feat: read subscriptions from an Atom feed when a page lists no dated links

app/lib/application/actors/fetch_actor.dart

@@ -1,6 +1,7 @@
 import 'package:actors/actors.dart';
 import 'package:http/http.dart' as http;
 
+import '../../mews/atom.dart';
 import '../../mews/document.dart';
 import '../../mews/parser.dart';
 
@@ -31,6 +32,14 @@ class SubmitForm extends FetchCommand {
   final Map<String, String> fields;
 }
 
+/// Fetch and read a subscribed page's Atom feed (spec 6.2). [url] is absolute
+/// and on the page's own site, which the parser has already checked.
+class FetchFeed extends FetchCommand {
+  const FetchFeed({required this.url});
+
+  final String url;
+}
+
 /// Parses a page's source without touching the network; used to prepare the
 /// neighboring pages that swipes reveal.
 class ParseSource extends FetchCommand {
@@ -44,6 +53,14 @@ sealed class FetchResult {
   const FetchResult();
 }
 
+/// The posts an Atom feed lists.
+class FetchFeedOk extends FetchResult {
+  const FetchFeedOk({required this.url, required this.entries});
+
+  final String url;
+  final List<MewsEntry> entries;
+}
+
 class FetchPageOk extends FetchResult {
   const FetchPageOk({
     required this.page,
@@ -103,6 +120,7 @@ class FetchHandler with Handler<FetchCommand, FetchResult> {
         await _fetchPage(url, etag, lastModified, cachedSource),
       SubmitForm(:final url, :final method, :final fields) =>
         await _submitForm(url, method, fields),
+      FetchFeed(:final url) => await _fetchFeed(url),
       ParseSource(:final url, :final source) => _parse(source, url),
     };
   }
@@ -149,6 +167,28 @@ class FetchHandler with Handler<FetchCommand, FetchResult> {
     }
   }
 
+  Future<FetchResult> _fetchFeed(String url) async {
+    final http.Response response;
+    try {
+      response = await _client
+          .get(Uri.parse(url), headers: {'User-Agent': _userAgent})
+          .timeout(const Duration(seconds: 20));
+    } catch (_) {
+      return const FetchNetworkError();
+    }
+    if (response.statusCode != 200) {
+      return FetchHttpStatusError(response.statusCode);
+    }
+    try {
+      return FetchFeedOk(
+        url: url,
+        entries: parseAtomEntries(source: response.body, feedUrl: url),
+      );
+    } on MewsParseException catch (e) {
+      return FetchParseError(e.message, response.body);
+    }
+  }
+
   Future<FetchResult> _submitForm(String url, String method, Map<String, String> fields) async {
     final headers = {'User-Agent': _userAgent};
     final http.Response response;

app/lib/application/controllers/subscriptions_controller.dart

@@ -43,9 +43,14 @@ class SubscriptionsController {
     }
     await _db.addSubscription(page.url, page.title, page.atomFeedHref);
     subscribedUrls.value = {...subscribedUrls.value, page.url};
-    // The posts already on the page are the site's back catalogue, not news,
-    // so they are stored read and only later posts are flagged.
+    // Whatever the site has already published is its back catalogue, not
+    // news, so it is stored read and only later posts are flagged. A site
+    // that publishes only through a feed is seeded the same way.
     await _db.addEntries(page.url, page.entries, read: true);
+    final feed = page.atomFeedHref;
+    if (page.entries.isEmpty && feed != null) {
+      await _checkFeed(page.url, feed, read: true);
+    }
   }
 
   Future<void> remove(String siteUrl) async {
@@ -127,6 +132,13 @@ class SubscriptionsController {
         await _db.describeSubscription(siteUrl,
             title: page.title, atomHref: page.atomFeedHref);
         await _db.addEntries(siteUrl, page.entries);
+        // Dated links are the normative way a site publishes (spec 6.1), so
+        // the feed is only read when the page itself listed nothing. A site
+        // where both agree is then fetched once, not twice.
+        final feed = page.atomFeedHref;
+        if (page.entries.isEmpty && feed != null) {
+          await _checkFeed(siteUrl, feed);
+        }
         if (!notModified) {
           // Keep the cache current so the site also reads offline, but do not
           // record a visit: the reader did not open it.
@@ -144,4 +156,14 @@ class SubscriptionsController {
     }
     await _db.touchSubscription(siteUrl, DateTime.now());
   }
+
+  /// Reads a site's Atom feed (spec 6.2). Entries are keyed by address, so a
+  /// post listed by both the page and the feed is stored once.
+  Future<void> _checkFeed(String siteUrl, String feedUrl,
+      {bool read = false}) async {
+    final result = await _fetch.send(FetchFeed(url: feedUrl));
+    if (result case FetchFeedOk(:final entries)) {
+      await _db.addEntries(siteUrl, entries, read: read);
+    }
+  }
 }