/mark unhandled error: "TypeError: Cannot read property 'id' of undefined"

[danoswaltCL]

Describe the bug
Seeing this for markExperimentPoint sometimes in QA/Prod logs:

"TypeError: Cannot read property 'id' of undefined"

To Reproduce
I'm not sure, it's easy to see exactly where and why the error happens codewise, but I'm not sure how to reproduce or what it means if this is unexpectedly undefined.

if (experimentDecisionPoint.length) {
        let selectedExperimentDP = experimentDecisionPoint.filter( dp => dp.experiment.id === experimentId);
        const decisionPointId = selectedExperimentDP[0].id; //<---- oops, selectedExperimentDP can be undefined

Expected behavior
Handle the error w/o 500 internal error throw

Screenshots

[danoswaltCL]

I was able to reproduce this. This would be when marking without sending the experimentId, as is how it is currently used by Mathia.

A user who is technically eligible for at least 1 experiment loaded into upgrade in that environment that marks a valid decision point in an experiment that they are excluded from will get the above error.

The experiment they are eligible for can be in "Enrolling" or "Enrollment Complete" state.
The experiment they are excluded from can be in any state, even in "Inactive".

This allows them to follow a specific path through the code where they will hit this line of code in the else, where experimentId will be set to an empty string:

      if (!experimentId) { 
        if (filteredExperiments.length > 1) {
          const random = seedrandom(userId)();
          experimentId = filteredExperiments[Math.floor(random * experiments.length)].id;
        } else {
          experimentId = filteredExperiments[0] ? filteredExperiments[0].id : ''; // <--- filteredExperiments can be empty 
        }
      }

Which then fails on this line, because it can't find any experiment id that equals empty string. (Sidebar, how come typescript allows this?? The types are defined for these arrays just fine...)

if (experimentDecisionPoint.length) {
        let selectedExperimentDP = experimentDecisionPoint.filter( dp => dp.experiment.id === experimentId);
        const decisionPointId = selectedExperimentDP[0].id; // <---- undefined, throws 500 error

in local, you can try this:

1. Create an experiment with include ALL and a single decision point (Experiment A)
2. Create another experiment with a different decision point, Include ALL except a user with id "Test" (Experiment B)
3. Start Experiment A enrolling.
4. Initialize "Test" user.
5. /mark user at the valid decision point in experiment B
6. You should see this error.

If you delete Experiment A, the error will not happen.

I can think of quick fixes for this, but would prefer if someone who knows the logic here took a look into this, there's a lot going on tied together here and I'm not sure I can see what is "supposed" to happen in this scenario per the code.

[zackcl]

I was able to reproduce this error even without excluding the user in the second experiment.

selectedExperimentDP.error.mov

[danoswaltCL]

@ppratikcr7 is this something you can look into as a priority? It may be a simple fix, but it feels like it should get a deeper look to make sure things are getting handled correctly. We may need to have this patched into the current release.

[ppratikcr7]

@danoswaltCL Yes, looking into this.

[jreddig]

Seeing this in production again, both /mark and v1/mark apis are failing (rarely, but still failing) with Cannot read property 'id' of undefined. Haven't tried to reproduce locally yet, but wanted to call it out asap