check if Memory is not nil for container stats

[yastij]

/kind bug
/sig node
What this PR does / why we need it:

Which issue(s) this PR fixes: Fixes #77655

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Check if container memory stats are available before accessing it

[yastij]

/priority critical-urgent

[yastij]

/cc @tallclair

[mattjmcnaughton]

/lgtm

Nice work here :)

I'm curious. do you think its worth adding a unit test ensuring that we don't panic when cstat.Memory is nil? That'd also give me slightly more confidence that this lack of nil check was the cause of the error this pr claims to fix.

Additionally, for more own understanding, can you share more about why cstat.Memory would sometimes be nil? Do we want to address the times when cstat.Memory is nil?

[mattjmcnaughton]

/assign @dchen1107

[mattjmcnaughton]

Also, we can talk about this once its merged, but this seems like a good candidate for cherry picking to earlier releases.

[neolit123]

thanks @yastij

/lgtm
/assign @yujuhong

[yastij]

I’ll also raise an issue to increase test coverage for some parts related to cadvisor in the kubelet.

[tallclair]

/approve
/cc @dashpole

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tallclair, yastij

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[dashpole]

/lgtm

[yastij]

@mattjmcnaughton - when getting stats for docker for example (or libcontainer in general) cadvisor talks to a cgroupManager IIRC (e.g. cgroupfs).
When opening the memory.stat file if it dosent exist we return just nil with no err, leading the cgroupManager to not error, and thus having stat with a nil memory.
On my cluster this was the issue, but another logic can lead to something like this I think