See this error (somehow triggered by a new version of boringssl):
https://grpc-testing.appspot.com/job/gRPC_pull_requests_msan_c/1154/testReport/junit/(root)/c_linux_msan/bins_msan_handshake_client_GRPC_POLL_STRATEGY_poll_cv/
In the alpn callback, in_len is the size of the in buffer and not the number of alpn elements.