fix(dl): resolve download links in arr (#100)

Transmission has no route to Prowlarr, and Prowlarr redirects a
.torrent link to a magnet that a file fetch cannot follow. arr fetches
the link itself and sends Transmission a magnet or the torrent bytes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.lock

@@ -88,6 +88,7 @@ dependencies = [
  "arr-parse",
  "arr-probe",
  "axum",
+ "base64",
  "chrono",
  "include_dir",
  "mime_guess",

crates/arr-daemon/Cargo.toml

@@ -36,6 +36,7 @@ tracing = { workspace = true }
 tracing-subscriber = { workspace = true }
 
 [dev-dependencies]
+base64 = { workspace = true }
 tempfile = { workspace = true }
 wiremock = { workspace = true }
 

crates/arr-daemon/src/grab.rs

@@ -22,7 +22,7 @@ use arr_core::policy::{evaluate, Candidate};
 use arr_core::{score::score, Language, Policy, TitleOverrides, Verdict};
 use arr_db::{blacklist, Blacklist, Db, MoviePolicy};
 use arr_dl::{AddTorrent, TorrentSource, TransmissionClient};
-use arr_indexer::{ProwlarrClient, SearchRelease, SearchRequest};
+use arr_indexer::{Download, ProwlarrClient, SearchRelease, SearchRequest};
 use arr_meta::TmdbClient;
 
 use crate::indexers::{DiscoveryError, IndexerDirectory};
@@ -78,6 +78,8 @@ pub enum GrabError {
     InvalidTmdbId(i64),
     #[error("transmission: {0}")]
     Transmission(#[from] arr_dl::Error),
+    #[error("download link: {0}")]
+    Download(#[from] arr_indexer::DownloadError),
     #[error("release {name}: {source}")]
     Parsed {
         name: String,
@@ -105,8 +107,8 @@ impl GrabAction {
     ) -> Self {
         Self {
             indexers: IndexerDirectory::new(prowlarr.clone()),
+            grabber: Grabber::new(prowlarr.clone(), transmission, download_dir, seeding),
             prowlarr,
-            grabber: Grabber::new(transmission, download_dir, seeding),
             tmdb: None,
         }
     }
@@ -380,6 +382,9 @@ impl GrabAction {
 /// release onward they are the same writes, so they share this.
 #[derive(Debug)]
 pub(crate) struct Grabber {
+    /// Resolves the winner's indexer link before it is sent on: Transmission
+    /// cannot reach Prowlarr and arr can (issue #100).
+    prowlarr: ProwlarrClient,
     transmission: TransmissionClient,
     download_dir: PathBuf,
     seeding: SeedingRules,
@@ -441,11 +446,13 @@ impl GrabScope {
 
 impl Grabber {
     pub(crate) fn new(
+        prowlarr: ProwlarrClient,
         transmission: TransmissionClient,
         download_dir: PathBuf,
         seeding: SeedingRules,
     ) -> Self {
         Self {
+            prowlarr,
             transmission,
             download_dir,
             seeding,
@@ -553,6 +560,30 @@ impl Grabber {
         }
     }
 
+    /// Resolve the winner's indexer link and add it to Transmission.
+    ///
+    /// The link is resolved here rather than passed on, because Transmission
+    /// has no route to Prowlarr and cannot follow its redirect to a magnet
+    /// (issue #100).
+    async fn send_to_transmission(
+        &self,
+        winner: &Eligible,
+        loaded: &MoviePolicy,
+    ) -> Result<arr_dl::AddedTorrent, GrabError> {
+        let seeding = self.seeding.for_indexer(winner.indexer_id);
+        let source = torrent_source(self.prowlarr.download(&winner.download_url).await?);
+        Ok(self
+            .transmission
+            .add_torrent(AddTorrent {
+                source,
+                label: label(loaded),
+                download_dir: self.download_dir.clone(),
+                seed_ratio_limit: seeding.ratio,
+                seed_idle_limit_minutes: seeding.idle_minutes,
+            })
+            .await?)
+    }
+
     /// Add the winning release to Transmission and record the grab.
     ///
     /// The search still counts as an attempt (§6.2) on every exit that is
@@ -567,22 +598,11 @@ impl Grabber {
         blacklist: &Blacklist,
         winner: Eligible,
     ) -> Result<Option<Outcome>, GrabError> {
-        let seeding = self.seeding.for_indexer(winner.indexer_id);
-        let added = match self
-            .transmission
-            .add_torrent(AddTorrent {
-                source: torrent_source(&winner.download_url),
-                label: label(loaded),
-                download_dir: self.download_dir.clone(),
-                seed_ratio_limit: seeding.ratio,
-                seed_idle_limit_minutes: seeding.idle_minutes,
-            })
-            .await
-        {
+        let added = match self.send_to_transmission(&winner, loaded).await {
             Ok(added) => added,
             Err(error) => {
                 self.record_attempt(database, target).await?;
-                return Err(error.into());
+                return Err(error);
             }
         };
         let infohash = added.hash.to_ascii_lowercase();
@@ -1120,13 +1140,11 @@ pub(crate) fn label_for_root(kind: &str, audience: &str) -> String {
     format!("{kind}-{audience}")
 }
 
