Start work on Monitor Album

src/magnet_resolver.rs

@@ -0,0 +1,74 @@
+//! Magnet-link resolver backed by librqbit.
+//!
+//! Turns a `magnet:` URI into the same bencoded torrent bytes a `.torrent`
+//! download would yield, by resolving metadata over the BitTorrent DHT and
+//! the BEP-9 `ut_metadata` peer extension. The bytes are then handed to
+//! [`crate::torrent_parser::TorrentParser`] so magnet and torrent sources are
+//! parsed identically.
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use anyhow::{Context, bail};
+use librqbit::{
+    AddTorrent, AddTorrentOptions, AddTorrentResponse, ListOnlyResponse, Session, SessionOptions,
+};
+use tempfile::TempDir;
+
+const RESOLVE_TIMEOUT: Duration = Duration::from_secs(90);
+
+pub struct MagnetResolver {
+    session: Arc<Session>,
+    _scratch_dir: TempDir,
+}
+
+impl std::fmt::Debug for MagnetResolver {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("MagnetResolver").finish_non_exhaustive()
+    }
+}
+
+impl MagnetResolver {
+    pub async fn new() -> anyhow::Result<Self> {
+        let scratch_dir = tempfile::TempDir::new()?;
+        let opts = SessionOptions {
+            disable_dht_persistence: true,
+            persistence: None,
+            enable_upnp_port_forwarding: false,
+            ..Default::default()
+        };
+        let session = Session::new_with_opts(scratch_dir.path().to_path_buf(), opts).await?;
+        Ok(Self {
+            session,
+            _scratch_dir: scratch_dir,
+        })
+    }
+
+    pub async fn resolve(&self, magnet: &str) -> anyhow::Result<Vec<u8>> {
+        let added = tokio::time::timeout(
+            RESOLVE_TIMEOUT,
+            self.session.add_torrent(
+                AddTorrent::from_url(magnet),
+                Some(AddTorrentOptions {
+                    list_only: true,
+                    ..Default::default()
+                }),
+            ),
+        )
+        .await
+        .context("magnet resolution timed out (>90s, likely no peers)")?
+        .context("error adding magnet to session")?;
+
+        match added {
+            AddTorrentResponse::ListOnly(ListOnlyResponse { torrent_bytes, .. }) => {
+                Ok(torrent_bytes.to_vec())
+            }
+            AddTorrentResponse::AlreadyManaged(_, handle) => {
+                handle.with_metadata(|m| m.torrent_bytes.to_vec())
+            }
+            AddTorrentResponse::Added(_, _) => {
+                bail!("torrent unexpectedly added under list_only")
+            }
+        }
+    }
+}