Check whether Mesos task labels are available

[changlan]

This patch fixes the panic issue when mesos.Task does not have a Labels reference:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xded5e4]

goroutine 1 [running]:
github.com/google/cadvisor/container/mesos.(*state).fetchLabelsFromTask(0xc000a0b378, 0xc000988270, 0x30, 0xc000980ae0, 0xc00087c1b8, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/container/mesos/mesos_agent.go:116 +0xf4
github.com/google/cadvisor/container/mesos.(*state).FetchLabels(0xc000a0b378, 0xc000988240, 0x29, 0xc000988270, 0x30, 0x4, 0x4, 0xc000a0b388)
        /home/lanchang/go/src/github.com/google/cadvisor/container/mesos/mesos_agent.go:141 +0x3dd
github.com/google/cadvisor/container/mesos.(*client).getLabels(0xc00000e2b8, 0xc00097e4c0, 0x24, 0xc00097e4c0, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/container/mesos/client.go:181 +0x175
github.com/google/cadvisor/container/mesos.(*client).ContainerInfo(0xc00000e2b8, 0xc0007c710d, 0x24, 0x24, 0x40c888, 0x10)
        /home/lanchang/go/src/github.com/google/cadvisor/container/mesos/client.go:89 +0x73
github.com/google/cadvisor/container/mesos.(*mesosFactory).CanHandleAndAccept(0xc000142780, 0xc0007c7100, 0x31, 0xc000a0b530, 0x2, 0x2)
        /home/lanchang/go/src/github.com/google/cadvisor/container/mesos/factory.go:110 +0x8a
github.com/google/cadvisor/container.NewContainerHandler(0xc0007c7100, 0x31, 0x0, 0x1e55101, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/container/factory.go:107 +0x124
github.com/google/cadvisor/manager.(*manager).createContainerLocked(0xc0005101e0, 0xc0007c7100, 0x31, 0x0, 0x0, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/manager/manager.go:965 +0xd6
github.com/google/cadvisor/manager.(*manager).createContainer(0xc0005101e0, 0xc0007c7100, 0x31, 0x0, 0x0, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/manager/manager.go:952 +0x94
github.com/google/cadvisor/manager.(*manager).detectSubcontainers(0xc0005101e0, 0x11b9325, 0x1, 0x0, 0x0)
        /home/lanchang/go/src/github.com/google/cadvisor/manager/manager.go:1145 +0x199
github.com/google/cadvisor/manager.(*manager).Start(0xc0005101e0, 0xc0005bd170, 0x133f180)
        /home/lanchang/go/src/github.com/google/cadvisor/manager/manager.go:366 +0x50e
main.main()
        /home/lanchang/go/src/cadvisor/cadvisor.go:173 +0x4b6

[k8s-ci-robot]

Hi @changlan. Thanks for your PR.

I'm waiting for a google or kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[dashpole]

/ok-to-test

[dashpole]

lgtm

[dashpole]

container/mesos/mesos_agent.go:43:43: undefined: mesos

[changlan]

/retest

[changlan]

@dashpole Fixed.