## What changes were proposed in this pull request?
Make it possible for the master to enable TCP keep alive on the RPC connections with clients.
## How was this patch tested?
Manually tested.
Added the following:
```
spark.rpc.io.enableTcpKeepAlive true
```
to spark-defaults.conf.
Observed the following on the Spark master:
```
$ netstat -town | grep 7077
tcp6 0 0 10.240.3.134:7077 10.240.1.25:42851 ESTABLISHED keepalive (6736.50/0/0)
tcp6 0 0 10.240.3.134:44911 10.240.3.134:7077 ESTABLISHED keepalive (4098.68/0/0)
tcp6 0 0 10.240.3.134:7077 10.240.3.134:44911 ESTABLISHED keepalive (4098.68/0/0)
```
Which proves that the keep alive setting is taking effect.
It's currently possible to enable TCP keep alive on the worker / executor, but is not possible to configure on other RPC connections. It's unclear to me why this could be the case. Keep alive is more important for the master to protect it against suddenly departing workers / executors, thus I think it's very important to have it. Particularly this makes the master resilient in case of using preemptible worker VMs in GCE. GCE has the concept of shutdown scripts, which it doesn't guarantee to execute. So workers often don't get shutdown gracefully and the TCP connections on the master linger as there's nothing to close them. Thus the need of enabling keep alive.
This enables keep-alive on connections besides the master's connections, but that shouldn't cause harm.
Closes#20512 from peshopetrov/master.
Authored-by: Petar Petrov <petar.petrov@leanplum.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
c01152dd22