Allow MTU to be specified on Route

[muff1nman]

This is a pick-up of the initial effort to add mtu as done here: #831

[muff1nman]

cc @mccv1r0

[coveralls]

Coverage decreased (-0.6%) to 71.681% when pulling 8b4c508 on muff1nman:route-mtu into 3ec1919 on containernetworking:main.

[mccv1r0]

Needs rebase

[muff1nman]

Rebased, no modifications made.

[muff1nman]

@mccv1r0 friendly ping

[mccv1r0]

We should have a test for when mtu is actually set as well. e.g. set to e.g. 1400 (adjust variable names to taste.)

+
+       Context("when setting custom routes with mtu 1400", func() {
+               notzero := 1400
+
+               exampleMTU := current.Route{
+                       Dst: net.IPNet{
+                               IP:   net.ParseIP("1.2.3.0"),
+                               Mask: net.CIDRMask(24, 32),
+                       },
+                       GW:  net.ParseIP("1.2.3.1"),
+                       MTU: &notzero,
+               }
+
+               It("marshals to JSON equivalent to an omitted mtu ", func() {
+                       jsonBytes, err := json.Marshal(&exampleMTU)
+                       Expect(err).NotTo(HaveOccurred())
+                       Expect(jsonBytes).To(MatchJSON(`{ "dst": "1.2.3.0/24", "gw": "1.2.3.1", "mtu": 1400 }`))
+               })
+       })

[muff1nman]

There were other tests that had covered this path (testResult), but I've added it to make the tests in this function complete.

[mccv1r0]

Those tests didn't check to see if the mtu was what we expected.

[mccv1r0]

Have you tested your changes with cnitool or something similar using currently shipping plugins?

I checked out this PR and tried a quick test (emphasis on quick) pointing plugins to use your cni repo changes. I get compile errors when trying to build existing plugs that expect: Routes []types.Route not Routes []Route

e.g.

$ ./build_linux.sh 
Building plugins 
  bandwidth
# github.com/containernetworking/plugins/pkg/ip
pkg/ip/utils_linux.go:79:44: undefined: types.Route

# github.com/containernetworking/plugins/plugins/meta/sbr
plugins/meta/sbr/main.go:208:57: undefined: types.Route

# github.com/containernetworking/plugins/plugins/main/bridge
plugins/main/bridge/bridge.go:176:7: undefined: types.Route

[dcbw]

@muff1nman Let's acutally just use the 020 implementation since they are the same. 040 types import the 020 so it's fine to do that.

So everything here in the 040 directory would just use types020.Route

[dcbw]

Nevermind... do what Casey suggests with the MTU *uint

[squeed]

Unfortunately this change breaks Semver promises (as @mccv1r0 correctly observes) -- not of the spec, but of the go library version. Namely, this removes existing types / fields. We would have to tag this as v2.0.0 in Git, which we're not going to do.

So, the best bet is probably to make MTU a *uint on the Route type. I can't think of any other way to solve this.