-/// Indexers hand out magnets and `.torrent` links interchangeably;
-/// Transmission takes either in the same field.
-fn torrent_source(download_url: &str) -> TorrentSource {
-    if download_url.starts_with("magnet:") {
-        TorrentSource::Magnet(download_url.to_owned())
-    } else {
-        TorrentSource::Url(download_url.to_owned())
+/// A resolved download, in the shape Transmission takes it.
+fn torrent_source(download: Download) -> TorrentSource {
+    match download {
+        Download::Magnet(uri) => TorrentSource::Magnet(uri),
+        Download::Torrent(bytes) => TorrentSource::Metainfo(bytes),
     }
 }
 
@@ -1151,9 +1169,61 @@ fn rfc3339(time: SystemTime) -> Option<String> {
     chrono::DateTime::from_timestamp(seconds, 0).map(|date| date.to_rfc3339())
 }
 
+/// Fixture support for the download-link resolve step (issue #100). Grab
+/// fixtures carry indexer links, and every lane now fetches them, so the mock
+/// indexer has to answer the way Prowlarr does.
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+pub(crate) mod test_downloads {
+    use wiremock::matchers::{method, path_regex};
+    use wiremock::{Mock, MockServer, Request, Respond, ResponseTemplate};
+
+    /// Where rewritten fixture links point on the mock indexer.
+    const PREFIX: &str = "/dl/";
+
+    /// Rewrites the download links in a recorded feed onto `server`.
+    pub(crate) fn rewrite(feed: &str, links: &str, server: &MockServer) -> String {
+        feed.replace(links, &format!("{}{PREFIX}", server.uri()))
+    }
+
+    /// Prowlarr's live behaviour: the download endpoint 302s to a magnet.
+    pub(crate) async fn mount(server: &MockServer) {
+        Mock::given(method("GET"))
+            .and(path_regex(format!("^{PREFIX}")))
+            .respond_with(MagnetRedirect)
+            .mount(server)
+            .await;
+    }
+
+    struct MagnetRedirect;
+
+    impl Respond for MagnetRedirect {
+        fn respond(&self, request: &Request) -> ResponseTemplate {
+            let name = request
+                .url
+                .path()
+                .rsplit('/')
+                .next()
+                .unwrap_or_default()
+                .to_owned();
+            // Deterministic stand-in for the infohash the tracker would give,
+            // and distinct per release so duplicates still collapse.
+            let mut hash: u64 = 5381;
+            for byte in name.as_bytes() {
+                hash = hash.wrapping_mul(33) ^ u64::from(*byte);
+            }
+            ResponseTemplate::new(302).insert_header(
+                "location",
+                format!("magnet:?xt=urn:btih:{hash:040x}&dn={name}"),
+            )
+        }
+    }
+}
+
 #[cfg(test)]
 #[allow(clippy::unwrap_used)]
 mod tests {
+    use base64::Engine as _;
     use std::sync::{Arc, Mutex};
     use std::time::Duration;
 
