Allow : colons in data value flags yaml keys

[cari-lynn]

Describe the problem/challenge you have
It is valid yaml to have a colon in a YAML key and the -f flag allows it:

$ ytt -f a:key: valid
a:key: valid

However, when using data values flags, this is disallowed:

$ ytt --data-value a:key=valid

$ ytt --data-value-yaml a:key=valid

Note: the below : in the environment variable key is invalid in unix, but may be valid in windows

$ export KEYVAL_a:key=valid
$ ytt --data-values-env KEYVAL

$ export KEYVAL_a:key=valid
$ ytt --data-values-env-yaml KEYVAL

all result in an error:

ytt: Error: Extracting data value from KV:
  Expected library ref to start with '@'

Describe the solution you'd like
Do not report this as an error.

a:key: valid

Anything else you would like to add:
YAML separates a key and a value using a colon and a space. So a colon can appear in a key as long as it's not followed by a space.

Considerations

I believe we should allow data values flags to contain : because it is valid yaml and I want to hear if users are running into this. However, it currently serves a purpose to catch syntax errors when incorrectly using a library reference. For example:
We catch this syntax error: if there are two colons in the library reference: ytt --data-value @libraryName::key=value. If we decide to allow : in keys then this becomes valid.

Current workaround

If you are unable to do this:

$ ytt --data-value a:key=valid

Then put the key in a file and load it

$ cat filename.yml
a:key=valid

$ ytt --data-values-file filename.yml

Please share you thoughts on this.

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[cppforlife]

im personally -1 on supporting :. there are other chars that we do not support (intentionally) such as __ (double underscore) or . (period) since we use them in special ways to figure out key structure. additionally env vars on various OSes do not support various special chars.

it currently serves a purpose to catch syntax errors when incorrectly using a library reference

this check seems to be quite important as well.

[cari-lynn]

Intentionally not supporting special chars in the key makes sense in the context of these flags. They all have a flag value of key=value and it's just the key portion of that we are talking about here.

• --data-values-env
• --data-values-env-yaml
• --data-value
• --data-value-yaml
• --data-value-file

That just leaves --data-values-file, which does not allow a key in the flag value, so it is not applicable here.

That reasoning makes sense, we currently disallow other 'valid yaml' syntax in this context. Unix disallows : in env vars, and we haven't heard a use case for it, and that there is a workaround if you must use : in a key of a data value flag. These are reasons to choose providing an error message designed to be friendly to user over allowing this value.

[cari-lynn]

During testing for : in --data-value-env keys I learned that this currently is allowed. This issue is not meant to change that.