findIdsByQuery() can return duplicate message IDs when UID input is chunked

Steps to reproduce

1. Use a mailbox with more than 1000 locally known messages.
2. Perform a filtered mailbox sync so that SyncService calls MessageMapper::findIdsByQuery() with a large UID array.
3. Instrument the number of UIDs passed to findIdsByQuery() and the number of message IDs returned.
4. Compare the returned IDs before and after deduplication.

In our production case:

changedUids = 40810
changedIds = 49010

After deduplicating the IDs returned from all UID chunks:

changedIds = 40810

Expected behavior

findIdsByQuery() should return each matching message ID only once.

Chunking the UID input into groups of 1000 should not change the logical result set or introduce duplicate message IDs.

Actual behavior

When $uids contains more than 1000 entries, findIdsByQuery() executes the same query once per UID chunk and concatenates the results:

if ($uids !== null) {
    return array_flat_map(function (array $chunk) use ($qb, $select) {
        $qb->setParameter('uids', $chunk, IQueryBuilder::PARAM_INT_ARRAY);

        return array_map(
            static fn (Message $message) => $message->getId(),
            $this->findEntities($select)
        );
    }, array_chunk($uids, 1000));
}

There is no deduplication when results from the chunks are combined.

In our case:

changedUids = 40810
changedIds before deduplication = 49010
changedIds after deduplication = 40810

This also caused unnecessary message hydration and additional PHP memory usage.

During testing, memory usage around this stage dropped approximately from:

~224 MB

to:

~190 MB

after deduplicating the result.

A tested workaround is:

if ($uids !== null) {
    $resultIds = [];

    foreach (array_chunk($uids, 1000) as $chunk) {
        $select->setParameter('uids', $chunk, IQueryBuilder::PARAM_INT_ARRAY);

        foreach ($this->findIds($select) as $id) {
            $resultIds[$id] = $id;
        }
    }

    return array_values($resultIds);
}

Mail app version

5.12.0

Nextcloud version

34.0.2

Mailserver or service

IMAP GMAIL

Operating system

Debian GNU/Linux 13 (trixie)

PHP engine version

Other

Nextcloud memory caching

memcache.local: APCu memcache.distributed: Redis memcache.locking: Redis

Web server

Apache (supported)

Database

PostgreSQL

Additional info

PHP: 8.5.10
Web server: Apache 2.4.68
OS: Debian GNU/Linux 13 (trixie)

Nextcloud memory caching:
- memcache.local: \OC\Memcache\APCu
- memcache.distributed: \OC\Memcache\Redis
- memcache.locking: \OC\Memcache\Redis

PHP memory_limit: 1024M

The issue was observed while debugging mailbox sync memory consumption on a mailbox with approximately 41,000 locally known messages.

Start at MessageMapper::findIdsByQuery(), which SyncService calls during filtered mailbox syncs with large UID arrays. Reproduce the behavior using more than 1000 UIDs and inspect how chunk results are combined. Done means each matching message ID is returned once, chunking does not change the logical result set, and the behavior is covered by an appropriate test.