Alert/Report screenshots showing "Unexpected error" even though it shows no error on server side

[zhaorui2022]

We have some user reporting the screenshots they received for their superset alerts or reports show "Unexpected error. See more" where the see more is supposed to be a link we can click on.

Since we can't click on the "See More" in the screenshot, we are trying to figure out the root cause by ruling out causes. After some investigation, we think it might be caused by timeouts because:

1. The user we use to capture screenshots have admin role attached, so it should not be a permission issue
2. There was no issue on the backend database or query engine
3. Not all charts on those dashboard screenshots show this error, and it's a different set of charts showing the error every time users receive screenshot emails
4. If the user click on the "Explorer in Superset" in the email, the dashboards load immediately and we think it's because the data is cached.

However, while looking at the log on the server side, there was no error at all. And the only thing it goes the theory that this is caused by timeout is that the exception is supposed to be caught here https://github.com/apache/superset/blob/master/superset/reports/commands/execute.py#L229

How to reproduce the bug

It's a bit tricky to reproduce, but we have been seeing this happening consistently with charts that takes a long time to load. It happens even more with the reports scheduled to run weekly.

Expected results

Either show the correct content in the screenshots or throw an error on the server side.

Actual results

No error on the server side but shows an unexpected error in the screenshot.

Screenshots

Environment

(please complete the following information):

• browser type and version: We are using latest version of firefox to capture screenshots on the server side
• superset version: 2.0.0, but it has been happening for quite a while
• python version: 3.8
• node.js version: N/A
• any feature flags active: Alert and Reports are enabled

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.

[zhaorui2022]

I have some thought maybe the reason why there is no error in server log is because the chart times out because it takes longer than SUPERSET_WEBSERVER_TIMEOUT to load which has a lower value than SCREENSHOT_LOAD_WAIT. Because of that, "loading" elements will be gone eventually when we take screenshots. And then the server won't be able to catch those timeout exceptions. But since there is no log, it's just an assumption.