gutenberg: one bind() for the search and redeem paths

compute_outcomes still passed the identifier-only lookup's Option<String> to
into_discovery_record, which now takes the binding (CI: E0308). Both paths now
call bind(): the identifier, else the title match for an etext Open Library
lists under no work, so a redeemed record is bound exactly as its search record
was.

feeder-plugin/book-feeder/src/gutenberg.rs

@@ -44,6 +44,26 @@ struct Binding {
     by_title: bool,
 }
 
+/// One etext's binding: the identifier Open Library records, else — only for an
+/// etext it lists under no work — the title match. Shared by the search path and
+/// the redeem path so the two cannot bind the same book differently.
+async fn bind(
+    client: &OpenLibraryClient,
+    id: u64,
+    title: &str,
+    author: &str,
+    deadline: std::time::Duration,
+) -> Result<Option<Binding>, GatewayError> {
+    Ok(match client.gutenberg_etext(id, deadline).await? {
+        Etext::One(work) => Some(Binding { work, by_title: false }),
+        Etext::Several => None,
+        Etext::Unlisted => client
+            .title_work(title, author, deadline)
+            .await?
+            .map(|work| Binding { work, by_title: true }),
+    })
+}
+
 /// Canonical Gutendex base URL. Overridable via [`GutenbergPlugin::with_base_url`]
 /// for tests + private mirrors.
 const DEFAULT_BASE_URL: &str = "https://gutendex.com";
@@ -451,19 +471,7 @@ impl GutenbergPlugin {
             .collect();
         stream::iter(wanted)
             .map(|(id, title, author)| async move {
-                // The identifier first; a title match only for an etext Open
-                // Library lists under no work at all.
-                let lookup = async {
-                    let deadline = openlibrary_client::BINDING_DEADLINE;
-                    Ok::<_, GatewayError>(match client.gutenberg_etext(id, deadline).await? {
-                        Etext::One(work) => Some(Binding { work, by_title: false }),
-                        Etext::Several => None,
-                        Etext::Unlisted => client
-                            .title_work(&title, &author, deadline)
-                            .await?
-                            .map(|work| Binding { work, by_title: true }),
-                    })
-                };
+                let lookup = bind(client, id, &title, &author, openlibrary_client::BINDING_DEADLINE);
                 match tokio::time::timeout(BINDING_TIMEOUT, lookup).await {
                     Ok(Ok(binding)) => binding,
                     Ok(Err(e)) => {
@@ -628,9 +636,9 @@ impl FeederPlugin for GutenbergPlugin {
         // Not user-facing, so the binding lookup may queue for the full search
         // deadline. A failure still leaves the record unbound, never fails the
         // compute.
+        let author = book.authors.first().map(|a| a.name.clone()).unwrap_or_default();
         let work = match self.openlibrary.as_ref() {
-            Some(ol) => ol
-                .gutenberg_work(book.id, openlibrary_client::SEARCH_DEADLINE)
+            Some(ol) => bind(ol, book.id, &book.title, &author, openlibrary_client::SEARCH_DEADLINE)
                 .await
                 .unwrap_or_else(|e| {
                     debug!(
@@ -704,8 +712,8 @@ impl FeederPlugin for GutenbergPlugin {
 
 /// Convert a `GutendexBook` into our wire-level `DiscoveryRecord`.
 ///
-/// `openlibrary_work` is the binding from [`OpenLibraryClient::gutenberg_work`]:
-/// `Some` only when Open Library records exactly one work for this etext.
+/// `openlibrary_work` is the binding from [`bind`]: the one work Open Library
+/// records for this etext, or, when it records none, the title match.
 fn into_discovery_record(
     book: GutendexBook,
     cover_cid: Option<String>,