Daisy: confusing error when step type misspelled about compute-image-tools

[ericdand]

Code tracing: the first line of the error message is reported in daisy.go#main(), specifically around line 197, within if err:= w.Run(ctx);. Digging into workflow.go#Run(ctx), it calls Validate(ctx), which contains the string "error populating workflow". Validate(ctx) calls populate(ctx), which in turn calls populateStep(ctx, step), which finally calls step.go#stepImpl(), the source of the string "no step type defined". So that's where all the parts of this error message come from.

Before the workflow is run, however, it is unmarshalled from a JSON object. This includes unmarshalling all of its steps. This happens in workflow.go#readWorkflow(file, w). Unfortunately, it looks like invalid step types are simply ignored. https://golang.org/pkg/encoding/json/#Unmarshal says:

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match. Unmarshal will only set exported fields of the struct.

Furthermore:

If a JSON value is not appropriate for a given target type ... Unmarshal skips that field and completes the unmarshaling as best it can.

So, as it stands, misspelled step types are just dropped silently on the floor.

But there is hope:

To unmarshal JSON into a value implementing the Unmarshaler interface, Unmarshal calls that value's UnmarshalJSON method, including when the input is a JSON null. Otherwise, if the value implements encoding.TextUnmarshaler and the input is a JSON quoted string, Unmarshal calls that value's UnmarshalText method with the unquoted form of the string.

So, if we add an UnmarshalJSON method to our Workflow type, then we could do the error checking. I'm yet unsure how much work this would take.

[ericdand]

After further consideration, I have two conclusions:

1. It makes more sense to only make Step implement the Unmarshaler interface, and
2. It does indeed make sense, given the funny way we represent steps (specifically their type) in JSON, to write custom unmarshalling logic for steps.

Happily, to implement Unmarshaler, it only takes one method. I'll get right to it.

[adjackura]

See this, should be in go 1.10:
https://go-review.googlesource.com/c/go/+/74830

[ericdand]

Awesome! Alright, I'll just wait until that's released, then we can use that.

[crunk1]

Partial fix out: #255

Reports the offending step name when there is a step population error.