Allow . in Environment Variable Names

[angrycub]

From https://github.com/appc/spec/blob/master/spec/aci.md:

environment (list of objects, optional) represents the app's environment variables (ACE can append). The listed objects must have two key-value pairs: name and value. The name must consist solely of letters, digits, and underscores '_' as outlined in IEEE Std 1003.1-2008, 2016 Edition, with practical considerations dictating that the name may also include periods '.' and hyphens '-'. The value is an arbitrary string. These values are not evaluated in any way, and no substitutions are made.

Dotted environment variables are frequently used as a part of the Spring Boot pattern. (re: ZD-6116)

This PR specifically doesn't address the conversion of hyphens (-) due to an issue with rkt [Nomad GH # 2358].

[schmichael]

Can you also fix our environment variable parsing library? https://github.com/hashicorp/go-envparse/blob/master/envparse.go#L116-L144

I don't want to merge this until we have parity in our environment variable parsing library.

[angrycub]

@schmichael, upon further reflection, based on the ask to adjust go-envparse, I ended up thinking that this might not be the best course of action. In the case of shell environment variables, dots have mixed support, tending to the unsupported. So for drivers like exec and raw_exec this type of change could allow for unreadable variables. I was able to find what appears to be the Spring Boot workaround using the SPRING_APPLICATION_JSON environment variable to work around the naming situation there.

Docker seems to happily allow the creation of a dotted environment variable via their -e parameter, so perhaps a solution is to create a strict and relaxed version of the check?

[angrycub]

An additional comment I have received on this issue for further discussion.

[This process] is not a shell script and . is certainly legal for ENV vars passed to processes. Perhaps this should be handled in the driver layer and then drivers can override any default behavior. Because docker is constrained because of [other drivers'] limitations.

[schmichael]

This sample job outputs (as viewed with nomad logs ...):

foo
foo.BAR
foo.bar

Meaning ${FOO.BAR} is a valid env var for dash (Debian/Ubuntu's version of /bin/sh).

Switching to /bin/bash produces the same logs.

So I'm inclined to accept this PR (if we also update the library as stated before) as it seems dots are fairly widely supported even if bash does not let you interactively set them.

[angrycub]

@schmichael - Made PR for go-envparse hashicorp/go-envparse#2

[schmichael]

Mind updating go-envparse in this PR? Should be:

go get -u github.com/hashicorp/go-envparse
govendor update github.com/hashicorp/go-envparse
# git commit + push

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