break: refreshable/v2 with Generic type handling

refreshable/async.go

@@ -0,0 +1,100 @@
+// Copyright (c) 2022 Palantir Technologies. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package refreshable
+
+import (
+	"context"
+	"time"
+)
+
+type ready[T any] struct {
+	in     Updatable[T]
+	readyC <-chan struct{}
+	cancel context.CancelFunc
+}
+
+func newReady[T any](in Updatable[T]) *ready[T] {
+	ctx, cancel := context.WithCancel(context.Background())
+	return &ready[T]{
+		in:     in,
+		readyC: ctx.Done(),
+		cancel: cancel,
+	}
+}
+
+func (r *ready[T]) Current() T {
+	return r.in.Current()
+}
+
+func (r *ready[T]) Subscribe(consumer func(T)) UnsubscribeFunc {
+	return r.in.Subscribe(consumer)
+}
+
+func (r *ready[T]) ReadyC() <-chan struct{} {
+	return r.readyC
+}
+
+func (r *ready[T]) Update(val T) {
+	r.cancel()
+	r.in.Update(val)
+}
+
+// NewFromChannel populates an Updatable with the values channel.
+// If an element is already available, the returned Value is guaranteed to be populated.
+// The channel should be closed when no longer used to avoid leaking resources.
+func NewFromChannel[T any](values <-chan T) Ready[T] {
+	out := newReady(newZero[T]())
+	select {
+	case initial, ok := <-values:
+		if !ok {
+			return out // channel already closed
+		}
+		out.Update(initial)
+	default:
+	}
+	go func() {
+		for value := range values {
+			out.Update(value)
+		}
+	}()
+	return out
+}
+
+// NewFromTickerFunc returns a Ready Refreshable populated by the result of the provider called each interval.
+// If the providers bool return is false, the value is ignored.
+// The result's ReadyC channel is closed when a new value is populated.
+func NewFromTickerFunc[T any](interval time.Duration, provider func() (T, bool)) (Ready[T], UnsubscribeFunc) {
+	out := newReady(newZero[T]())
+	ctx, cancel := context.WithCancel(context.Background())
+	values := make(chan T)
+	go func() {
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+		defer close(values)
+		for {
+			if value, ok := provider(); ok {
+				out.Update(value)
+			}
+			select {
+			case <-ticker.C:
+				continue
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+	return out, UnsubscribeFunc(cancel)
+}
+
+// Wait waits until the Ready has a current value or the context expires.
+func Wait[T any](ctx context.Context, ready Ready[T]) (T, bool) {
+	select {
+	case <-ready.ReadyC():
+		return ready.Current(), true
+	case <-ctx.Done():
+		var zero T
+		return zero, false
+	}
+}

refreshable/go.mod

@@ -1,4 +1,4 @@
-module github.com/palantir/pkg/refreshable
+module github.com/palantir/pkg/refreshable/v2
 
 go 1.20
 

refreshable/refreshable.go

@@ -4,14 +4,91 @@
 
 package refreshable
 
-type Refreshable interface {
+import (
+	"context"
+)
+
+// A Refreshable is a generic container type for a volatile underlying value.
+// It supports atomic access and user-provided callback "subscriptions" on updates.
+type Refreshable[T any] interface {
 	// Current returns the most recent value of this Refreshable.
-	Current() interface{}
+	// If the value has not been initialized, returns T's zero value.
+	Current() T
+
+	// Subscribe calls the consumer function when Value updates until stop is closed.
+	// The consumer must be relatively fast: Updatable.Set blocks until all subscribers have returned.
+	// Expensive or error-prone responses to refreshed values should be asynchronous.
+	// Updates considered no-ops by reflect.DeepEqual may be skipped.
+	Subscribe(consumer func(T)) UnsubscribeFunc
+}
+
+// A Updatable is a Refreshable which supports setting the value with a user-provided value.
+// When a utility returns a (non-Updatable) Refreshable, it implies that value updates are handled internally.
+type Updatable[T any] interface {
+	Refreshable[T]
+	// Update updates the Refreshable with a new T.
+	// It blocks until all subscribers have completed.
+	Update(T)
+}
+
+// A Validated is a Refreshable capable of rejecting updates according to validation logic.
+// Its Current method returns the most recent value to pass validation.
+type Validated[T any] interface {
+	Refreshable[T]
+	// Validation returns the result of the most recent validation.
+	// If the last value was valid, Validation returns the same value as Current and a nil error.
+	// If the last value was invalid, it and the error are returned. Current returns the most recent valid value.
+	Validation() (T, error)
+}
+
+// Ready extends Refreshable for asynchronous implementations which may not have a value when they are constructed.
+// Callers should check that the Ready channel is closed before using the Current value.
+type Ready[T any] interface {
+	Refreshable[T]
+	// ReadyC returns a channel which is closed after a value is successfully populated.
+	ReadyC() <-chan struct{}
+}
 
