Handle process termination during read from /proc

[noidi]

Fixes #2815.

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• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
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[phemmer]

This might be worth reporting upstream to GO that it should be handled by os.IsNotExist(). Yes os.IsNotExist() is typically only useful on Open(), but I think the semantics of the function apply here, as the ESRCH means that the file has disappeared.

[noidi]

Personally, I don't think os.IsNotExist() should check for ESRCH. Currently it only checks for ENOENT [1], which (at least on Linux) is reserved for path operations [2], so I think it's meant to detect errors caused by operations on non-existent paths. In this case the path does exist during open(), but the corresponding process is terminated while (or before) read() is being called on the opened file descriptor, causing something akin to an I/O error. (I think it's like removing a USB drive while a file is being read: the file becomes unreadable even though there's nothing wrong with the path that the program opened.)

[1] https://github.com/golang/go/blob/964639cc338db650ccadeafb7424bc8ebb2c0f6c/src/os/error_unix.go#L18
[2] https://lkml.org/lkml/2012/12/23/75

[danielnelson]

Interesting issue, I always thought that if open succeeded you were good to read. It sounds like this could occur with any file in /proc/<pid>/, Are there any other files this could happen with?

[danielnelson]

Whoops, I forgot to ask you to add it the changelog, I'll add it

[noidi]

@danielnelson Thanks for the merge! I don't have much experience with /proc/<pid>/ but I'd be surprised if the other files didn't work this way. I did some cursory grepping and I think this was the only place in the Telegraf source code where this could be a problem.