unable to create a cluster/plan to a remote server in R - r

Question

Trying (still) to get a cluster running. I have two Linux (Ubuntu) computers, the server is "laptop". Pretty sure ssh works, with key authentication, I can ssh to "laptop" without a password from command line. ssh_exec_wait works, e.g.,
session <- ssh_connect("laptop")
a <- ssh_exec_wait(session, command = "whoami")
ssh_disconnect(session)
works. I've gone through the doc (A future for R), works fine until I get to cluster futures.
plan(cluster, workers = c("localhost"))
works fine, but
plan(cluster, workers = c("laptop"))
gives the error:
Warning: remote port forwarding failed for listen port 11968
List of 2
$ node_idx: int 1
$ node :List of 5
..$ con : 'sockconn' int 3
.. ..- attr(*, "conn_id")=<externalptr>
..$ host : chr "laptop"
.. ..- attr(*, "localhost")= logi FALSE
..$ rank : int 1
..$ rshlogfile : NULL
..$ session_info:List of 6
.. ..$ r :List of 15
.. .. ..$ platform : chr "x86_64-pc-linux-gnu"
.. .. ..$ arch : chr "x86_64"
.. .. ..$ os : chr "linux-gnu"
.. .. ..$ system : chr "x86_64, linux-gnu"
.. .. ..$ status : chr ""
.. .. ..$ major : chr "3"
.. .. ..$ minor : chr "6.3"
.. .. ..$ year : chr "2020"
.. .. ..$ month : chr "02"
.. .. ..$ day : chr "29"
.. .. ..$ svn rev : chr "77875"
.. .. ..$ language : chr "R"
.. .. ..$ version.string: chr "R version 3.6.3 (2020-02-29)"
.. .. ..$ nickname : chr "Holding the Windsock"
.. .. ..$ os.type : chr "unix"
.. ..$ system :List of 8
.. .. ..$ sysname : chr "Linux"
.. .. ..$ release : chr "5.3.0-46-generic"
.. .. ..$ version : chr "#38~18.04.1-Ubuntu SMP Tue Mar 31 04:17:56 UTC 2020"
.. .. ..$ nodename : chr "laptop"
.. .. ..$ machine : chr "x86_64"
.. .. ..$ login : chr "mark"
.. .. ..$ user : chr "mark"
.. .. ..$ effective_user: chr "mark"
.. ..$ libs : chr [1:4] "/home/mark/R/x86_64-pc-linux-gnu-library/3.6" "/usr/local/lib/R/site-library" "/usr/lib/R/site-library" "/usr/lib/R/library"
.. ..$ pkgs : NULL
.. ..$ pwd : chr "/home/mark"
.. ..$ process:List of 1
.. .. ..$ pid: int 2809
..- attr(*, "class")= chr "SOCKnode"
Error: Initialization of plan() failed, because the test future used for validation failed. The reason was: Unexpected result (of class ‘NULL’ != ‘FutureResult’) retrieved for ClusterFuture future (label = ‘future-plan-test’, expression = ‘NA’): . This suggests that the communication with ClusterFuture worker (‘SOCKnode’ #1) is out of sync.
I assume the first line is the real error message:
Warning: remote port forwarding failed for listen port 11968
Do I need to manually set the port?

Answer 1

In my experience, the warning
Warning: remote port forwarding failed for listen port 11968
is caused by a hanging R process on that port (it could be another process of course but it's likely that the future package created it). If you kill the process on laptop, the port should become available again.
You can check by manually running
ssh -R 11968:localhost:11968 laptop