test: clean up yarn test

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

pkg/dagger.io/dagger/utils.cue

@@ -8,6 +8,12 @@ import (
 // This may safely be called from any package
 #Source: engine.#Source
 
+// A (best effort) persistent cache dir
+#CacheDir: engine.#CacheDir
+
+// A temporary directory for command execution
+#TempDir: engine.#TempDir
+
 // Select a subdirectory from a filesystem tree
 #Subdir: {
 	// Input tree

pkg/universe.dagger.io/yarn/test/yarn-test.cue

@@ -2,11 +2,9 @@ package yarn
 
 import (
 	"dagger.io/dagger"
-	"dagger.io/dagger/engine"
-
 	"universe.dagger.io/yarn"
-	// "universe.dagger.io/alpine"
-	// "universe.dagger.io/bash"
+	"universe.dagger.io/alpine"
+	"universe.dagger.io/bash"
 )
 
 dagger.#Plan & {
@@ -16,8 +14,7 @@ dagger.#Plan & {
 	}
 
 	actions: {
-		TestReact: {
-			cache: engine.#CacheDir & {
+		cache: dagger.#CacheDir & {
 			id: "yarn cache"
 		}
 
@@ -26,24 +23,19 @@ dagger.#Plan & {
 			"cache": cache
 		}
 
-			_image: engine.#Pull & {
-				source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		_image: alpine.#Build & {
+			packages: bash: {}
 		}
 
-			// FIXME: use bash.#Script
-			test: engine.#Exec & {
-				input: _image.output
+		test: bash.#Run & {
+			image: _image.output
 			mounts: build: {
 				dest:     "/build"
 				contents: pkg.output
 			}
-				args: [
-					"sh", "-c",
-					#"""
+			script: #"""
 				test "$(cat /build/test)" = "output"
-						"""#,
-				]
-			}
+				"""#
 		}
 
 		// FIXME: re-enable?

plan/task/exec.go

@@ -190,7 +190,12 @@ func (t *execTask) mountTmp(_ *plancontext.Context, dest string, _ *compiler.Val
 
 func (t *execTask) mountCache(_ *plancontext.Context, dest string, mnt *compiler.Value) (llb.RunOption, error) {
 	contents := mnt.Lookup("contents")
-	id, err := contents.Lookup("id").String()
+
+	idValue := contents.Lookup("id")
+	if !idValue.IsConcrete() {
+		return nil, fmt.Errorf("cache %q is not set", mnt.Path().String())
+	}
+	id, err := idValue.String()
 	if err != nil {
 		return nil, err
 	}