Result of calls to enif_send

[evnu]

enif_send returns a flag, indicating if the send succeeded or not. From the documentation:

Returns true if the message is successfully sent. Returns false if the send operation fails, that is:

• *to_pid does not refer to an alive local process.
• The currently executing process (that is, the sender) is not alive.

Functions such as Env::send() or OwnedEnv::send_and_clear() could indicate the success with Result.

[evnu]

Currently, enif_send is only used in Env::send() and OwnedEnv::send_and_clear(). For OwnedEnv, we know that a failure indicates that to_pid is dead, as we do not send from a process here. For Env::send(), both the sending process or the receiver could be dead, but we cannot reliably know which of them is gone. We could therefore distinguish two error cases:

pub enum SendError {
  ReceiverDead,
  SenderOrReceiverDead
}

Then, the signature for OwnedEnv::send_and_clear() would become:

pub fn send_and_clear<F>(&mut self, recipient: &Pid, closure: F) -> Result<(), SendError>
where
   F: for<'a> FnOnce(Env<'a>) -> Term<'a>,

And for Env::send():

pub fn send(self, pid: &Pid, message: Term<'a>) -> Result<(), SendError>

[evnu]

See also this PR: #563