gs: Fix LNS-initiated time transfers for Basic Station

[reissjason]

Summary

References #4852

All Basic Station gateways send timesync requests to correlate the concentrator counter with GPS time, regardless of PPS/GPS availability. The current code incorrectly assumes that a gateway sending a timesync request has PPS and disables LNS-initiated time transfers for that session. This prevents non-GPS gateways from receiving periodic time correlation updates.

Additionally, the LNS-initiated time transfer ticker passes nil for both gpsTime and concentratorTime (with a TODO referencing #4852), so even when transfers are enabled, only MuxTime is sent — never the xtime+gpstime pair needed for direct concentrator-to-GPS mapping.

Changes

• Do not disable LNS-initiated time transfers when a gateway sends a timesync request — both round-trip and direct mapping modes are needed.
• Track xtime and gpstime from recent uplinks (jreq/updf) in session state.
• Use uplink-derived xtime+gpstime (extrapolated to current time) in LNS-initiated time transfers, per the Basic Station protocol specification.

For GPS-equipped gateways, this enables the full direct concentrator↔GPS time mapping that #4852 describes (for frames from the same gateway). For non-GPS gateways, the round-trip timesync mechanism remains the primary time source, and the fix ensures it stays active.

Note: This is a partial fix for #4852. The full cross-gateway GPS time propagation (transferring GPS time from a PPS-equipped gateway to a non-GPS gateway via overlapping frames) requires frame deduplication at the Gateway Server layer, which is out of scope for this change.

Motivation

We are developing Class B Network-Assisted Synchronization (NAS) for LoRaWAN, which uses multicast ping slot downlinks on non-GPS gateways (MultiTech MTCAP3 with cellular backhaul). Accurate timesync is critical for ping slot TX scheduling. Without this fix, the LNS disables time transfers after the first timesync exchange, degrading the concentrator↔GPS correlation over time.

Testing

• Existing TestTransferTime passes unchanged.
• New TestTimeSyncDoesNotDisableTransferTime: Verifies that TransferTime produces output after a timesync request (previously it returned nil).
• New TestTransferTimeWithUplinkXTime: Verifies the end-to-end flow of storing uplink timing and using it in time transfers, for both GPS and non-GPS gateways.
• New TestLastUplink: Unit test for UpdateLastUplink/GetLastUplink session state round-trip.
• Full go test ./pkg/gatewayserver/io/semtechws/... passes with zero regressions.

Regressions

Not expected. The change is additive — time transfers that were previously disabled are now enabled, and previously nil arguments are now populated when uplink data is available.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

Made with Cursor

[johanstokking]

Thanks @reissjason, one small thing and mostly one question.

[johanstokking]

Suggested change

	if time.Since(rxAt) < 10*time.Second {
	elapsed := time.Since(rxAt)
	if elapsed := time.Since(rxAt); elapsed < maxTimeSyncAfterUplink {

And declare maxTimeSyncAfterUplink as const.

The time sync happens by default every 200 seconds. There might not be an uplink message in the last 10 seconds every period. On busy gateways, it may take long for time sync to happen.

What's the reasoning for the 10 seconds time since the last uplink? Clock drift?

[reissjason]

Looks like thia is only a partial fix. I'll close this one and make a new request once it ia complete.