net: Consider having from_std panic on blocking sockets

[carllerche]

While Tokio technically accepts and can function using blocking sockets, it usually results in poor and unexpected runtime behaviors. There is most likely no good reason to pass a blocking socket to from_std. Accidentally passing blocking sockets to from_std is a common error that results in hard-to-debug runtime behaviors.

Because Tokio has historically technically accepted blocking sockets to be passed to from_std, changing the behavior to panic on blocking sockets could be considered a breaking change. We are considering making that change anyway because we are not aware of any valid usage.

As a first step, we changed Tokio to panic when passing a blocking socket to from_std and debug assertions are enabled. Use the tokio_allow_from_blocking_fd cfg flag to disable this assertion.

If you have a valid use case for passing blocking sockets to from_std, please comment in this thread.

[Darksonn]

This is being implemented in #7166. Please also see #5595 that tracks the problem that this proposal is intended to solve.