@@ -1192,8 +1262,11 @@ mod tests {
         }
 
         fn add(&self, arguments: &Value) -> ResponseTemplate {
+            // A magnet arrives as `filename`, a torrent body as base64
+            // `metainfo` — either identifies the torrent for the fake.
             let source = arguments["filename"]
                 .as_str()
+                .or_else(|| arguments["metainfo"].as_str())
                 .unwrap_or_default()
                 .to_owned();
             let labels: Vec<String> = arguments["labels"]
@@ -1317,13 +1390,21 @@ mod tests {
 
     async fn prowlarr() -> MockServer {
         let server = MockServer::start().await;
+        let feed = test_downloads::rewrite(RSS, "https://tracker/", &server);
+        test_downloads::mount(&server).await;
+        mount_indexer(&server, &feed).await;
+        server
+    }
+
+    /// One indexer advertising a text search, serving `feed` to every query.
+    async fn mount_indexer(server: &MockServer, feed: &str) {
         Mock::given(method("GET"))
             .and(path("/api/v1/indexer"))
             .respond_with(
                 ResponseTemplate::new(200)
                     .set_body_json(json!([{"id": 7, "name": "tracker", "enable": true}])),
             )
-            .mount(&server)
+            .mount(server)
             .await;
         Mock::given(method("GET"))
             .and(path("/7/api"))
@@ -1331,15 +1412,14 @@ mod tests {
             .respond_with(ResponseTemplate::new(200).set_body_string(
                 r#"<caps><searching><search available="yes" supportedParams="q"/></searching></caps>"#,
             ))
-            .mount(&server)
+            .mount(server)
             .await;
         Mock::given(method("GET"))
             .and(path("/7/api"))
             .and(query_param("t", "search"))
-            .respond_with(ResponseTemplate::new(200).set_body_string(RSS))
-            .mount(&server)
+            .respond_with(ResponseTemplate::new(200).set_body_string(feed))
+            .mount(server)
             .await;
-        server
     }
 
     async fn empty_prowlarr() -> MockServer {
@@ -2004,15 +2084,70 @@ mod tests {
         assert!(fake.torrents().is_empty());
     }
 
+    /// §100: Transmission has no route to the indexer, so a download link
+    /// that answers with the torrent itself is forwarded as inline metainfo
+    /// and the link never leaves arr.
+    #[tokio::test]
+    async fn a_torrent_body_is_forwarded_inline_and_the_link_never_leaves_arr() {
+        const TORRENT: &[u8] = b"d4:infod6:lengthi1e4:name8:good.mkvee";
+
+        let (_dir, database) = wanted_movie().await;
+        let indexer = MockServer::start().await;
+        let feed = RSS.replace("https://tracker/", &format!("{}/dl/", indexer.uri()));
+        mount_indexer(&indexer, &feed).await;
+        Mock::given(method("GET"))
+            .and(path("/dl/good.torrent"))
+            .respond_with(
+                ResponseTemplate::new(200).set_body_raw(TORRENT, "application/x-bittorrent"),
+            )
+            .mount(&indexer)
+            .await;
+        let (downloader, fake) = transmission().await;
+
+        action(&indexer, &downloader).tick(&database).await.unwrap();
+
+        let sent = base64::engine::general_purpose::STANDARD
+            .decode(&fake.torrents()[0].source)
+            .expect("the torrent is sent as base64 metainfo");
+        assert_eq!(sent, TORRENT);
+        let fetched_with_key = indexer
+            .received_requests()
+            .await
+            .unwrap()
+            .into_iter()
+            .filter(|request| request.url.path() == "/dl/good.torrent")
+            .all(|request| request.headers.contains_key("x-api-key"));
+        assert!(fetched_with_key, "arr resolves the link with its own key");
+    }
+
+    /// A link that redirects to a magnet — Prowlarr's usual answer — reaches
+    /// Transmission as the magnet, which it can act on without the indexer.
+    #[tokio::test]
+    async fn a_link_that_redirects_to_a_magnet_is_sent_as_the_magnet() {
+        let (_dir, database) = wanted_movie().await;
+        let indexer = prowlarr().await;
+        let (downloader, fake) = transmission().await;
+
+        action(&indexer, &downloader).tick(&database).await.unwrap();
+
+        assert!(
+            fake.torrents()[0]
+                .source
+                .starts_with("magnet:?xt=urn:btih:"),
+            "{}",
+            fake.torrents()[0].source
+        );
+    }
+
     #[test]
-    fn indexer_links_and_magnets_both_reach_transmission() {
+    fn a_resolved_download_keeps_its_shape() {
         assert_eq!(
-            torrent_source("magnet:?xt=urn:btih:abc"),
+            torrent_source(Download::Magnet("magnet:?xt=urn:btih:abc".into())),
             TorrentSource::Magnet("magnet:?xt=urn:btih:abc".into())
         );
         assert_eq!(
-            torrent_source("https://tracker/good.torrent"),
-            TorrentSource::Url("https://tracker/good.torrent".into())
+            torrent_source(Download::Torrent(b"d4:infodee".to_vec())),
+            TorrentSource::Metainfo(b"d4:infodee".to_vec())
         );
     }
 }

