ConfigureConnectionLanes: reconfiguring lanes can leave one lane in a stale priority class #453

What happens

The docs for ConfigureConnectionLanes say lanes may be reconfigured at any time. When a connection reconfigures its lanes, the lane with the lowest priority value (the one that sorts first) keeps whatever m_idxPriorityClass it had from the previous configuration. Every other lane gets its class index reassigned.

Swapping priorities on two lanes is enough to see it. Lane 1 should now be strictly ahead of lane 0, but it stays in class 1 next to lane 0. Class 0 ends up with no lanes, and the two lanes share bandwidth by weight instead of by priority.

If the stale index is greater than or equal to the new number of priority classes, m_vecPriorityClasses[ l.m_idxPriorityClass ] in the "Setup each lane" loop reads past the end of the vector. For example: priorities { 0, 1, 2 }, then { 1, 1, 0 }. Lane 2 keeps class 2, but there are now only 2 classes.

What should happen

After every call, each lane's priority class should match the priorities passed to that call.

Minimal repro

// hSender from CreateSocketPair( ..., true, ... ), fixed send rate of 128 KB/s
int prio1[2] = { 0, 1 };
ConfigureConnectionLanes( hSender, 2, prio1, nullptr );
int prio2[2] = { 1, 0 };
ConfigureConnectionLanes( hSender, 2, prio2, nullptr );
// queue 64 KB of unreliable messages on each lane, then:
GetConnectionRealTimeStatus( hSender, &status, 2, laneStatus );

Expected: lane 1 queue time is about 500 ms and lane 0 is about 1000 ms.
Actual: both lanes report 1000 ms, because they are scheduled in the same class.

Root cause

In CSteamNetworkConnectionBase::SNP_ConfigureLanes (steamnetworkingsockets_snp.cpp), the loop that merges sorted priority classes starts at j = 1 and only assigns m_idxPriorityClass for pPriorityClass[j]. The entry at sorted position 0 is never assigned. On the first configuration this goes unnoticed, because a newly created lane defaults to class 0. On a later reconfiguration, that lane may already exist with a nonzero class.