generate: set omitzero on specific types

Set `omitzero` on the following types to allow clients to set an empty
slice and have it serialized to `[]`, which is required by Oxide APIs to
unset a value.

* `[]VpcFirewallRuleUpdate`
* `[]NameOrId`

The previous behavior would omit an empty slice entirely during
serialization, causing the Oxide APIs to respond with an error.

Author: sudomateo

internal/generate/exceptions.go

@@ -4,6 +4,15 @@
 
 package main
 
+// omitzeroTypes returns a slice of types that should be tagged with omitzero
+// for serialization and deserialization.
+func omitzeroTypes() []string {
+	return []string{
+		"[]VpcFirewallRuleUpdate",
+		"[]NameOrId",
+	}
+}
+
 func emptyTypes() []string {
 	return []string{
 		"BgpMessageHistory",

internal/generate/types.go

@@ -542,9 +542,20 @@ func createTypeObject(schema *openapi3.Schema, name, typeName, description strin
 		field.Name = strcase.ToCamel(k)
 		field.Type = typeName
 
+		// TODO: Set omitzero on all types.
+		// Long-term, omitzero should be set on all types, even nullable arrays.
+		// In https://spec.openapis.org/oas/v3.0.4.html#appendix-b-data-type-conversion,
+		// which is one patch version ahead of Nexus' OpenAPI specification at the time
+		// of this writing, it notes that null is treated as undefined. That suggests
+		// that an omitted JSON attribute is treated the same as setting it to null.
+		//
+		// For now though, only set omitzero on certain types that are confirmed to be
+		// safe when omitted (e.g., types defined by omitzeroTypes).
 		serInfo := fmt.Sprintf("`json:\"%s,omitempty\" yaml:\"%s,omitempty\"`", k, k)
 		if isNullableArray(v) {
 			serInfo = fmt.Sprintf("`json:\"%s\" yaml:\"%s\"`", k, k)
+		} else if sliceContains(omitzeroTypes(), typeName) {
+			serInfo = fmt.Sprintf("`json:\"%s,omitzero\" yaml:\"%s,omitzero\"`", k, k)
 		}
 
 		field.SerializationInfo = serInfo