feat: add support for directives

Author: pavelnikolov

README.md

@@ -163,6 +163,8 @@ schema := graphql.MustParseSchema(sdl, &RootResolver{}, nil)
 - `DisableFieldSelections()` disables capturing child field selections used by helper APIs (see below).
 - `DisableMemoryPooling()` disables internal execution-path memory pooling. Pooling is enabled by default; this option is intended for diagnostics and benchmark comparisons.
 - `OverlapValidationLimit(n int)` sets a hard cap on examined overlap pairs during validation; exceeding it emits `OverlapValidationLimitExceeded` error.
+- `DirectiveVisitors(...)` registers directive visitors to inspect and validate directives during pre-execution analysis. Visitors can implement authorization, cost analysis, analytics, or custom logic. Users implement the `DirectiveVisitor` interface with `Name() string` and `Visit(ctx context.Context, d DirectiveContext) error`. Call `d.DecodeArgs(&args)` inside `Visit` to parse directive arguments, and `d.FieldArg(name, &value)` to read field arguments by name. Visitor names must be unique per schema.
+- `PreExecHook(...)` registers a callback that runs after validation and directive pre-execution checks, but before resolver execution. Returning an error aborts execution.
 
 ### Field Selection Inspection Helpers
 
@@ -283,4 +285,9 @@ type Tracer interface {
 }
 ```
 
+### Directive Visitors
+
+Directive visitors provide a generic, Go-idiomatic way to inspect and validate GraphQL directives during pre-execution analysis. Visitors can directly reject execution by returning an error.
+Each directive name can have at most one registered visitor per schema.
+
 ### [Examples](https://github.com/graph-gophers/graphql-go/wiki/Examples)

directives.go

@@ -0,0 +1,299 @@
+package graphql
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+
+	"github.com/graph-gophers/graphql-go/ast"
+	"github.com/graph-gophers/graphql-go/errors"
+	"github.com/graph-gophers/graphql-go/internal/exec/packer"
+	"github.com/graph-gophers/graphql-go/internal/exec/selected"
+)
+
+type directiveArgsCacheKey struct {
+	def *ast.DirectiveDefinition
+	typ reflect.Type
+}
+
+type DirectiveContext struct {
+	owner       *Schema
+	schema      *ast.Schema
+	directive   *ast.Directive
+	args        map[string]any
+	fieldArgs   map[string]any
+	decodedArgs map[reflect.Type]reflect.Value
+}
+
+func (c DirectiveContext) DecodeArgs(dst any) error {
+	if c.schema == nil || c.directive == nil {
+		return fmt.Errorf("directive context is missing schema metadata")
+	}
+	def := c.schema.Directives[c.directive.Name.Name]
+	if def == nil {
+		return fmt.Errorf("directive %q is not defined in the schema", c.directive.Name.Name)
+	}
+	if dst == nil {
+		return fmt.Errorf("destination must be a non-nil pointer")
+	}
+	typ := reflect.TypeOf(dst)
+	if typ.Kind() != reflect.Pointer {
+		return fmt.Errorf("destination must be a pointer, got %s", typ)
+	}
+	rv := reflect.ValueOf(dst)
+	if rv.IsNil() {
+		return fmt.Errorf("destination must be a non-nil pointer")
+	}
+	var (
+		sp  *packer.StructPacker
+		err error
+	)
+	if c.owner != nil {
+		sp, err = c.owner.directiveArgsPacker(def, typ)
+	} else {
+		b := packer.NewBuilder()
+		sp, err = b.MakeStructPacker(def.Arguments, typ)
+		if err != nil {
+			return err
+		}
+		err = b.Finish()
+	}
+	if err != nil {
+		return err
+	}
+	if c.decodedArgs != nil {
+		if packed, ok := c.decodedArgs[typ]; ok {
+			rv.Elem().Set(packed.Elem())
+			return nil
+		}
+	}
+	packed, err := sp.Pack(c.args)
+	if err != nil {
+		return err
+	}
+	if c.decodedArgs != nil {
+		c.decodedArgs[typ] = packed
+	}
+	rv.Elem().Set(packed.Elem())
+	return nil
+}
+
+func (c DirectiveContext) FieldArg(name string, dst any) error {
+	if c.fieldArgs == nil {
+		return fmt.Errorf("directive context is missing field arguments")
+	}
+	if dst == nil {
+		return fmt.Errorf("destination must be a non-nil pointer")
+	}
+	typ := reflect.TypeOf(dst)
+	if typ.Kind() != reflect.Pointer {
+		return fmt.Errorf("destination must be a pointer, got %s", typ)
+	}
+	rv := reflect.ValueOf(dst)
+	if rv.IsNil() {
+		return fmt.Errorf("destination must be a non-nil pointer")
+	}
+
+	value, ok := c.fieldArgs[name]
+	if !ok {
+		return fmt.Errorf("field argument %q not found", name)
+	}
+
+	packed, err := (&packer.ValuePacker{ValueType: typ.Elem()}).Pack(value)
+	if err != nil {
+		return err
+	}
+	rv.Elem().Set(packed)
+	return nil
+}
+
+// DirectiveVisitor defines the interface for directive visitors.
+type DirectiveVisitor interface {
+	// Name returns the name of the directive this visitor handles.
+	Name() string
+	// Visit is called when the directive is encountered during field definition traversal.
+	// Use [DirectiveContext.DecodeArgs] to parse the directive arguments.
+	Visit(ctx context.Context, d DirectiveContext) error
