client: setting empty cpu cgroup is broken

[shoenig]

Somewhere between the upgrade to runc v1.0.3 and testing on ubuntu 20.04, cpuset management got broken.

➜ sudo -E go test -v -run TestCpusetManager_AddAlloc
=== RUN   TestCpusetManager_AddAlloc
2022-01-25T16:46:21.938-0600 [INFO]  cgutil/cpuset_manager_linux.go:165: initialized cpuset cgroup manager: parent=/gotest-8d9ec808 cpuset=0-23
    cpuset_manager_linux_test.go:78: 
        	Error Trace:	cpuset_manager_linux_test.go:78
        	Error:      	Should be empty, but was [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23]
        	Test:       	TestCpusetManager_AddAlloc
--- FAIL: TestCpusetManager_AddAlloc (0.02s)
FAIL
exit status 1
FAIL	github.com/hashicorp/nomad/client/lib/cgutil	0.029s

I notice that by manually setting value written to the shared/cpuset.cpus to a space character " " instead of empty string "", it works.

 func (_ *cpusetManager) setCgroupCpusetCPUs(path, cpus string) error {
        currentCpusRaw, err := fscommon.ReadFile(path, "cpuset.cpus")
        if err != nil {
                return err
        }
 
        if cpus != strings.TrimSpace(currentCpusRaw) {
+               if cpus == "" {
+                       cpus = " "
+               }
                if err := fscommon.WriteFile(path, "cpuset.cpus", cpus); err != nil {
                        return err
                }
        }

➜ sudo -E go test -v -run TestCpusetManager_AddAlloc
=== RUN   TestCpusetManager_AddAlloc
2022-01-25T16:48:43.574-0600 [INFO]  cgutil/cpuset_manager_linux.go:168: initialized cpuset cgroup manager: parent=/gotest-af715183 cpuset=0-23
--- PASS: TestCpusetManager_AddAlloc (0.03s)
PASS
ok  	github.com/hashicorp/nomad/client/lib/cgutil	0.033s

But I don't understand why. The implementation of fscommon.WriteFile changed significantly from runc v1.0.0-rc92 to runc v1.0.0-rc93, but AFAICT this feature was introduced with the later version.

[shoenig]

Spun up a ubuntu 16.04 vm and indeed it seems to work there

ubuntu@ip-172-31-73-208:~/nomad/client/lib/cgutil$ cat /etc/os-release  | grep VERSION= 
VERSION="16.04.7 LTS (Xenial Xerus)"
ubuntu@ip-172-31-73-208:~/nomad/client/lib/cgutil$ git show | grep commit
commit 94b744c5394f9943158e737f4b53b99722122392
ubuntu@ip-172-31-73-208:~/nomad/client/lib/cgutil$ sudo -E /usr/local/go/bin/go test -v -run TestCpusetManager_AddAlloc
=== RUN   TestCpusetManager_AddAlloc
2022-01-25T23:32:09.086Z [INFO]  cgutil/cpuset_manager_linux.go:165: initialized cpuset cgroup manager: parent=/gotest-e7a6fdcd cpuset=0-3
--- PASS: TestCpusetManager_AddAlloc (0.00s)
PASS
ok  	github.com/hashicorp/nomad/client/lib/cgutil	0.014s

[shoenig]

Turns out this is a duplicate of #11289

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