ISteamNetworkingSockets: Trying to use over 8 network lanes/channels fails #455

Hello! We're pivoting our Godot project's network services to Steam's, using ISteamNetworkingSockets via GodotSteam (GodotSteam versions 4.17.1 and 4.23, if relevant) instead of EOS.

We'd split our traffic into 21 channels while using EOS to get around another issue we were having, but found when migrating to Steam that Steam appears to only support 8 - if you try to use more than 8 channels/lanes, you'll get a message like:

Steam connection 2199661473 status changed: 3 -> 4. End reason: 5002 (SNP decode unreliable msgnum underflow. 0 mod 10000, highest seen 0). Remote: 76561198127298424. Description: #2199661473 P2P ICE steamid:76561198127298424 vport 41500

30.300396825394, peer 1: Client left: 704154296

After some research (I assumed it was my code first, then GodotSteam's second, I now see this in GetLane() in src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp (main branch as of this writing):

// Get a conservative size for the header we will need to select the specified lane.
	// This assumes the lanes will be encoded in order.
	if ( nLaneID == 0 ) // Lane zero is the default
	{
		Assert( m_idxLane0 == -1 );
		pNewLane->m_cbHdr = 0;
		m_idxLane0 = len( m_vecLanes )-1;
	}
	else
	{
		if ( nLaneID <= 7 )
		{
			pNewLane->m_cbHdr = 1;
			pNewLane->m_hdr[0] = (uint8)( 0x87 + nLaneID );
		} else {
			pNewLane->m_hdr[0] = 0x8f;
			uint8 *p = SerializeVarInt( &pNewLane->m_hdr[1], (unsigned)nLaneID );
			pNewLane->m_cbHdr = p - &pNewLane->m_hdr[1];
		}
		m_cbRemainingForSegments -= pNewLane->m_cbHdr;
	}

Specifically, if I'm reading it right, this line may be in error:

pNewLane->m_cbHdr = p - &pNewLane->m_hdr[1];

It looks like this logic is trying to put the number of bytes for the lane being used into pNewLane->m_cbHdr - so for 8 lanes, this ought to be 2 bytes, but the above logic will return 1, and the connection will fail with the above error. I think that line should read:

pNewLane->m_cbHdr = p - &pNewLane->m_hdr[0];

Sorry if this isn't the right place to report this! Also happy to be wrong if I'm misreading how that works. Thanks!