bibfetch: the books a register has under a title

A book that arrives as a file often has no identifier to ask about, only
a title and perhaps a name. `bibfetch search --title T [--author A]
[--limit N]` asks Open Library's index which books it has under those,
and Google Books only when Open Library has nothing, and prints one
record per candidate in the order the register gave them. From Rust,
`bibfetch::search(&agent, title, author, limit, attempts)`.

Each candidate is one edition. The index answers with works and, asked
for `editions.*`, names the edition of each that best matches; the ISBN,
publisher, language, year and cover come from that edition, the title,
authors, subjects and length from the work. The work's own ISBN list
mixes editions: the first War and Peace work lists four, while the
edition matched is a 1917 printing with none.

`id` is the first ISBN whose check digit agrees, in thirteen digits,
thirteen-digit entries taken first. A candidate with none is named by
the register's key: `olid` (OL…M for the edition, OL…W for the work when
no edition was named) or `googlebooks` (a volume id). Google's `OTHER`
shelfmarks are never read as ISBNs.

`cover_id` keeps the meaning it has in a lookup record: the edition's
own first cover, taken from `editions.cover_i` and only when the record
names that edition. The work's `cover_i` and `cover_edition_key` are not
asked for. A work with no edition named, and every Google Books
candidate, has none.

Open Library having nothing stands when Google Books is down; both down
is exit 69. Exit 0 a candidate, 1 none, 65 an empty title.

The retrying GET that lookups use is now `ask::get`, shared by both, and
no longer sleeps after its last attempt.

crates/bibfetch/src/ask.rs

@@ -89,10 +89,6 @@ pub fn lookup(
 }
 
 /// Ask a register, retrying a failure that might not be one.
-///
-/// Waits a second before the second attempt and two before the third: long
-/// enough for a register that is rate-limiting, short enough that one which
-/// is simply down does not hold up a pipeline.
 fn ask(
     agent: &ureq::Agent,
     source: Source,
@@ -100,39 +96,48 @@ fn ask(
     kind: &str,
     attempts: u32,
 ) -> Result<crate::Record, Error> {
-    let mut last = Error::Unreachable("no attempt was made".into());
+    match get(agent, &source.url(id), attempts)? {
+        Some(body) => source.read(id, kind, &body),
+        // A register that does not have it says so with a status, and that is
+        // an answer rather than a failure.
+        None => Ok(crate::missing(id, kind, source)),
+    }
+}
 
-    for attempt in 0..attempts.max(1) {
-        match fetch(agent, source, id, kind) {
-            Ok(record) => return Ok(record),
-            // A shape this cannot read will not read any better next time.
-            Err(e @ Error::Shape(_)) => return Err(e),
+/// The body at an address, or `None` when the register says it has no such
+/// thing; a failure is asked again.
+///
+/// Waits a second before the second attempt and two before the third: long
+/// enough for a register that is rate-limiting, short enough that one which
+/// is simply down does not hold up a pipeline. Reading what came back is left
+/// to the caller, because an answer this cannot read will not read any better
+/// next time.
+pub(crate) fn get(agent: &ureq::Agent, url: &str, attempts: u32) -> Result<Option<String>, Error> {
+    let attempts = attempts.max(1);
+    let mut last = Error::Unreachable("no attempt was made".into());
+    for attempt in 0..attempts {
+        match once(agent, url) {
+            Ok(answer) => return Ok(answer),
             Err(e) => last = e,
         }
-        std::thread::sleep(Duration::from_secs(1 << attempt));
+        if attempt + 1 < attempts {
+            std::thread::sleep(Duration::from_secs(1 << attempt.min(5)));
+        }
     }
     Err(last)
 }
 
-fn fetch(
-    agent: &ureq::Agent,
-    source: Source,
-    id: &str,
-    kind: &str,
-) -> Result<crate::Record, Error> {
-    let response = agent.get(source.url(id)).call();
-    let mut response = match response {
+fn once(agent: &ureq::Agent, url: &str) -> Result<Option<String>, Error> {
+    let mut response = match agent.get(url).call() {
         Ok(response) => response,
-        // A register that does not have it says so with a status, and that is
-        // an answer rather than a failure.
-        Err(ureq::Error::StatusCode(404)) => return Ok(crate::missing(id, kind, source)),
+        Err(ureq::Error::StatusCode(404)) => return Ok(None),
         Err(e) => return Err(Error::Unreachable(e.to_string())),
     };
-    let body = response
+    response
         .body_mut()
         .read_to_string()
-        .map_err(|e| Error::Unreachable(e.to_string()))?;
-    source.read(id, kind, &body)
+        .map(Some)
+        .map_err(|e| Error::Unreachable(e.to_string()))
 }
 
 /// Who is asking.