Improvement in difference engine

[stevendborrelli]

1. I propose adding a new method to status.go:

// Difference returns the Current value of a Difference by key
func (t *Status) Difference(key string) (string, bool) {
    diff, ok := t.Differences[key]
    if !ok {
        return "", ok
    }
    return diff.Current(), diff.Changes()
}

Usage would be like this in an Apply() phase to check if file content has been modified:

    if content, ok := status.Difference("content"); ok && f.Type == "file" {
        err = ioutil.WriteFile(f.Destination, content, f.FileMode)

1. As part of this, the engine and Check() should use the status.go status.AnyChanges() method.

This means that a typical check would just AddDifference() for every field, and AnyChanges() is used to see if any of the defined differences indicate a change.

Instead of doing this in my Check() method:

    if f.State != actual.State {
        status.AddDifference("state", actual.State, f.State, "")
    }

I'd just do:

status.AddDifference("state", actual.State, f.State, "")

And expect anything else to call AnyChanges() for any change or Difference() to get the value of a specific change.

We'd have to be more aggressive about using Default values to fill in results when Original or Current is not defined.

For example of default, take the example of permissions. The logic would be:

1. If mode is set in the hcl file, use that value
2. if the mode is not set in the hcl file but the file exists, use the existing permissions
3. if mode is not set & the file doesn't exist, use 0750

[ryane]

👍 The docker container resource already follows the pattern of calling status.AddDifference without comparing first which mostly works today. But, other parts of the system need updating. For example, the printer doesn't check if the diffs have changes so you end up with output like described in #317 (comment).