safely write state files

[camerondavison]

Currently

nomad/client/util.go

Line 88 in 7db2eb0

if err := ioutil.WriteFile(path, buf, 0600); err != nil {

is not writing files to disk safely.

According to golang docs WriteFile https://golang.org/pkg/io/ioutil/#WriteFile will truncate a file that exists before writing to it.

Given that setState is getting called all over the place this introduces a whole bunch of potential states in history where the json state file is empty.

The code should write to a temp file and then mv the temp file over the existing file (which in unix can be an atomic operation) It is important that the temp file lives in the exact same directory as well, because it can only be somewhat assure to be atomic if it is just changing inode table entries.

[github-actions]

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