Setup SourceUnit interface (#1393)

* Test: Asymmetrical unmarshal API

* Test: Symmetric marshal API

* Revert "Test: Symmetric marshal API"

This reverts commit f51c64a.

* Cleanup test example and add SourceUnitUnmarshaller interface

* Add CommonSourceUnit implementation

* Update comments

* Remove UnmarshalJSON

pkg/sources/source_unit.go

@@ -0,0 +1,12 @@
+package sources
+
+// CommonSourceUnit is a common implementation of SourceUnit that Sources can
+// use instead of implementing their own types.
+type CommonSourceUnit struct {
+	ID string `json:"source_unit_id"`
+}
+
+// Implement the SourceUnit interface.
+func (c CommonSourceUnit) SourceUnitID() string {
+	return c.ID
+}

pkg/sources/sources.go

@@ -44,6 +44,19 @@ type Source interface {
 	GetProgress() *Progress
 }
 
+// SourceUnitUnmarshaller defines an optional interface a Source can implement
+// to support units coming from an external source.
+type SourceUnitUnmarshaller interface {
+	UnmarshalSourceUnit(data []byte) (SourceUnit, error)
+}
+
+// SourceUnit is an object that represents a Source's unit of work. This is
+// used as the output of enumeration, progress reporting, and job distribution.
+type SourceUnit interface {
+	// SourceUnitID uniquely identifies a source unit.
+	SourceUnitID() string
+}
+
 // GCSConfig defines the optional configuration for a GCS source.
 type GCSConfig struct {
 	// CloudCred determines whether to use cloud credentials.