New --context flag for pre-loading datasources into context

[hairyhenderson]

Fixes #404

Adds a new --context/-c commandline flag to allow pre-loading a datasource into the template's context.

This allows this sort of usage:

$ gomplate -c foo=https://example.com/foo.json -c bar.yml -i '{{ .foo.value }}, {{ .bar.value }}'
foo's value, bar's value
$ gomplate -c .=baz.yml -i '{{ .value }}'
baz's value

Caveats:

• all context datasources are loaded first - this can introduce latency
• setting the root (with alias of .) makes .Env unavailable
• sub-paths can't be given to context datasources in the template (i.e. {{ datasource "foo" "subpath" }})
• inline datasources can't be added to the context at runtime

Other:

• context datasources are still available by name as regular datasources:

  $ gomplate -c .=baz.yml -i '{{ .value }} - {{ (ds ".").value }}'
  baz's value - baz's value

Signed-off-by: Dave Henderson [email protected]

[vitalka200]

Hi @hairyhenderson,
when this PR can be merged?
I wanted to use gomplate to pre-render Helm Charts, and loading datasources is one that is missing.

Thank you!

[hairyhenderson]

@vitalka200 Soon! 😉

Keep in mind that gomplate is not a drop-in replacement for Helm. Helm and gomplate have different sets of functions that are supported, and I don't intend for gomplate to ever be fully Helm-compatible.

If you want true pre-rendering for Helm, I suggest using helm's --dry-run and --debug options.