Fix duplicate feed items from concurrent syncs

A sync button press racing a page-reload sync could run two sync
requests concurrently. create_feed_item used a check-then-insert
pattern (query for an existing item, then insert if none found), so
both requests could pass the check before either inserted, creating
duplicate items.

- Add a UNIQUE (feed_id, url) constraint on feed_item, keyed on the
  article's link rather than its title since that's RSS's stable
  article identity (a feed editing a headline after publishing, same
  link, would otherwise dupe under title-based matching).
- The migration first collapses any duplicates already created by the
  race, propagating read=true onto the surviving row when any
  duplicate in its group was already read, so the cleanup can't make
  an already-read article look unread.
- create_feed_item now does a single atomic
  INSERT ... ON CONFLICT (feed_id, url) DO NOTHING instead of
  select-then-insert, closing the race entirely.
- Add a test that races two threads on separate connections inserting
  the same item and asserts only one row survives.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CHcxDSPbHhe7sLJQBRVC9

migrations/2026-09-10-000000_add_unique_feed_item_url_per_feed/down.sql

@@ -0,0 +1,3 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE feed_item
+DROP CONSTRAINT feed_item_feed_id_url_unique;

migrations/2026-09-10-000000_add_unique_feed_item_url_per_feed/up.sql

@@ -0,0 +1,36 @@
+-- Your SQL goes here
+
+-- Concurrent syncs for the same feed could previously race past the
+-- application-level "does this article already exist" check and both
+-- insert, producing duplicate feed items. Before enforcing uniqueness,
+-- collapse any duplicates that already exist, keyed on the article's link
+-- (rather than its title) since that's the stable identity RSS items are
+-- built around — some feeds edit a headline after publishing while keeping
+-- the same link, which title-based matching would treat as a new article.
+
+-- If any duplicate in a group was already read, propagate that onto the
+-- surviving (lowest-id) row first, so this one-time cleanup can't make an
+-- already-read article look unread again.
+UPDATE feed_item AS keep
+SET read = true
+FROM feed_item AS dupe
+WHERE keep.feed_id = dupe.feed_id
+  AND keep.url = dupe.url
+  AND keep.id <> dupe.id
+  AND keep.id = (
+    SELECT MIN(candidate.id)
+    FROM feed_item AS candidate
+    WHERE candidate.feed_id = dupe.feed_id
+      AND candidate.url = dupe.url
+  )
+  AND dupe.read = true;
+
+-- Keep the oldest (lowest id) row of each duplicate set.
+DELETE FROM feed_item a
+USING feed_item b
+WHERE a.feed_id = b.feed_id
+  AND a.url = b.url
+  AND a.id > b.id;
+
+ALTER TABLE feed_item
+ADD CONSTRAINT feed_item_feed_id_url_unique UNIQUE (feed_id, url);

src/reader/sync.rs

@@ -4,8 +4,7 @@ use crate::error::AppError;
 use crate::json_serialization::user::JsonUser;
 use crate::models::feed::rss_feed::Feed;
 use crate::models::feed_item::new_feed_item::NewFeedItem;
-use crate::models::feed_item::rss_feed_item::FeedItem;
-use crate::schema::feed_item::{feed_id, title};
+use crate::schema::feed_item::{feed_id, url};
 use crate::{
     database::establish_connection,
     schema::{
@@ -143,24 +142,29 @@ fn create_feed_item(item: Item, feed: &Feed, connection: &mut PgConnection) -> a
         }
     }
 
-    let existing_item: Vec<FeedItem> = feed_item::table
-        .filter(feed_id.eq(feed.id))
-        .filter(title.eq(&item_title))
-        .load(connection)?;
+    let new_feed_item = NewFeedItem::new(
+        feed.id,
+        content.clone(),
+        item_title.clone(),
+        item.link.expect("checked above"),
+        Some(time),
+    );
 
-    if existing_item.is_empty() {
-        let new_feed_item = NewFeedItem::new(
-            feed.id,
-            content.clone(),
-            item_title.clone(),
-            item.link.expect("checked above"),
-            Some(time),
-        );
-        let insert_result = diesel::insert_into(feed_item::table)
-            .values(&new_feed_item)
-            .execute(connection);
+    // `on_conflict` on the (feed_id, url) unique constraint makes this
+    // insert idempotent, so two syncs for the same feed running concurrently
+    // (e.g. a sync button press racing a page-reload sync) can't both pass a
+    // check-then-insert race and create duplicate items. Keying on the
+    // article's link rather than its title also handles feeds that edit a
+    // headline after publishing while keeping the same link — the link is
+    // the stable identity.
+    let inserted_rows = diesel::insert_into(feed_item::table)
+        .values(&new_feed_item)
+        .on_conflict((feed_id, url))
+        .do_nothing()
+        .execute(connection)?;
 
-        log::info!("Insert Result: {:?}", insert_result);
+    if inserted_rows > 0 {
+        log::info!("Inserted item: {}", item_title);
     } else {
         log::info!("Item {} already exists.", item_title);
     }
@@ -210,10 +214,11 @@ pub async fn sync(
 #[cfg(test)]
 mod tests {
     use crate::models::feed::new_feed::NewFeed;
+    use crate::models::feed_item::rss_feed_item::FeedItem;
     use crate::models::user::new_user::NewUser;
     use crate::models::user::rss_user::User;
     use crate::schema::users;
-    use crate::test_helpers::unique_suffix;
+    use crate::test_helpers::{delete_feed, delete_user, insert_feed, insert_user, unique_suffix};
     use chrono::Duration;
 
     use super::*;
@@ -440,6 +445,58 @@ mod tests {
             .ok();
     }
 
+    #[actix_web::test]
+    async fn create_feed_item_is_race_safe_under_concurrent_syncs() {
+        let mut connection = establish_connection();
+        let suffix = unique_suffix();
+
+        let user = insert_user(&mut connection, "secret");
+        let feed = insert_feed(&mut connection, user.id);
+
+        let mut item = Item::default();
+        item.set_title(Some(format!("Race test article {suffix}")));
+        item.set_link(Some(format!("https://example.test/race/{suffix}")));
+        item.set_content(Some("<p>Hello world</p>".to_string()));
+
+        // Simulate two concurrent syncs for the same feed (e.g. a sync
+        // button press racing a page-reload sync) — each on its own
+        // connection/thread, released together so they genuinely overlap —
+        // both inserting the same item.
+        let barrier = std::sync::Arc::new(std::sync::Barrier::new(2));
+        let handles: Vec<_> = (0..2)
+            .map(|_| {
+                let feed = feed.clone();
+                let item = item.clone();
+                let barrier = barrier.clone();
+                std::thread::spawn(move || {
+                    let mut connection = establish_connection();
+                    barrier.wait();
+                    create_feed_item(item, &feed, &mut connection)
+                })
+            })
+            .collect();
+
+        for handle in handles {
+            handle.join().unwrap().unwrap();
+        }
+
+        let items: Vec<FeedItem> = feed_item::table
+            .filter(feed_id.eq(feed.id))
+            .load(&mut connection)
+            .unwrap();
+        assert_eq!(
+            1,
+            items.len(),
+            "concurrent syncs must not create duplicate feed items"
+        );
+
+        diesel::delete(feed_item::table.filter(feed_id.eq(feed.id)))
+            .execute(&mut connection)
+            .ok();
+        delete_feed(&mut connection, feed.id);
+        delete_user(&mut connection, user.id);
+    }
+
     #[actix_web::test]
     async fn create_feed_item_strips_onerror_from_feed_image() {
         let mut connection = establish_connection();