crates/arr-daemon/src/rss.rs

@@ -44,8 +44,8 @@ impl RssAction {
     ) -> Self {
         Self {
             indexers: IndexerDirectory::new(prowlarr.clone()),
+            grabber: Grabber::new(prowlarr.clone(), transmission, download_dir, seeding),
             prowlarr,
-            grabber: Grabber::new(transmission, download_dir, seeding),
         }
     }
 
@@ -304,7 +304,7 @@ mod tests {
     use wiremock::{Mock, MockServer, Request, Respond, ResponseTemplate};
 
     use super::*;
-    use crate::grab::SeedingLimits;
+    use crate::grab::{test_downloads, SeedingLimits};
     use arr_dl::TransmissionClient;
 
     /// The recorded feed: two releases of one wanted title, one that only a
@@ -352,6 +352,8 @@ mod tests {
     /// feed to an empty query.
     async fn prowlarr() -> MockServer {
         let server = MockServer::start().await;
+        let feed = test_downloads::rewrite(FEED, "https://indexer.invalid/download/", &server);
+        test_downloads::mount(&server).await;
         Mock::given(method("GET"))
             .and(path("/api/v1/indexer"))
             .respond_with(ResponseTemplate::new(200).set_body_json(json!([
@@ -374,7 +376,7 @@ mod tests {
                 .and(path(format!("/{id}/api")))
                 .and(query_param("t", "search"))
                 .and(query_param_is_missing("q"))
-                .respond_with(ResponseTemplate::new(200).set_body_string(FEED))
+                .respond_with(ResponseTemplate::new(200).set_body_string(&feed))
                 .mount(&server)
                 .await;
         }

crates/arr-daemon/src/tv_grab.rs

@@ -53,8 +53,8 @@ impl TvGrabAction {
     ) -> Self {
         Self {
             indexers: IndexerDirectory::new(prowlarr.clone()),
+            grabber: Grabber::new(prowlarr.clone(), transmission, download_dir, seeding),
             prowlarr,
-            grabber: Grabber::new(transmission, download_dir, seeding),
         }
     }
 
@@ -678,7 +678,7 @@ mod tests {
     use wiremock::matchers::{method, path, query_param};
     use wiremock::{Mock, MockServer, Request, Respond, ResponseTemplate};
 
-    use crate::grab::SeedingLimits;
+    use crate::grab::{test_downloads, SeedingLimits};
 
     use super::*;
 
@@ -786,6 +786,8 @@ mod tests {
 
     async fn prowlarr() -> MockServer {
         let server = MockServer::start().await;
+        let feed = test_downloads::rewrite(TV_RSS, "https://tracker/", &server);
+        test_downloads::mount(&server).await;
         Mock::given(method("GET"))
             .and(path("/api/v1/indexer"))
             .respond_with(
@@ -805,7 +807,7 @@ mod tests {
         Mock::given(method("GET"))
             .and(path("/7/api"))
             .and(query_param("t", "search"))
-            .respond_with(ResponseTemplate::new(200).set_body_string(TV_RSS))
+            .respond_with(ResponseTemplate::new(200).set_body_string(feed))
             .mount(&server)
             .await;
         server

crates/arr-dl/src/lib.rs

@@ -16,12 +16,14 @@ const CONNECT_TIMEOUT: Duration = Duration::from_secs(5);
 const REQUEST_TIMEOUT: Duration = Duration::from_secs(30);
 
