WISH: isAlive()

[HenrikBengtsson]

Add S3 methods `isAlive() for 'cluster' to check whether or not the R process of a node is still running, e.g.

> cl <- makeClusterPSOCK(4L)
> isAlive(cl)
[1] TRUE TRUE TRUE TRUE
> ans <- clusterEvalQ(cl[3], { quit("no") })
> isAlive(cl)
[1] TRUE TRUE FALSE TRUE

Not all types of 'cluster' support this, or it will be complicated to achieve. For instance, for cluster nodes running on remote machines, we would have to SSH into each of the remote machines and query the process via a remote ps --pid=PID system call.

When not supported for some types of node types, isAlive() could return NA, e.g.

> isAlive(cl)
[1] TRUE NA NA FALSE