, hashes: Vec<Hash>) -> Self {
+        // TODO(@bpeetz): The number should not be hardcoded. <2025-06-14>
+        let pool = LocalPoolHandle::new(16);
+
         Self {
             max_backlog,
             hashes,
+            pool,
         }
     }
 
-    pub(super) async fn update(
-        &mut self,
-        app: &App,
-        subscriptions: &[&Subscription],
-    ) -> Result<()> {
+    pub(super) async fn update(self, app: &App, subscriptions: Vec<Subscription>) -> Result<()> {
         let mut stream = stream::iter(subscriptions)
             .map(|sub| self.get_new_entries(sub))
-            .buffer_unordered(100);
+            .buffer_unordered(16 * 4);
 
         while let Some(output) = stream.next().await {
             let mut entries = output?;
 
-            if entries.is_empty() {
-                continue;
-            }
+            if let Some(next) = entries.next() {
+                let (sub, entry) = next;
+                process_subscription(app, sub, entry).await?;
 
-            let (sub, entry) = entries.remove(0);
-            process_subscription(app, sub, entry).await?;
-
-            let entry_stream: Result<()> = stream::iter(entries)
-                .map(|(sub, entry)| process_subscription(app, sub, entry))
-                .buffer_unordered(100)
-                .try_collect()
-                .await;
-            entry_stream?;
+                join_all(entries.map(|(sub, entry)| process_subscription(app, sub, entry)))
+                    .await
+                    .into_iter()
+                    .collect::<Result<(), _>>()?;
+            }
         }
 
         Ok(())
@@ -73,11 +71,14 @@ impl<'a> Updater<'a> {
 
     async fn get_new_entries(
         &self,
-        sub: &'a Subscription,
-    ) -> Result<Vec<(&'a Subscription, InfoJson)>> {
+        sub: Subscription,
+    ) -> Result<impl Iterator<Item = (Subscription, InfoJson)>> {
+        let max_backlog = self.max_backlog;
+        let hashes = self.hashes.clone();
+
         let yt_dlp = YoutubeDLOptions::new()
             .set("playliststart", 1)
-            .set("playlistend", self.max_backlog)
+            .set("playlistend", max_backlog)
             .set("noplaylist", false)
             .set(
                 "extractor_args",
@@ -88,70 +89,83 @@ impl<'a> Updater<'a> {
             .set("match-filter", "availability=public")
             .build()?;
 
-        if !log_enabled!(Level::Debug) {
-            clear_whole_line();
-            move_to_col(1);
-            eprint!("Checking playlist {}...", sub.name);
-            move_to_col(1);
-            stderr().flush()?;
-        }
-
-        let info = yt_dlp
-            .extract_info(&sub.url, false, false)
-            .with_context(|| format!("Failed to get playlist '{}'.", sub.name))?;
-
-        let empty = vec![];
-        let entries = info
-            .get("entries")
-            .map_or(&empty, |val| json_cast!(val, as_array));
-
-        let valid_entries: Vec<(&Subscription, InfoJson)> = entries
-            .iter()
-            .take(self.max_backlog)
-            .filter_map(|entry| -> Option<(&Subscription, InfoJson)> {
-                let id = json_get!(entry, "id", as_str);
-                let extractor_hash = blake3::hash(id.as_bytes());
-                if self.hashes.contains(&extractor_hash) {
-                    debug!("Skipping entry, as it is already present: '{extractor_hash}'",);
-                    None
-                } else {
-                    Some((sub, json_cast!(entry, as_object).to_owned()))
-                }
-            })
-            .collect();
-
-        let processed_entries: Vec<(&Subscription, InfoJson)> = stream::iter(valid_entries)
-            .map(
-                async |(sub, entry)| match yt_dlp.process_ie_result(entry, false) {
-                    Ok(output) => Ok((sub, output)),
-                    Err(err) => Err(err),
-                },
-            )
-            .buffer_unordered(100)
-            .collect::<Vec<_>>()
-            .await
-            .into_iter()
-            // Don't fail the whole update, if one of the entries fails to fetch.
-            .filter_map(|base| match base {
-                Ok(ok) => Some(ok),
-                Err(err) => {
-                    let process_ie_result::Error::Python(err) = &err;
-
-                    if err.contains("Join this channel to get access to members-only content ") {
-                        // Hide this error
-                    } else {
-                        // Show the error, but don't fail.
-                        let error = err
-                            .strip_prefix("DownloadError: \u{1b}[0;31mERROR:\u{1b}[0m ")
-                            .unwrap_or(err);
-                        error!("{error}");
+        self.pool
+            .spawn_pinned(move || {
+                async move {
+                    if !log_enabled!(Level::Debug) {
+                        clear_whole_line();
+                        move_to_col(1);
+                        eprint!(
+                            "Checking playlist {}...",
+                            sub.name
+                        );
+                        move_to_col(1);
+                        stderr().flush()?;
                     }
 
-                    None
+                    let info = yt_dlp
+                        .extract_info(&sub.url, false, false)
+                        .with_context(|| format!("Failed to get playlist '{}'.", sub.name))?;
+
+                    let empty = vec![];
+                    let entries = info
+                        .get("entries")
+                        .map_or(&empty, |val| json_cast!(val, as_array));
+
+                    let valid_entries: Vec<(Subscription, InfoJson)> = entries
+                        .iter()
+                        .take(max_backlog)
+                        .filter_map(|entry| -> Option<(Subscription, InfoJson)> {
+                            let id = json_get!(entry, "id", as_str);
+                            let extractor_hash = blake3::hash(id.as_bytes());
+
+                            if hashes.contains(&extractor_hash) {
+                                debug!(
+                                    "Skipping entry, as it is already present: '{extractor_hash}'",
+                                );
+                                None
+                            } else {
+                                Some((sub.clone(), json_cast!(entry, as_object).to_owned()))
+                            }
+                        })
+                        .collect();
+
+                    Ok(valid_entries
+                        .into_iter()
+                        .map(|(sub, entry)| {
+                            let inner_yt_dlp = YoutubeDLOptions::new()
+                                .set("noplaylist", true)
+                                .build()
+                                .expect("Worked before, should work now");
+
+                            match inner_yt_dlp.process_ie_result(entry, false) {
+                                Ok(output) => Ok((sub, output)),
+                                Err(err) => Err(err),
+                            }
+                        })
+                        // Don't fail the whole update, if one of the entries fails to fetch.
+                        .filter_map(|base| match base {
+                            Ok(ok) => Some(ok),
+                            Err(err) => {
+                                let process_ie_result::Error::Python(err) = &err;
+
+                                if err.contains(
+                                    "Join this channel to get access to members-only content ",
+                                ) {
+                                    // Hide this error
+                                } else {
+                                    // Show the error, but don't fail.
+                                    let error = err
+                                        .strip_prefix("DownloadError: \u{1b}[0;31mERROR:\u{1b}[0m ")
+                                        .unwrap_or(err);
+                                    error!("{error}");
+                                }
+
+                                None
+                            }
+                        }))
                 }
             })
-            .collect();
-
-        Ok(processed_entries)
+            .await?
     }
 }