Pace configuration-packet sends to avoid device UDP buffer overrun

[cboulay]

Problem

Configuring a device via Orion (Observatory) failed on a Gemini Hub running firmware 7.7.0 with repeated Response timeout on the runlevel sync barrier, and channel configuration never completed:

[CereLink] Clear-config sync failed: Response timeout   (×5)
[CereLink] Timed out waiting to confirm cleared channel configuration
[CereLink] Channel configuration failed

Root cause (verified on real Hub1 hardware)

Not a protocol issue — the Hub negotiates protocol 4.1. Orion's clear-config phase sends one setChannelConfig() per channel (272 channels × 2 = ~544 UDP datagrams) back-to-back with no pacing, then calls sync(). That burst overruns the device's small UDP receive buffer (~8 CHANINFO packets — the exact limit DeviceSession::sendPackets() already documents), dropping packets including the runlevel sync barrier, so sync() times out.

Newer firmware drains fast enough to survive the burst, which is why this looked like a 7.7.0 backward-compat regression.

Reproduced and fixed on hardware: unpaced → times out every retry; with pacing → clear-config + sync succeeds on the first attempt (3/3 runs).

Fix

DeviceSession::sendPacket() enforces a 200 µs minimum gap between consecutive configuration-channel sends (chid & cbPKTCHAN_CONFIGURATION). Data/streaming packets and isolated config sends (gap already elapsed) incur no delay. This extends the pacing that sendPackets() already does for its own batches to the per-packet path that callers use — so it protects every caller with no change required in Orion.

Testing

• New unit test ConfigSends_ArePaced_DataSends_AreNot — uses sleep_for's guaranteed-minimum semantics, so the timing assertion cannot be flaky.
• New examples/ClearConfigRepro/clear_config_repro.cpp reproduces the failure against real hardware (clear_config_repro HUB1, env PACE=<n> to compare).
• Full build + cbdev_device_tests / cbdev_tests / cbsdk_tests pass. (ConnectionParams_Predefined_NPlay fails on master too — pre-existing, unrelated.)

🤖 Generated with Claude Code