Factor out a method to parse termination messages

[imjasonh]

This reduces duplication, and allows us to make future changes to how
termination messages are written in lock-step with how they're parsed.

cc @othomann @chengjingtao

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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/termination/parse.go	Do not exist	75.0%
pkg/termination/write.go	Do not exist	68.4%

[vdemeester]

/lgtm

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[othomann]

@imjasonh I think the ParseMessage function needs to handle an empty string, this might explain the problem in the integration tests. So the parse tests should have a test with an empty string.
Something like this:

// ParseMessage parses a termination message as results.
func ParseMessage(msg string) ([]PipelineResourceResult, error) {
	if len(msg) == 0 {
		return nil, nil
	}
	var r []PipelineResourceResult
	if err := json.Unmarshal([]byte(msg), &r); err != nil {
		return nil, err
	}
	return r, nil
}

Could be nil or an empty slice.

[imjasonh]

@imjasonh I think the ParseMessage function needs to handle an empty string

Thanks, that was definitely the bug. Handled, and tested

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/termination/parse.go	Do not exist	83.3%
pkg/termination/write.go	Do not exist	68.4%

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/termination/parse.go	Do not exist	100.0%
pkg/termination/write.go	Do not exist	68.4%

[bobcatfish]

/lgtm
/meow space

[tekton-robot]

@bobcatfish:

In response to this:

/lgtm
/meow space

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[othomann]

/test pull-tekton-pipeline-build-tests