rkt: Don't require port_map with host networking

[schmichael]

Also don't try to return a DriverNetwork with host networking. None will
ever exist as that's the point of host networking: rkt won't create a
network namespace.

[preetapan]

I am concerned about nil pointer dereferencing in downstream code like

nomad/command/agent/consul/client.go

Line 575 in a52cb34

if net.Advertise() {

that use the DriverNetwork struct, isn't that going to panic when network == "host" ?

[schmichael]

So I have regrets about how I implemented DriverNetwork, but it was designed to gracefully handle nils since the vast majority of drivers don't implement it (so they return nil).

nomad/client/driver/driver.go

Lines 199 to 201 in 730e2aa

	// Network may be nil as not all drivers or configurations create
	// networks.
	Network *cstructs.DriverNetwork

And if you look at each of the methods on DriverNetwork they handle nil:

nomad/client/structs/structs.go

Lines 134 to 186 in 730e2aa

	// DriverNetwork is the network created by driver's (eg Docker's bridge
	// network) during Prestart.
	type DriverNetwork struct {
	// PortMap can be set by drivers to replace ports in environment
	// variables with driver-specific mappings.
	PortMap map[string]int
	// IP is the IP address for the task created by the driver.
	IP string
	// AutoAdvertise indicates whether the driver thinks services that
	// choose to auto-advertise-addresses should use this IP instead of the
	// host's. eg If a Docker network plugin is used
	AutoAdvertise bool
	}
	// Advertise returns true if the driver suggests using the IP set. May be
	// called on a nil Network in which case it returns false.
	func (d *DriverNetwork) Advertise() bool {
	return d != nil && d.AutoAdvertise
	}
	// Copy a DriverNetwork struct. If it is nil, nil is returned.
	func (d *DriverNetwork) Copy() *DriverNetwork {
	if d == nil {
	return nil
	}
	pm := make(map[string]int, len(d.PortMap))
	for k, v := range d.PortMap {
	pm[k] = v
	}
	return &DriverNetwork{
	PortMap: pm,
	IP: d.IP,
	AutoAdvertise: d.AutoAdvertise,
	}
	}
	// Hash the contents of a DriverNetwork struct to detect changes. If it is nil,
	// an empty slice is returned.
	func (d *DriverNetwork) Hash() []byte {
	if d == nil {
	return []byte{}
	}
	h := md5.New()
	io.WriteString(h, d.IP)
	io.WriteString(h, strconv.FormatBool(d.AutoAdvertise))
	for k, v := range d.PortMap {
	io.WriteString(h, k)
	io.WriteString(h, strconv.Itoa(v))
	}
	return h.Sum(nil)
	}

Definitely something worth changing/fixing in the future, but I think it's out of scope for this PR.

[preetapan]

Agreed, thanks for the explain.

[ashald]

Actually, port_map doesn't make much sense with macvlan and ipvlan based networks neither as with those modes containers can get their own IPs on the same network as host and might not even be able to talk to the host. IMO, there shouldn't be any enforcement of port_map - maybe just a warning would be good enough.

[schmichael]

@ashald excellent point. I was just dealing with a similar issue in #3380, so I'll try to fixup the port_map check in rkt too.

[schmichael]

@ashald Should be fixed by #3637

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