jsonschema: validate logic keywords

Validate keywords that combine schemas with various logical operators.

Also add iterators over schemas. Now we only need them to skip some
tests, but later they will come in handy for non-test code.

Change-Id: Idaf41efea4d693b6be0ee5c6c3a527664b8c472f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668115
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/jsonschema/validate.go

@@ -166,6 +166,70 @@ func (st *state) validate(instance reflect.Value, schema *Schema, path []any) (e
 		}
 	}
 
+	// logic
+	// https://json-schema.org/draft/2020-12/json-schema-core#section-10.2
+	// These must happen before arrays and objects because if they evaluate an item or property,
+	// then the unevaluatedItems/Properties schemas don't apply to it.
+	// See https://json-schema.org/draft/2020-12/json-schema-core#section-11.2, paragraph 4.
+	//
+	// If any of these fail, then validation fails, even if there is an unevaluatedXXX
+	// keyword in the schema. The spec is unclear about this, but that is the intention.
+
+	valid := func(s *Schema) bool { return st.validate(instance, s, path) == nil }
+
+	if schema.AllOf != nil {
+		for _, ss := range schema.AllOf {
+			if err := st.validate(instance, ss, path); err != nil {
+				return err
+			}
+		}
+	}
+	if schema.AnyOf != nil {
+		ok := false
+		for _, ss := range schema.AnyOf {
+			if valid(ss) {
+				ok = true
+				break
+			}
+		}
+		if !ok {
+			return fmt.Errorf("anyOf: did not validate against any of %v", schema.AnyOf)
+		}
+	}
+	if schema.OneOf != nil {
+		// Exactly one.
+		var okSchema *Schema
+		for _, ss := range schema.OneOf {
+			if valid(ss) {
+				if okSchema != nil {
+					return fmt.Errorf("oneOf: validated against both %v and %v", okSchema, ss)
+				}
+				okSchema = ss
+			}
+		}
+		if okSchema == nil {
+			return fmt.Errorf("oneOf: did not validate against any of %v", schema.OneOf)
+		}
+	}
+	if schema.Not != nil {
+		if valid(schema.Not) {
+			return fmt.Errorf("not: validated against %v", schema.Not)
+		}
+	}
+	if schema.If != nil {
+		var ss *Schema
+		if valid(schema.If) {
+			ss = schema.Then
+		} else {
+			ss = schema.Else
+		}
+		if ss != nil {
+			if err := st.validate(instance, ss, path); err != nil {
+				return err
+			}
+		}
+	}
+
 	return nil
 }