libp2p-mplex decode performance issue

[twoeths]

Describe the bug

• With subscribeToAllSubnets() flag, it takes 5% of cpu time to decode libp2p-mplex chunks

• 90% of the time was used to throw RangeError

1130_lodestar_subscribeAllSubnets_local.cpuprofile.zip

[dapplion]

Oh interesting, could you check the metrics to see if there were many invalid messages during the time that the CPU profile was taken? How could it be that we get sent so many invalid messages? Or the consumer code of this library is doing something strange and passing bad data to the decoder?

[twoeths]

@dapplion it's by design, after decoding a chunk's header successfully, the buffer is consumed the whole length, and it has 0 length, and varint.decode() will just throw RangeError in that case. This happens 50% of the time libp2p-mplex uses varint.decode() in that function

this could be a simple fix, will create a PR for it