Fix duplicate messages at backup page boundaries

Co-authored-by: Rolfe Schmidt <141083381+rolfe-signal@users.noreply.github.com>

ts/sql/Interface.std.ts

@@ -589,7 +589,7 @@ export type PageMessagesResultType = Readonly<{
 export type PageBackupMessagesCursorType = Readonly<{
   __page_backup_messages_cursor: never;
 
-  nextRowid: number;
+  lastRowId: number;
   done: boolean;
 }>;
 

ts/sql/Server.node.ts

@@ -9186,7 +9186,8 @@ function pageBackupMessages(
       ${MESSAGE_COLUMNS_FRAGMENT}
     FROM messages
     WHERE
-      rowid >= ${cursor?.nextRowid ?? 0}
+      rowid > ${cursor?.lastRowId ?? 0}
+    ORDER BY rowid ASC
     LIMIT ${LIMIT}
   `;
   const rows: Array<MessageTypeUnhydrated & { rowid: number }> = db
@@ -9194,7 +9195,7 @@ function pageBackupMessages(
     .all(params);
   return {
     cursor: {
-      nextRowid: rows.at(-1)?.rowid ?? 0,
+      lastRowId: rows.at(-1)?.rowid ?? 0,
       done: rows.length < LIMIT,
     } as PageBackupMessagesCursorType,
     messages: hydrateMessages(db, rows),

ts/test-node/sql/pageBackupMessages_test.node.ts

@@ -0,0 +1,126 @@
+// Copyright 2026 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { cwd } from 'node:process';
+import { assert } from 'chai';
+
+import type {
+  PageBackupMessagesCursorType,
+  WritableDB,
+} from '../../sql/Interface.std.ts';
+import { DataReader, setupTests } from '../../sql/Server.node.ts';
+import { createDB } from './helpers.node.ts';
+
+describe('SQL/pageBackupMessages', () => {
+  // Must match the LIMIT inside pageBackupMessages.
+  const PAGE_SIZE = 1000;
+
+  let db: WritableDB;
+
+  beforeEach(() => {
+    db = createDB();
+    setupTests(db, { userDataPath: cwd() });
+  });
+
+  afterEach(() => {
+    db.close();
+  });
+
+  function insertMessages(count: number): void {
+    const stmt = db.prepare(`
+      INSERT INTO messages (id, conversationId, json)
+      VALUES ($id, $conversationId, $json)
+    `);
+    db.transaction(() => {
+      for (let i = 0; i < count; i += 1) {
+        stmt.run({
+          id: `m${i}`,
+          conversationId: 'c1',
+          json: JSON.stringify({}),
+        });
+      }
+    })();
+  }
+
+  function pageAll(): {
+    ids: Array<string>;
+    pages: number;
+  } {
+    const ids: Array<string> = [];
+    let pages = 0;
+    let cursor: PageBackupMessagesCursorType | undefined;
+
+    while (!cursor?.done) {
+      const { messages, cursor: nextCursor } = DataReader.pageBackupMessages(
+        db,
+        cursor
+      );
+      ids.push(...messages.map(message => message.id));
+      cursor = nextCursor;
+      pages += 1;
+
+      assert.isBelow(pages, 100, 'pagination did not terminate');
+    }
+
+    return { ids, pages };
+  }
+
+  it('returns every message exactly once when paging', () => {
+    const count = PAGE_SIZE * 2 + 500;
+    insertMessages(count);
+
+    const { ids } = pageAll();
+
+    assert.strictEqual(ids.length, count, 'wrong number of messages');
+    assert.strictEqual(
+      new Set(ids).size,
+      count,
+      'a message was returned more than once'
+    );
+  });
+
+  it('does not repeat the row on a page boundary', () => {
+    insertMessages(PAGE_SIZE + 1);
+
+    const first = DataReader.pageBackupMessages(db);
+    assert.strictEqual(first.messages.length, PAGE_SIZE);
+    assert.isFalse(first.cursor.done);
+
+    const second = DataReader.pageBackupMessages(db, first.cursor);
+    assert.deepStrictEqual(
+      second.messages.map(message => message.id),
+      [`m${PAGE_SIZE}`],
+      'second page must start after the last row of the first page'
+    );
+    assert.isTrue(second.cursor.done);
+  });
+
+  it('terminates on an exact multiple of the page size', () => {
+    const count = PAGE_SIZE * 2;
+    insertMessages(count);
+
+    const { ids, pages } = pageAll();
+
+    assert.strictEqual(ids.length, count);
+    assert.strictEqual(new Set(ids).size, count);
+    // Two full pages, then one empty page to learn that we are done.
+    assert.strictEqual(pages, 3);
+  });
+
+  it('returns messages in ascending rowid order', () => {
+    const count = PAGE_SIZE + 10;
+    insertMessages(count);
+
+    const { ids } = pageAll();
+
+    const expected = Array.from({ length: count }, (_, i) => `m${i}`);
+    assert.deepStrictEqual(ids, expected);
+  });
+
+  it('handles an empty table', () => {
+    const { ids, pages } = pageAll();
+
+    assert.deepStrictEqual(ids, []);
+    assert.strictEqual(pages, 1);
+  });
+});