 /// Where Transmission is to get the torrent from.
+///
+/// Deliberately not an indexer URL: Transmission has no route to Prowlarr and
+/// cannot follow its redirect to a magnet, so arr resolves the link itself and
+/// hands over the result (issue #100).
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum TorrentSource {
     Magnet(String),
-    /// An HTTP link to a `.torrent`, fetched by Transmission itself. Indexer
-    /// download links arrive in this form.
-    Url(String),
+    /// The bytes of a `.torrent` file, sent inline.
     Metainfo(Vec<u8>),
 }
 
@@ -162,7 +164,7 @@ impl TransmissionClient {
         });
 
         match request.source {
-            TorrentSource::Magnet(uri) | TorrentSource::Url(uri) => {
+            TorrentSource::Magnet(uri) => {
                 arguments["filename"] = json!(uri);
             }
             TorrentSource::Metainfo(bytes) => {

crates/arr-indexer/src/lib.rs

@@ -7,10 +7,14 @@ pub use search::{IndexerSearch, SearchError, SearchRelease, SearchRequest, TvSel
 use std::{collections::BTreeSet, time::Duration};
 
 use quick_xml::events::Event;
-use reqwest::{Client, StatusCode};
+use reqwest::{header::LOCATION, Client, StatusCode, Url};
 use serde::Deserialize;
 use thiserror::Error;
 
+/// How many redirects a download link may take before arr gives up. Prowlarr
+/// takes one hop to a magnet or to the tracker's own file; the rest is slack.
+const MAX_DOWNLOAD_REDIRECTS: usize = 5;
+
 /// A Prowlarr indexer and the Torznab capabilities it advertises.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Indexer {
@@ -62,6 +66,9 @@ pub struct ProwlarrClient {
     base_url: String,
     api_key: String,
     client: Client,
+    /// Kept apart from `client` because resolving a download link depends on
+    /// seeing the redirect rather than following it (§7.1, issue #100).
+    downloads: Client,
 }
 
 impl ProwlarrClient {
@@ -75,11 +82,17 @@ impl ProwlarrClient {
             .timeout(Duration::from_secs(30))
             .build()
             .map_err(|_| Error::Client)?;
+        let downloads = Client::builder()
+            .timeout(Duration::from_secs(30))
+            .redirect(reqwest::redirect::Policy::none())
+            .build()
+            .map_err(|_| Error::Client)?;
 
         Ok(Self {
             base_url: base_url.into().trim_end_matches('/').to_owned(),
             api_key: api_key.into(),
             client,
+            downloads,
         })
     }
 
