Make the --cluster-ip flag optional and check Kingpin parse errors

- This flag shouldn't be used when SIDECAR_SEEDS is set, so making
it mandatory will break this functionality.

Author: mihaitodor

README.md

@@ -145,8 +145,9 @@ $ go run *.go --cluster-ip <boostrap_host>
 ```
 
 You always need to supply at least one IP address or hostname with the
-`--cluster-ip` argument. If are running solo, or are the first member, this can
-be your own hostname. You may specify the argument multiple times to have
+`--cluster-ip` argument (or via the `SIDECAR_SEEDS` environment variable).
+If are running solo, or are the first member, this can be your own
+hostname. You may specify the argument multiple times to have
 multiple hosts. It is recommended to use more than one when possible.
 
 Note: `--cluster-ip` will overwrite the values passed into the `SIDECAR_SEEDS` environment variable.

cli.go

@@ -28,13 +28,14 @@ func parseCommandLine() *CliOpts {
 	app := kingpin.New("sidecar", "")
 
 	opts.AdvertiseIP = app.Flag("advertise-ip", "The address to advertise to the cluster").Short('a').String()
-	opts.ClusterIPs = app.Flag("cluster-ip", "The cluster seed addresses").Required().Short('c').NoEnvar().Strings()
+	opts.ClusterIPs = app.Flag("cluster-ip", "The cluster seed addresses").Short('c').NoEnvar().Strings()
 	opts.ClusterName = app.Flag("cluster-name", "The cluster we're part of").Short('n').String()
 	opts.CpuProfile = app.Flag("cpuprofile", "Enable CPU profiling").Short('p').Bool()
 	opts.Discover = app.Flag("discover", "Method of discovery").Short('d').NoEnvar().Strings()
 	opts.LoggingLevel = app.Flag("logging-level", "Set the logging level").Short('l').String()
 
-	app.Parse(os.Args[1:])
+	_, err := app.Parse(os.Args[1:])
+	exitWithError(err, "Failed to parse CLI opts")
 
 	return &opts
 }