internal/mcp/jsonschema: validate defaults

Optionally validate a schema's defaults when resolving it.

We simply walk the tree of schemas, validating each locally. This can't
handle dynamic references, which require maintaing the stack from the
root. It seems unlikely that a default would require one, however.

Since there may be other options to Resolve, use an options struct
to future-proof it.

Also, fix a bug: you can't resolve a schema twice.

Change-Id: I374c582d9cc49d66d4ed56233d1070b8b3263bdf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676395
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/validate.go

@@ -30,6 +30,31 @@ func (rs *Resolved) Validate(instance any) error {
 	return st.validate(reflect.ValueOf(instance), st.rs.root, nil)
 }
 
+// validateDefaults walks the schema tree. If it finds a default, it validates it
+// against the schema containing it.
+//
+// TODO(jba): account for dynamic refs. This algorithm simple-mindedly
+// treats each schema with a default as its own root.
+func (rs *Resolved) validateDefaults() error {
+	if s := rs.root.Schema; s != "" && s != draft202012 {
+		return fmt.Errorf("cannot validate version %s, only %s", s, draft202012)
+	}
+	st := &state{rs: rs}
+	for s := range rs.root.all() {
+		// We checked for nil schemas in [Schema.Resolve].
+		assert(s != nil, "nil schema")
+		if s.DynamicRef != "" {
+			return fmt.Errorf("jsonschema: %s: validateDefaults does not support dynamic refs", s)
+		}
+		if s.Default != nil {
+			if err := st.validate(reflect.ValueOf(*s.Default), s, nil); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
 // state is the state of single call to ResolvedSchema.Validate.
 type state struct {
 	rs *Resolved