pkg/asset: introduce AssetGenerator

Introduces the asset generator interface, which allows generating
assets with a passed in context. Provides an adapter for assets
that do not implement GenerateWithContext. The adapter simply
wraps the asset and calls the original Generate (without context)
function.

Author: patrickdillon

pkg/asset/generator.go

@@ -0,0 +1,31 @@
+package asset
+
+import (
+	"context"
+)
+
+// Generator is used to generate assets.
+type Generator interface {
+	// Generate generates this asset given
+	// the states of its parent assets.
+	GenerateWithContext(context.Context, Parents) error
+}
+
+// generatorAdapter wraps an asset to provide the
+// Generate with context function.
+type generatorAdapter struct {
+	a Asset
+}
+
+// NewDefaultGenerator creates a new adapter to generate
+// an asset with a context.
+func NewDefaultGenerator(a Asset) Generator {
+	return &generatorAdapter{a: a}
+}
+
+// Generate calls Generate on an asset, dropping the context
+// to maintain compatibility with assets that do not implement
+// generate with context.
+func (a *generatorAdapter) GenerateWithContext(_ context.Context, p Parents) error {
+	return a.a.Generate(p)
+}