+}
+
+// DirectiveVisitors registers one or more directive visitors with the schema.
+// Visitor names must be unique within a schema.
+func DirectiveVisitors(visitors ...DirectiveVisitor) SchemaOpt {
+	return func(s *Schema) {
+		for _, v := range visitors {
+			visitor, err := newDirectiveVisitor(v)
+			if err != nil {
+				s.optErr = err
+				return
+			}
+			s.directiveVisitors = append(s.directiveVisitors, visitor)
+		}
+	}
+}
+
+func newDirectiveVisitor(visitor DirectiveVisitor) (DirectiveVisitor, error) {
+	if visitor == nil {
+		return nil, fmt.Errorf("directive visitor is nil")
+	}
+
+	name := visitor.Name()
+	if name == "" {
+		return nil, fmt.Errorf("directive visitor must have a non-empty name")
+	}
+
+	return visitor, nil
+}
+
+func (s *Schema) validateDirectiveVisitors() error {
+	seen := make(map[string]struct{}, len(s.directiveVisitors))
+	for _, v := range s.directiveVisitors {
+		name := v.Name()
+		def := s.schema.Directives[name]
+		if def == nil {
+			return fmt.Errorf("directive %q is not defined in the schema", name)
+		}
+		if _, ok := seen[name]; ok {
+			return fmt.Errorf("directive visitor %q is already registered", name)
+		}
+		seen[name] = struct{}{}
+	}
+
+	return nil
+}
+
+func directiveArgs(schema *ast.Schema, d *ast.Directive, vars map[string]any) (map[string]any, error) {
+	if d == nil {
+		return nil, fmt.Errorf("directive is nil")
+	}
+	def := schema.Directives[d.Name.Name]
+	if def == nil {
+		return nil, fmt.Errorf("directive %q is not defined in the schema", d.Name.Name)
+	}
+	args := make(map[string]any, len(def.Arguments))
+	for _, arg := range def.Arguments {
+		if v, ok := d.Arguments.Get(arg.Name.Name); ok {
+			if isNilValue(v) {
+				args[arg.Name.Name] = nil
+				continue
+			}
+			args[arg.Name.Name] = v.Deserialize(vars)
+			continue
+		}
+		if arg.Default != nil {
+			args[arg.Name.Name] = arg.Default.Deserialize(nil)
+			continue
+		}
+		args[arg.Name.Name] = nil
+	}
+	return args, nil
+}
+
+func isNilValue(v ast.Value) bool {
+	if v == nil {
+		return true
+	}
+
+	rv := reflect.ValueOf(v)
+	switch rv.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
+		return rv.IsNil()
+	default:
+		return false
+	}
+}
+
+func (s *Schema) directiveArgsPacker(def *ast.DirectiveDefinition, typ reflect.Type) (*packer.StructPacker, error) {
+	if def == nil {
+		return nil, fmt.Errorf("directive definition is nil")
+	}
+	key := directiveArgsCacheKey{def: def, typ: typ}
+
+	s.directiveArgsMu.RLock()
+	sp := s.directiveArgsPackers[key]
+	s.directiveArgsMu.RUnlock()
+	if sp != nil {
+		return sp, nil
+	}
+
+	b := packer.NewBuilder()
+	sp, err := b.MakeStructPacker(def.Arguments, typ)
+	if err != nil {
+		return nil, err
+	}
+	if err := b.Finish(); err != nil {
+		return nil, err
+	}
+
+	s.directiveArgsMu.Lock()
+	defer s.directiveArgsMu.Unlock()
+	if s.directiveArgsPackers == nil {
+		s.directiveArgsPackers = make(map[directiveArgsCacheKey]*packer.StructPacker)
+	}
+	if cached := s.directiveArgsPackers[key]; cached != nil {
+		return cached, nil
+	}
+	s.directiveArgsPackers[key] = sp
+	return sp, nil
+}
+
+func (s *Schema) buildDirectiveCaches() {
+	if s.directiveArgsPackers == nil {
+		s.directiveArgsPackers = make(map[directiveArgsCacheKey]*packer.StructPacker)
+	}
+	s.directiveVisitorsByName = make(map[string]DirectiveVisitor, len(s.directiveVisitors))
+	for _, v := range s.directiveVisitors {
+		name := v.Name()
+		s.directiveVisitorsByName[name] = v
+	}
+}
+
+func (s *Schema) runDirectiveVisitors(ctx context.Context, vars map[string]any, sels []selected.Selection) []*errors.QueryError {
+	if len(s.directiveVisitorsByName) == 0 {
+		return nil
+	}
+
+	var errs []*errors.QueryError
+
+	path := make([]any, 0, 8)
+	var walk func([]selected.Selection)
+	walk = func(selections []selected.Selection) {
+		for _, sel := range selections {
+			switch sel := sel.(type) {
+			case *selected.SchemaField:
+				path = append(path, sel.Alias)
+				for _, d := range sel.Directives {
+					hook, ok := s.directiveVisitorsByName[d.Name.Name]
+					if !ok {
+						continue
+					}
+					args, err := directiveArgs(s.schema, d, vars)
+					if err != nil {
+						errs = append(errs, &errors.QueryError{Message: err.Error(), Path: append([]any(nil), path...)})
+						continue
+					}
+					dctx := DirectiveContext{
+						owner:     s,
+						schema:    s.schema,
+						fieldArgs: sel.Args,
+						directive: d,
+						args:      args,
+					}
+					err = hook.Visit(ctx, dctx)
+					if err != nil {
+						errs = append(errs, &errors.QueryError{Message: err.Error(), Path: append([]any(nil), path...)})
+					}
+				}
+				walk(sel.Sels)
+				path = path[:len(path)-1]
+			case *selected.TypeAssertion:
+				walk(sel.Sels)
+			}
+		}
+	}
+
+	walk(sels)
+	if len(errs) != 0 {
+		return errs
+	}
+	return nil
+}