BUG: Session cache with PDO backend is deadlock prone

[martinlipp]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Writing two different keys simultaneously into the session cache, can lead to a sql deadlock (when using a PDO backend for the Flow_Session_Storage cache).

Expected Behavior

The cache writing should be in such a way, that the possibility of deadlocks is as small as possible.

Steps To Reproduce

It is hard to reproduce reliably, since it depends on the current cache contents.

Environment

- Flow: 8.4
- PHP: 8.3

Anything else?

This is happening because the keys used for Session cache entries are in large parts not unique enough.
See

flow-development-collection/Neos.Flow/Classes/Session/Session.php

Line 477 in 2205332

$this->storageCache->set($this->storageIdentifier . md5($key), $data, [$this->storageIdentifier], 0);

For the key $this->storageIdentifier . md5($key) is used. The first part ($this->storageIdentifier) is always the same for the same Session. This can lead to deadlocks, while the database is trying to insert the index for the new entry (since the probability that the index needs to be placed in the same open spot is really high, with this not very well distributed key naming strategy).
A possible fix might be to md5 the whole combined string, not just the $key variable: md5($this->storageIdentifier . $key)

The same might be true for Flow 9.0:

flow-development-collection/Neos.Flow/Classes/Session/Data/SessionKeyValueStore.php

Line 95 in 7d1c46b

return $storageIdentifier->value . md5($key);