Merge resources recursively (GoogleCloudPlatform#7783)

tools/missing-test-detector/reader.go

@@ -277,11 +277,7 @@ func readHCLBlockBody(body hcl.Body, fileBytes []byte) (Resource, error) {
 		if existing, ok := m[block.Type]; ok {
 			// Merge the fields from the current block into the existing resource config.
 			if existingResource, ok := existing.(Resource); ok {
-				for k, v := range blockConfig {
-					if _, ok := existingResource[k]; !ok {
-						existingResource[k] = v
-					}
-				}
+				mergeResources(existingResource, blockConfig)
 			}
 		} else {
 			m[block.Type] = blockConfig
@@ -292,3 +288,18 @@ func readHCLBlockBody(body hcl.Body, fileBytes []byte) (Resource, error) {
 	}
 	return m, nil
 }
+
+// Perform a recursive one-way merge of b into a.
+func mergeResources(a, b Resource) {
+	for k, bv := range b {
+		if av, ok := a[k]; ok {
+			if avr, ok := av.(Resource); ok {
+				if bvr, ok := bv.(Resource); ok {
+					mergeResources(avr, bvr)
+				}
+			}
+		} else {
+			a[k] = bv
+		}
+	}
+}