@@ -147,6 +160,101 @@ impl ProwlarrClient {
             })?;
         parse_capabilities(&body)
     }
+
+    /// Resolves an indexer download link into something the download client
+    /// can take without reaching the indexer itself.
+    ///
+    /// Handing Transmission the Prowlarr link fails twice over: Transmission
+    /// has no route to Prowlarr, and Prowlarr answers a `.torrent` link with a
+    /// redirect to a magnet, which a plain file fetch cannot follow. arr has
+    /// the route and the API key, so it resolves the link here and passes on
+    /// the result (issue #100).
+    ///
+    /// # Errors
+    ///
+    /// Returns an error when the link cannot be fetched or resolves to
+    /// something that is neither a magnet nor a torrent.
+    pub async fn download(&self, download_url: &str) -> Result<Download, DownloadError> {
+        if let Some(magnet) = as_magnet(download_url) {
+            return Ok(Download::Magnet(magnet));
+        }
+
+        let mut url = Url::parse(download_url).map_err(|_| DownloadError::InvalidUrl)?;
+        for _ in 0..MAX_DOWNLOAD_REDIRECTS {
+            let mut request = self.downloads.get(url.clone());
+            // Only ever to Prowlarr itself: a redirect can leave for a tracker
+            // and the key must not go with it.
+            if self.is_prowlarr(&url) {
+                request = request.header("X-Api-Key", &self.api_key);
+            }
+            let response = request
+                .send()
+                .await
+                .map_err(|error| DownloadError::Request {
+                    status: error.status(),
+                })?;
+
+            if response.status().is_redirection() {
+                let location = response
+                    .headers()
+                    .get(LOCATION)
+                    .and_then(|value| value.to_str().ok())
+                    .ok_or(DownloadError::UnusableRedirect)?;
+                if let Some(magnet) = as_magnet(location) {
+                    return Ok(Download::Magnet(magnet));
+                }
+                url = url
+                    .join(location)
+                    .map_err(|_| DownloadError::UnusableRedirect)?;
+                continue;
+            }
+
+            let body = response
+                .error_for_status()
+                .map_err(|error| DownloadError::Request {
+                    status: error.status(),
+                })?
+                .bytes()
+                .await
+                .map_err(|error| DownloadError::Request {
+                    status: error.status(),
+                })?;
+
+            // Some indexers answer with the magnet as the body rather than as
+            // a redirect.
+            if let Some(magnet) = std::str::from_utf8(&body).ok().and_then(as_magnet) {
+                return Ok(Download::Magnet(magnet));
+            }
+            // A torrent is a bencoded dictionary. An HTML error page or a
+            // login form must not reach the download client as metainfo.
+            if body.first() != Some(&b'd') {
+                return Err(DownloadError::NotATorrent);
+            }
+            return Ok(Download::Torrent(body.to_vec()));
+        }
+
+        Err(DownloadError::TooManyRedirects(MAX_DOWNLOAD_REDIRECTS))
+    }
+
+    fn is_prowlarr(&self, url: &Url) -> bool {
+        Url::parse(&self.base_url).is_ok_and(|base| base.origin() == url.origin())
+    }
+}
+
+/// What an indexer's download link resolved to.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum Download {
+    Magnet(String),
+    /// The bytes of a `.torrent` file.
+    Torrent(Vec<u8>),
+}
+
+fn as_magnet(value: &str) -> Option<String> {
+    let value = value.trim();
+    value
+        .get(..7)
+        .is_some_and(|scheme| scheme.eq_ignore_ascii_case("magnet:"))
+        .then(|| value.to_owned())
 }
 
 /// Errors returned while discovering Prowlarr indexers.
@@ -160,6 +268,21 @@ pub enum Error {
     Response,
 }
 
+/// A download link that could not be turned into a magnet or a torrent.
+#[derive(Clone, Debug, Eq, Error, PartialEq)]
+pub enum DownloadError {
+    #[error("download link request failed (status: {status:?})")]
+    Request { status: Option<StatusCode> },
+    #[error("download link is not a valid URL")]
+    InvalidUrl,
+    #[error("download link redirected to a location that cannot be used")]
+    UnusableRedirect,
+    #[error("download link returned neither a torrent nor a magnet")]
+    NotATorrent,
+    #[error("download link still redirected after {0} hops")]
+    TooManyRedirects(usize),
+}
+
 /// A failed capabilities request for an otherwise usable Prowlarr indexer.
 #[derive(Clone, Debug, Eq, Error, PartialEq)]
 pub enum CapabilityError {
@@ -290,8 +413,8 @@ fn is_available(value: &str) -> bool {
 #[cfg(test)]
 mod tests {
     use super::{
-        parse_capabilities, Capabilities, CapabilityError, ProwlarrClient, SearchRequest,
-        TvSelector, TvTarget,
+        parse_capabilities, Capabilities, CapabilityError, Download, DownloadError, ProwlarrClient,
+        SearchRequest, TvSelector, TvTarget,
     };
     use reqwest::StatusCode;
     use wiremock::{
@@ -301,6 +424,10 @@ mod tests {
 
     const API_KEY: &str = "test-api-key";
 
+    fn client(server: &MockServer) -> ProwlarrClient {
+        ProwlarrClient::new(server.uri(), API_KEY).expect("HTTP client can be created")
+    }
+
     #[tokio::test]
     async fn enumerates_indexers_with_recorded_capabilities() {
         let server = MockServer::start().await;
@@ -369,6 +496,145 @@ mod tests {
         );
     }
 
