access-engine/internal/schemaguard/schemaguard.go

// Package schemaguard validates published example documents against the JSON
// Schemas this repository publishes.
//
// It exists because of a defect class that hit three repositories in one week:
// a contract whose examples contradict its prose gets implemented as its
// examples. flex-auth shipped a caring fixture asserting an authority the
// contract's own ownership section denied, and a decision record naming
// flex-auth.decision-record.v1 while omitting three of that contract's
// provenance fields. approval-engine suggested the check after the same class
// caught its own two published claim examples.
//
// This is a deliberately small subset of JSON Schema — only the keywords the
// schemas here actually use. The property that makes it trustworthy is that an
// unrecognised keyword is a FAILURE, not a skip: a validator that silently
// approves what it does not understand is worse than no validator, because it
// invites reliance it cannot support.
package schemaguard

import (
	"encoding/json"
	"fmt"
	"os"
	"path"
	"path/filepath"
	"regexp"
	"sort"
	"strings"
)

// known lists every keyword this validator implements. Anything else in a
// schema is reported rather than ignored.
var known = map[string]bool{
	"$schema": true, "$id": true, "title": true, "description": true,
	"$defs": true, "$ref": true,
	"type": true, "required": true, "properties": true,
	"additionalProperties": true, "const": true, "enum": true,
	"pattern": true, "items": true, "minLength": true, "format": true,
	"if": true, "then": true, "allOf": true,
	"uniqueItems": true, "minimum": true, "maximum": true, "not": true,
	"minItems": true, "maxItems": true,
}

// Schema is a parsed JSON Schema document.
//
// Sibling schemas are resolved by file name from the directory the schema was
// loaded from. These schemas reference each other by absolute $id URL, but the
// URLs are identifiers rather than fetchable locations, so resolving them over
// the network would be both wrong and untestable.
type Schema struct {
	root    map[string]any
	dir     string
	sibling map[string]map[string]any
}

// Load reads a schema from disk.
func Load(path string) (*Schema, error) {
	root, err := loadDoc(path)
	if err != nil {
		return nil, err
	}
	return &Schema{root: root, dir: filepath.Dir(path), sibling: map[string]map[string]any{}}, nil
}

func loadDoc(path string) (map[string]any, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var root map[string]any
	if err := json.Unmarshal(raw, &root); err != nil {
		return nil, fmt.Errorf("parse %s: %w", path, err)
	}
	return root, nil
}

// Validate checks doc against the schema, returning every problem found rather
// than stopping at the first.
func (s *Schema) Validate(doc any) []string {
	return s.check("", s.root, doc)
}

// ValidateFile reads a JSON document and validates it.
func (s *Schema) ValidateFile(path string) ([]string, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var doc any
	if err := json.Unmarshal(raw, &doc); err != nil {
		return nil, fmt.Errorf("parse %s: %w", path, err)
	}
	return s.Validate(doc), nil
}

func at(path string) string {
	if path == "" {
		return "(root)"
	}
	return path
}

func (s *Schema) resolve(doc0 map[string]any, node map[string]any) (map[string]any, map[string]any, []string) {
	ref, ok := node["$ref"].(string)
	if !ok {
		return doc0, node, nil
	}
	doc := doc0
	frag := ref
	if !strings.HasPrefix(ref, "#") {
		loc, rest, _ := strings.Cut(ref, "#")
		name := path.Base(loc)
		other, cached := s.sibling[name]
		if !cached {
			loaded, err := loadDoc(filepath.Join(s.dir, name))
			if err != nil {
				return nil, nil, []string{fmt.Sprintf("$ref %q: sibling schema %s not readable in %s: %v", ref, name, s.dir, err)}
			}
			s.sibling[name] = loaded
			other = loaded
		}
		doc = other
		if rest == "" {
			return other, other, nil
		}
		frag = "#" + rest
	}
	const prefix = "#/$defs/"
	if !strings.HasPrefix(frag, prefix) {
		return nil, nil, []string{fmt.Sprintf("unsupported $ref fragment %q — schemaguard resolves only %s*", frag, prefix)}
	}
	defs, _ := doc["$defs"].(map[string]any)
	target, ok := defs[strings.TrimPrefix(frag, prefix)].(map[string]any)
	if !ok {
		return nil, nil, []string{fmt.Sprintf("$ref %q does not resolve", ref)}
	}
	return doc, target, nil
}

func (s *Schema) check(path string, node map[string]any, doc any) []string {
	return s.checkIn(s.root, path, node, doc)
}

func (s *Schema) checkIn(doc0 map[string]any, path string, node map[string]any, doc any) []string {
	doc0, node, probs := s.resolve(doc0, node)
	if probs != nil {
		return probs
	}

	for k := range node {
		if !known[k] {
			probs = append(probs, fmt.Sprintf("%s: schema uses keyword %q, which schemaguard does not implement — extend it rather than trusting this pass", at(path), k))
		}
	}

