MW-1026: Avoiding session reset upon database preview

[lajosarpad]

This is a performance patch, a direct follow-up of my chat with @Shnoulle about session reset in setVariableAndTokenMappingsForExpressionManager being executed too often, slowing down the entire system. According to my tests, there are cases when we set the preview mode to 'database' and therefore we reset for each group because of the criteria at

LimeSurvey/application/helpers/expressions/em_manager_helper.php

Line 3279 in ad0a1d6

} elseif ($forceRefresh === false && !empty($this->knownVars) && !$this->sPreviewMode) {

We can see that if the preview is not equivalent to false, then we end up doing the entirety of the reset for each and every call, so when we loop the groups at

LimeSurvey/application/models/services/Proxy/ProxyExpressionManager.php

Line 63 in ad0a1d6

LimeExpressionManager::StartProcessingGroup(

force refresh being false, but at

LimeSurvey/application/helpers/expressions/em_manager_helper.php

Line 4539 in ad0a1d6

$LEM->setVariableAndTokenMappingsForExpressionManager($_SESSION['LEMsid']);

which is being executed for each group we already have the preview mode being set to 'database' because of

LimeSurvey/application/models/services/Proxy/ProxyExpressionManager.php

Line 49 in ad0a1d6

LimeExpressionManager::SetPreviewMode('database');

so we do a reset for each and every group and that eats up all our memory and time resources, creating spikes.

We will need to check other use-cases of this feature as well whether we have similar preview mode sets that slow down the entire system.

Measurements:

Survey1 Text Element Save 1 15.49s - 3.25
Survey1 Reload: 1.85 - 0.34s
Survey1 Start: 2.62s - 1.94s
Survey2 Reload: 11.56s -> 0.81s
Survey2 Text Element Save 4.3 minutes - 40.68s
Survey2 Start: 21.6s - 20 s

[Shnoulle]

Seems really great for improvement, we can do the same for previewmode == 'logicfile' (unsure on the code here).

But a better improvement can be : add a var to know if it's already done ?

Something like, using global var (then the var don't enter in EM)

    public function setVariableAndTokenMappingsForExpressionManager($surveyid, $forceRefresh = false, $anonymized = false)
    {
        if (isset($_SESSION['LEMforceRefresh'])) {
            unset($_SESSION['LEMforceRefresh']);
            $forceRefresh = true;
        } elseif ($forceRefresh === false && !empty($this->knownVars) && !$this->sPreviewMode && !App()->getConfig('FreshVariableAndTokenMappingsForExpressionManager')) {
            return false;   // means that those variables have been cached and no changes needed
        }
        /* Set this variable is refreshed */
        App()->setConfig('FreshVariableAndTokenMappingsForExpressionManager') = true;
        // Here we do all action : refresh or create VariableAndTokenMappingsForExpressionManager
    }