@ -6,13 +6,13 @@
// Example usage:
//
// // Create a context key.
// var TimeoutKey = ctxkey.New(" fsrv .Timeout", 5*time.Second)
// var TimeoutKey = ctxkey.New(" mapreduce .Timeout", 5*time.Second)
//
// // Store a context value.
// ctx = fsrv .TimeoutKey.WithValue(ctx, 10*time.Second)
// ctx = mapreduce .TimeoutKey.WithValue(ctx, 10*time.Second)
//
// // Load a context value.
// timeout := fsrv .TimeoutKey.Value(ctx)
// timeout := mapreduce .TimeoutKey.Value(ctx)
// ... // use timeout of type time.Duration
//
// This is inspired by https://go.dev/issue/49189.
@ -24,20 +24,23 @@ import (
"reflect"
)
// TODO(https://go.dev/issue/60088): Use reflect.TypeFor instead.
func reflectTypeFor [ T any ] ( ) reflect . Type {
return reflect . TypeOf ( ( * T ) ( nil ) ) . Elem ( )
}
// Key is a generic key type associated with a specific value type.
//
// A zero Key is valid where the Value type itself is used as the context key.
// This pattern should only be used with locally declared Go types.
// The Value type must not be an interface type.
// This pattern should only be used with locally declared Go types ,
// otherwise different packages risk producing key conflicts .
//
// Example usage:
//
// type peerInfo struct { ... } // peerInfo is a n unexport ed type
// var peerInfoKey = ctxkey.Key[peerInfo]
// type peerInfo struct { ... } // peerInfo is a locally declar ed type
// var peerInfoKey ctxkey.Key[peerInfo]
// ctx = peerInfoKey.WithValue(ctx, info) // store a context value
// info = peerInfoKey.Value(ctx) // load a context value
//
// In general, any exported keys should be produced using [New].
type Key [ Value any ] struct {
name * stringer [ string ]
defVal * Value
@ -49,6 +52,7 @@ type Key[Value any] struct {
// The provided name is an arbitrary name only used for human debugging.
// As a convention, it is recommended that the name be the dot-delimited
// combination of the package name of the caller with the variable name.
// If the name is not provided, then the name of the Value type is used.
// Every key is unique, even if provided the same name.
//
// Example usage:
@ -56,32 +60,25 @@ type Key[Value any] struct {
// package mapreduce
// var NumWorkersKey = ctxkey.New("mapreduce.NumWorkers", runtime.NumCPU())
func New [ Value any ] ( name string , defaultValue Value ) Key [ Value ] {
// Allocate a new stringer to ensure that every invocation of New
// creates a universally unique context key even for the same name
// since newly allocated pointers are globally unique within a process.
key := Key [ Value ] { name : new ( stringer [ string ] ) }
if name == "" {
var v Value
name = reflect . TypeOf ( v ) . String ( ) // TODO(https://go.dev/issue/60088): Use reflect.TypeFor.
name = reflectTypeFor [ Value ] ( ) . String ( )
}
var defVal * Value
switch v := reflect . ValueOf ( & defaultValue ) . Elem ( ) ; {
case v . Kind ( ) == reflect . Interface :
panic ( fmt . Sprintf ( "value type %v must not be an interface" , v . Type ( ) ) )
case ! v . IsZero ( ) :
defVal = & defaultValue
key . name . v = name
if v := reflect . ValueOf ( defaultValue ) ; v . IsValid ( ) && ! v . IsZero ( ) {
key . defVal = & defaultValue
}
// Allocate a *stringer to ensure that every invocation of New
// creates a universally unique context key even for the same name.
return Key [ Value ] { name : & stringer [ string ] { name } , defVal : defVal }
return key
}
// contextKey returns the context key to use.
func ( key Key [ Value ] ) contextKey ( ) any {
if key . name == nil {
// Use the reflect.Type of the Value (implies key not created by New).
var v Value
t := reflect . TypeOf ( v )
if t == nil {
panic ( fmt . Sprintf ( "value type %v must not be an interface" , reflect . TypeOf ( & v ) . Elem ( ) ) )
}
return t
return reflectTypeFor [ Value ] ( )
} else {
// Use the name pointer directly (implies key created by New).
return key . name
@ -122,8 +119,7 @@ func (key Key[Value]) Has(ctx context.Context) (ok bool) {
// String returns the name of the key.
func ( key Key [ Value ] ) String ( ) string {
if key . name == nil {
var v Value
return reflect . TypeOf ( v ) . String ( ) // TODO(https://go.dev/issue/60088): Use reflect.TypeFor.
return reflectTypeFor [ Value ] ( ) . String ( )
}
return key . name . String ( )
}
@ -134,6 +130,11 @@ func (key Key[Value]) String() string {
// Note that the [context] package lacks a dependency on [reflect],
// so it cannot print arbitrary values. By implementing [fmt.Stringer],
// we functionally teach a context how to print itself.
//
// Wrapping values within a struct has an added bonus that interface kinds
// are properly handled. Without wrapping, we would be unable to distinguish
// between a nil value that was explicitly set or not.
// However, the presence of a stringer indicates an explicit nil value.
type stringer [ T any ] struct { v T }
func ( v stringer [ T ] ) String ( ) string { return fmt . Sprint ( v . v ) }
Joe Tsai
4 months ago
committed by