HTTP/3, Safari, & Google App Engine

Just as an FYI since I wasn’t able to find this information elsewhere…

Our team spent the last ~24h frantically debugging a gnarly issue where iOS Safari was sporadically having trouble fetching information from our Google App Engine backend (using Apollo GraphQL).

The error in question showed “The network connection was lost” in Safari Web Inspector.

We found this issue challenging to debug for a few different reasons:

We tried several things to mitigate and understand what might have gone wrong:

Finally, we found that on the phone having the issues, HTTP/3 was enabled in Safari iOS (in Experimental Settings). Disabling HTTP/3 on Safari iOS fixed the problem.

Things to look into:

Your inputs, ideas, and feedback are welcome. It’s still unclear to me which party is at fault, though Safari certainly could use more detailed error logging…!

[Apple friends; this is filed in Feedback as FB8924034]