+    /// #100: Prowlarr answers a `.torrent` link with a redirect to a magnet.
+    /// The download client cannot follow that, so arr does it here.
+    #[tokio::test]
+    async fn a_redirect_to_a_magnet_resolves_to_the_magnet() {
+        let server = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/download"))
+            .and(header("X-Api-Key", API_KEY))
+            .respond_with(
+                ResponseTemplate::new(302)
+                    .insert_header("location", "magnet:?xt=urn:btih:abc&dn=release"),
+            )
+            .mount(&server)
+            .await;
+
+        let download = client(&server)
+            .download(&format!("{}/download", server.uri()))
+            .await
+            .expect("the redirect resolves");
+
+        assert_eq!(
+            download,
+            Download::Magnet("magnet:?xt=urn:btih:abc&dn=release".to_owned())
+        );
+    }
+
+    #[tokio::test]
+    async fn a_torrent_body_resolves_to_its_bytes() {
+        let body: &[u8] = b"d4:infod6:lengthi1e4:name7:one.mkvee";
+        let server = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/download"))
+            .respond_with(ResponseTemplate::new(200).set_body_raw(body, "application/x-bittorrent"))
+            .mount(&server)
+            .await;
+
+        let download = client(&server)
+            .download(&format!("{}/download", server.uri()))
+            .await
+            .expect("the body is a torrent");
+
+        assert_eq!(download, Download::Torrent(body.to_vec()));
+    }
+
+    /// A magnet in the feed needs no request at all.
+    #[tokio::test]
+    async fn a_magnet_link_resolves_without_a_request() {
+        let server = MockServer::start().await;
+
+        let download = client(&server)
+            .download("magnet:?xt=urn:btih:abc")
+            .await
+            .expect("a magnet is already resolved");
+
+        assert_eq!(
+            download,
+            Download::Magnet("magnet:?xt=urn:btih:abc".to_owned())
+        );
+        assert!(server
+            .received_requests()
+            .await
+            .expect("recorded")
+            .is_empty());
+    }
+
+    /// The API key belongs to Prowlarr. A redirect that leaves for a tracker
+    /// must not carry it there.
+    #[tokio::test]
+    async fn the_api_key_never_follows_a_redirect_off_prowlarr() {
+        let tracker = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/file.torrent"))
+            .respond_with(
+                ResponseTemplate::new(200)
+                    .set_body_raw(b"d4:infodee".as_slice(), "application/x-bittorrent"),
+            )
+            .mount(&tracker)
+            .await;
+        let server = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/download"))
+            .respond_with(
+                ResponseTemplate::new(302)
+                    .insert_header("location", format!("{}/file.torrent", tracker.uri())),
+            )
+            .mount(&server)
+            .await;
+
+        let download = client(&server)
+            .download(&format!("{}/download", server.uri()))
+            .await
+            .expect("the tracker serves the file");
+
+        assert_eq!(download, Download::Torrent(b"d4:infodee".to_vec()));
+        let sent_key = tracker
+            .received_requests()
+            .await
+            .expect("recorded")
+            .into_iter()
+            .any(|request| request.headers.contains_key("x-api-key"));
+        assert!(!sent_key);
+    }
+
+    /// An HTML error page or a login form is not a torrent, and passing it on
+    /// as metainfo would only fail later and less clearly.
+    #[tokio::test]
+    async fn a_body_that_is_not_a_torrent_is_refused() {
+        let server = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/download"))
+            .respond_with(ResponseTemplate::new(200).set_body_string("<html>login</html>"))
+            .mount(&server)
+            .await;
+
+        let error = client(&server)
+            .download(&format!("{}/download", server.uri()))
+            .await
+            .expect_err("HTML is not a torrent");
+
+        assert_eq!(error, DownloadError::NotATorrent);
+    }
+
+    #[tokio::test]
+    async fn a_redirect_loop_gives_up() {
+        let server = MockServer::start().await;
+        Mock::given(method("GET"))
+            .and(path("/download"))
+            .respond_with(ResponseTemplate::new(302).insert_header("location", "/download"))
+            .mount(&server)
+            .await;
+
+        let error = client(&server)
+            .download(&format!("{}/download", server.uri()))
+            .await
+            .expect_err("the link never resolves");
+
+        assert_eq!(error, DownloadError::TooManyRedirects(5));
+    }
+
     #[test]
     fn tv_requests_degrade_to_what_each_indexer_accepts() {
         let alpha = capabilities(include_str!("../tests/fixtures/alpha-caps.xml"));