	if want, ok := node["const"]; ok && !equal(want, doc) {
		probs = append(probs, fmt.Sprintf("%s: must be %v, got %v", at(path), want, doc))
	}
	if raw, ok := node["enum"].([]any); ok {
		hit := false
		for _, v := range raw {
			if equal(v, doc) {
				hit = true
				break
			}
		}
		if !hit {
			probs = append(probs, fmt.Sprintf("%s: %v is not one of %v", at(path), doc, raw))
		}
	}
	if t, ok := node["type"]; ok {
		if err := checkType(t, doc); err != "" {
			probs = append(probs, fmt.Sprintf("%s: %s", at(path), err))
			return probs // further checks would be noise
		}
	}
	if pat, ok := node["pattern"].(string); ok {
		if str, isStr := doc.(string); isStr {
			re, err := regexp.Compile(pat)
			switch {
			case err != nil:
				probs = append(probs, fmt.Sprintf("%s: schema pattern %q does not compile: %v", at(path), pat, err))
			case !re.MatchString(str):
				probs = append(probs, fmt.Sprintf("%s: %q does not match %s", at(path), str, pat))
			}
		}
	}
	if min, ok := node["minimum"].(float64); ok {
		if n, isNum := doc.(float64); isNum && n < min {
			probs = append(probs, fmt.Sprintf("%s: %v is below minimum %v", at(path), n, min))
		}
	}
	if max, ok := node["maximum"].(float64); ok {
		if n, isNum := doc.(float64); isNum && n > max {
			probs = append(probs, fmt.Sprintf("%s: %v is above maximum %v", at(path), n, max))
		}
	}
	if not, ok := node["not"].(map[string]any); ok {
		if len(s.checkIn(doc0, path, not, doc)) == 0 {
			probs = append(probs, fmt.Sprintf("%s: matches a schema it must not match", at(path)))
		}
	}
	if min, ok := node["minLength"].(float64); ok {
		if str, isStr := doc.(string); isStr && len(str) < int(min) {
			probs = append(probs, fmt.Sprintf("%s: shorter than minLength %d", at(path), int(min)))
		}
	}

	if arr, ok := doc.([]any); ok {
		if unique, has := node["uniqueItems"].(bool); has && unique {
			seen := map[string]bool{}
			for _, v := range arr {
				key := fmt.Sprintf("%v", v)
				if seen[key] {
					probs = append(probs, fmt.Sprintf("%s: duplicate item %v but uniqueItems is set", at(path), v))
				}
				seen[key] = true
			}
		}
		if min, has := node["minItems"].(float64); has && len(arr) < int(min) {
			probs = append(probs, fmt.Sprintf("%s: %d items, below minItems %d", at(path), len(arr), int(min)))
		}
		if max, has := node["maxItems"].(float64); has && len(arr) > int(max) {
			probs = append(probs, fmt.Sprintf("%s: %d items, above maxItems %d", at(path), len(arr), int(max)))
		}
		if items, has := node["items"].(map[string]any); has {
			for i, v := range arr {
				probs = append(probs, s.checkIn(doc0, fmt.Sprintf("%s[%d]", path, i), items, v)...)
			}
		}
	}

	obj, isObj := doc.(map[string]any)
	if !isObj {
		return probs
	}

	if req, ok := node["required"].([]any); ok {
		for _, r := range req {
			name, _ := r.(string)
			if _, present := obj[name]; !present {
				probs = append(probs, fmt.Sprintf("%s: missing required field %q", at(path), name))
			}
		}
	}
	props, _ := node["properties"].(map[string]any)
	if allow, ok := node["additionalProperties"].(bool); ok && !allow {
		var extra []string
		for k := range obj {
			if _, declared := props[k]; !declared {
				extra = append(extra, k)
			}
		}
		sort.Strings(extra)
		for _, k := range extra {
			probs = append(probs, fmt.Sprintf("%s: undeclared field %q (additionalProperties is false)", at(path), k))
		}
	}
	for name, sub := range props {
		v, present := obj[name]
		if !present {
			continue
		}
		subSchema, ok := sub.(map[string]any)
		if !ok {
			continue
		}
		probs = append(probs, s.checkIn(doc0, join(path, name), subSchema, v)...)
	}

	// if/then: apply then only when if validates cleanly.
	if cond, ok := node["if"].(map[string]any); ok {
		if then, hasThen := node["then"].(map[string]any); hasThen {
			if len(s.checkIn(doc0, path, cond, doc)) == 0 {
				probs = append(probs, s.checkIn(doc0, path, then, doc)...)
			}
		}
	}
	if all, ok := node["allOf"].([]any); ok {
		for _, sub := range all {
			if subSchema, isMap := sub.(map[string]any); isMap {
				probs = append(probs, s.checkIn(doc0, path, subSchema, doc)...)
			}
		}
	}
	return probs
}

func join(path, name string) string {
	if path == "" {
		return name
	}
	return path + "." + name
}

func checkType(t any, doc any) string {
	var want []string
	switch v := t.(type) {
	case string:
		want = []string{v}
	case []any:
		for _, x := range v {
			if s, ok := x.(string); ok {
				want = append(want, s)
			}
		}
	default:
		return fmt.Sprintf("schema type %v is neither a string nor a list", t)
	}
	for _, w := range want {
		if matchesType(w, doc) {
			return ""
		}
	}
	return fmt.Sprintf("expected type %s, got %T", strings.Join(want, "|"), doc)
}

func matchesType(want string, doc any) bool {
	switch want {
	case "object":
		_, ok := doc.(map[string]any)
		return ok
	case "array":
		_, ok := doc.([]any)
		return ok
	case "string":
		_, ok := doc.(string)
		return ok
	case "boolean":
		_, ok := doc.(bool)
		return ok
	case "number":
		_, ok := doc.(float64)
		return ok
	case "integer":
		f, ok := doc.(float64)
		return ok && f == float64(int64(f))
	case "null":
		return doc == nil
	}
	return false
}

func equal(a, b any) bool {
	return fmt.Sprintf("%v", a) == fmt.Sprintf("%v", b)
}