v5 sync takes 5s when no new events

[Batmaev]

When I open Element X, it says "Sync..." for around 5s.

Investigation

curl -s -X POST "http://localhost:8008/_matrix/client/unstable/org.matrix.simplified_msc3575/sync?timeout=0" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"lists":{}}'

# timeout=0    -> Returns in 5s
# timeout=100  -> Returns in 5s
# timeout=6000 -> Returns in 6s
# timeout none -> Returns in ~12ms
#
# same empty {"pos":"..."} response

When using v3 (curl -s "http://localhost:8008/_matrix/client/v3/sync?timeout=100" -H "Authorization: Bearer $TOKEN"), it returns in ~12ms. Probably v3 checks for additional types of events -> they are present -> no waiting.

Conclusion: sync ignores the timeout parameter if it is less than 5s.

Environment: tuwunel v1.5.1, Element Pro v26.03.3 (61)

[jevolk]

Might be related to #386 but we have to adjust the specific approach there to merge it.

[jevolk]

Alright I have verified now that there is a configured minimum imposed on the timeout, and it's not related to #386. This didn't occur to me before because non-blocking behavior is indeed specified and I recall implementing that. Apparently that was reverted.

I have followed up on why that happened and that we can bring this into compliance, and I will be doing so shortly.

Thanks for reporting.