Add config.maxItemsSync schema traverser for syncing the MaxItems

constraints between the JSON & Go schemas.

- We've observed that some MaxItems constraints in the JSON schemas are not set
  where the corresponding MaxItems constraints in the Go schemas are set to 1.
- This inconsistency results in some singleton lists not being properly converted
  in the MR API.
- This traverser can mitigate such inconsistencies and can also embed
  the singleton lists.

Signed-off-by: Alper Rifat Ulucinar <ulucinar@users.noreply.github.com>

pkg/config/schema_conversions.go

@@ -72,3 +72,47 @@ func (l *SingletonListEmbedder) VisitResource(r *traverser.ResourceNode) error {
 	l.r.AddSingletonListConversion(traverser.FieldPathWithWildcard(r.TFPath), traverser.FieldPathWithWildcard(r.CRDPath))
 	return nil
 }
+
+// maxItemsSync is a visitor to sync the MaxItems constraints from the
+// Go schema to the JSON schema. We've observed that some MaxItems constraints
+// in the JSON schemas are not set where the corresponding MaxItems constraints
+// in the Go schemas are set to 1. This inconsistency results in some singleton
+// lists not being properly converted in the MR API whereas at runtime we may
+// try to invoke the corresponding Terraform conversion functions. This
+// traverser can mitigate such inconsistencies and can also embed the
+// singleton lists.
+type maxItemsSync struct {
+	traverser.NoopTraverser
+
+	embedder   SingletonListEmbedder
+	jsonSchema map[string]*Resource
+}
+
+// NewListEmbedderWithMaxItemsSync returns a new schema traverser capable of
+// syncing the MaxItems constraints from the Go schema to the specified JSON
+// schema and can embed singleton lists as objects after the sync. This will
+// ensure the generation time and the runtime schema MaxItems constraints will
+// match while embedding singleton lists. If both the generation time and
+// runtime use the same schema, then SingletonListEmbedder should be used
+// instead.
+func NewListEmbedderWithMaxItemsSync(s map[string]*Resource) traverser.SchemaTraverser {
+	return &maxItemsSync{
+		jsonSchema: s,
+	}
+}
+
+func (m *maxItemsSync) VisitResource(r *traverser.ResourceNode) error {
+	// this visitor only works on singleton lists
+	if (r.Schema.Type != schema.TypeList && r.Schema.Type != schema.TypeSet) || r.Schema.MaxItems != 1 {
+		return nil
+	}
+
+	if err := traverser.AccessSchema(m.jsonSchema[r.TFName].TerraformResource, r.TFPath, func(s *schema.Schema) error {
+		s.MaxItems = 1
+		return nil
+	}); err != nil {
+		return errors.Wrapf(err, "failed to access the schema element at path %v for resource %q", r.TFPath, r.TFName)
+	}
+	m.embedder.SetResource(m.jsonSchema[r.TFName])
+	return errors.Wrapf(m.embedder.VisitResource(r), "failed to embed the singleton list at path %v for resource %q", r.TFPath, r.TFName)
+}

pkg/schema/traverser/access.go

@@ -18,7 +18,7 @@ type SchemaAccessor func(*schema.Schema) error
 // the accessors. The terminal node at the end of the specified path must be
 // a *schema.Resource or an error will be reported. The specified path must
 // have at least one component.
-func AccessSchema(sch any, path []string, accessors ...SchemaAccessor) error {
+func AccessSchema(sch any, path []string, accessors ...SchemaAccessor) error { //nolint:gocyclo // easier to follow the flow
 	if len(path) == 0 {
 		return errors.New("empty path specified while accessing the Terraform resource schema")
 	}