Back the catalog up daily, not only before migrations

NFR-R2 asks for the catalog to be backed up on a schedule and before
schema migrations. Only the second half existed: every backup on disk
was a pre-migration copy, and a library that never migrated was never
backed up at all.

A backup is now also taken at the end of a library sweep when the newest
one is more than a day old — the moment the catalog is quiet and a day's
collection and people edits have just been folded in — on its own
thread and its own connection, so the copy of a 130 MB file is not spent
on the UI. Whether one is due is read from the backup directory, not
the catalog, so the ordinary case costs nothing. An empty catalog is
skipped: there is nothing in it a rescan would not rebuild. Pruning to
KEEP_BACKUPS applies as before.

core/dr-catalog/src/recovery.rs

@@ -198,6 +198,54 @@ pub fn backup_before_migration(conn: &Connection, catalog: &Path) -> Result<(),
     Ok(())
 }
 
+/// How long a catalog may go without a backup before the next opportunity
+/// takes one.
+///
+/// A day. The catalog is an index, so what a backup protects is the day's
+/// worth of collection and people edits the sidecars do not hold — and a
+/// second copy of a 130 MB file per launch would be a cost with nothing to
+/// show for it when the user launches four times in an afternoon.
+pub const BACKUP_EVERY: i64 = 24 * 60 * 60;
+
+/// Whether [`BACKUP_EVERY`] has passed since the newest backup, or there is
+/// none.
+///
+/// Read from the filenames, like [`backups`], so a restored or copied backup
+/// directory answers the same way it did on the machine it came from.
+pub fn backup_due(catalog: &Path) -> bool {
+    match backups(catalog).first() {
+        Some(newest) => now() - newest.taken_at >= BACKUP_EVERY,
+        None => true,
+    }
+}
+
+/// TRACES: NFR-R2
+/// Take the scheduled backup, if one is due. Returns the file written, or
+/// `None` when the newest is recent enough.
+///
+/// The scheduled half of NFR-R2 — the migration half is
+/// [`backup_before_migration`]. "On a schedule" for an application that runs
+/// when the user opens it means "at the next chance after a day has passed",
+/// and the chance the caller picks is the end of a library sweep: the
+/// catalog is quiet, the work is already off the UI thread, and it is the
+/// moment a day's edits have just been consolidated.
+///
+/// A brand-new catalog with no images is not backed up: there is nothing in
+/// it yet that a rescan would not rebuild, and the first backup would only be
+/// a copy of an empty schema.
+pub fn backup_if_due(conn: &Connection, catalog: &Path) -> Result<Option<PathBuf>, CatalogError> {
+    if !backup_due(catalog) {
+        return Ok(None);
+    }
+    let images: i64 = conn.query_row("SELECT count(*) FROM images", [], |r| r.get(0))?;
+    if images == 0 {
+        return Ok(None);
+    }
+    let path = backup(conn, catalog)?;
+    log::info!("scheduled backup of the catalog to {}", path.display());
+    Ok(Some(path))
+}
+
 /// The backups available for `catalog`, newest first.
 ///
 /// Never fails: an unreadable or absent backup directory means there are no
@@ -386,6 +434,49 @@ mod tests {
         base
     }
 
+    #[test]
+    fn a_scheduled_backup_is_taken_once_a_day_and_not_more() {
+        let dir = tempdir("scheduled");
+        let path = dir.join("catalog.sqlite");
+        fixture(&path, 3);
+        let cat = Catalog::open(&path).unwrap();
+
+        // Nothing yet: due.
+        assert!(backup_due(&path));
+        let first = backup_if_due(cat.connection(), &path).unwrap();
+        assert!(first.is_some(), "the first opportunity takes one");
+
+        // Taken just now: not due, and a second call does nothing.
+        assert!(!backup_due(&path));
+        assert_eq!(backup_if_due(cat.connection(), &path).unwrap(), None);
+        assert_eq!(backups(&path).len(), 1);
+
+        // Age the one backup past the interval by renaming it, since the
+        // timestamp is read from the name. Now it is due again.
+        let old = first.unwrap();
+        let aged = old
+            .parent()
+            .unwrap()
+            .join(format!("catalog-{}.sqlite", now() - BACKUP_EVERY - 1));
+        std::fs::rename(&old, &aged).unwrap();
+        assert!(backup_due(&path));
+        assert!(backup_if_due(cat.connection(), &path).unwrap().is_some());
+        assert_eq!(backups(&path).len(), 2);
+
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn an_empty_catalog_is_not_worth_backing_up() {
+        let dir = tempdir("empty");
+        let path = dir.join("catalog.sqlite");
+        let cat = Catalog::open(&path).unwrap();
+        assert!(backup_due(&path), "due in principle");
+        assert_eq!(backup_if_due(cat.connection(), &path).unwrap(), None);
+        assert!(backups(&path).is_empty());
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     /// A catalog on disk with enough rows to span several pages, closed.
     ///
     /// Closed matters: WAL means the rows are in `catalog.sqlite-wal` until