-	// Subscribe subscribes to changes of this Refreshable. The provided function is called with the value of Current()
-	// whenever the value changes.
-	Subscribe(consumer func(interface{})) (unsubscribe func())
+// UnsubscribeFunc removes a subscription from a refreshable's internal tracking and/or stops its update routine.
+// It is safe to call multiple times.
+type UnsubscribeFunc func()
+
+func New[T any](val T) Updatable[T] {
+	return newDefault(val)
+}
+
+// Map returns a new Refreshable based on the current one that handles updates based on the current Refreshable.
+func Map[T any, M any](original Refreshable[T], mapFn func(T) M) (Refreshable[M], UnsubscribeFunc) {
+	out := New(mapFn(original.Current()))
+	stop := original.Subscribe(func(v T) {
+		out.Update(mapFn(v))
+	})
+	out.Update(mapFn(original.Current()))
+	return out, stop
+}
+
+// MapContext is like Map but unsubscribes when the context is cancelled.
+func MapContext[T any, M any](ctx context.Context, original Refreshable[T], mapFn func(T) M) Refreshable[M] {
+	out, stop := Map(original, mapFn)
+	go func() {
+		<-ctx.Done()
+		stop()
+	}()
+	return out
+}
+
+// MapWithError is similar to Validate but allows for the function to return a mapping/mutation
+// of the input object in addition to returning an error. The returned validRefreshable will contain the mapped value.
+// An error is returned if the current original value fails to map.
+func MapWithError[T any, M any](original Refreshable[T], mapFn func(T) (M, error)) (Validated[M], UnsubscribeFunc, error) {
+	v, stop := newValidRefreshable(original, mapFn)
+	_, err := v.Validation()
+	return v, stop, err
+}
 
-	// Map returns a new Refreshable based on the current one that handles updates based on the current Refreshable.
-	Map(func(interface{}) interface{}) Refreshable
+// Validate returns a new Refreshable that returns the latest original value accepted by the validatingFn.
+// If the upstream value results in an error, it is reported by Validation().
+// An error is returned if the current original value is invalid.
+func Validate[T any](original Refreshable[T], validatingFn func(T) error) (Validated[T], UnsubscribeFunc, error) {
+	return MapWithError(original, identity(validatingFn))
 }

refreshable/refreshable_default.go

@@ -5,84 +5,71 @@
 package refreshable
 
 import (
-	"fmt"
 	"reflect"
 	"sync"
 	"sync/atomic"
 )
 
-type DefaultRefreshable struct {
-	typ     reflect.Type
-	current *atomic.Value
-
-	sync.Mutex  // protects subscribers
-	subscribers []*func(interface{})
+type defaultRefreshable[T any] struct {
+	mux         sync.Mutex
+	current     atomic.Value
+	subscribers []*func(T)
 }
 
-func NewDefaultRefreshable(val interface{}) *DefaultRefreshable {
-	current := atomic.Value{}
-	current.Store(val)
-
-	return &DefaultRefreshable{
-		current: &current,
-		typ:     reflect.TypeOf(val),
-	}
+func newDefault[T any](val T) Updatable[T] {
+	d := new(defaultRefreshable[T])
+	d.current.Store(&val)
+	return d
 }
 
-func (d *DefaultRefreshable) Update(val interface{}) error {
-	d.Lock()
-	defer d.Unlock()
-
-	if valType := reflect.TypeOf(val); valType != d.typ {
-		return fmt.Errorf("new refreshable value must be type %s: got %s", d.typ, valType)
-	}
+func newZero[T any]() Updatable[T] {
+	d := new(defaultRefreshable[T])
+	var zero T
+	d.current.Store(&zero)
+	return d
+}
 
-	if reflect.DeepEqual(d.current.Load(), val) {
-		return nil
+// Update changes the value of the Refreshable, then blocks while subscribers are executed.
+func (d *defaultRefreshable[T]) Update(val T) {
+	d.mux.Lock()
+	defer d.mux.Unlock()
+	old := d.current.Swap(&val)
+	if reflect.DeepEqual(*(old.(*T)), val) {
+		return
 	}
-	d.current.Store(val)
-
 	for _, sub := range d.subscribers {
 		(*sub)(val)
 	}
-	return nil
 }
 
-func (d *DefaultRefreshable) Current() interface{} {
-	return d.current.Load()
+func (d *defaultRefreshable[T]) Current() T {
+	return *(d.current.Load().(*T))
 }
 
-func (d *DefaultRefreshable) Subscribe(consumer func(interface{})) (unsubscribe func()) {
-	d.Lock()
-	defer d.Unlock()
+func (d *defaultRefreshable[T]) Subscribe(consumer func(T)) UnsubscribeFunc {
+	d.mux.Lock()
+	defer d.mux.Unlock()
 
 	consumerFnPtr := &consumer
 	d.subscribers = append(d.subscribers, consumerFnPtr)
-	return func() {
-		d.unsubscribe(consumerFnPtr)
-	}
+	return d.unsubscribe(consumerFnPtr)
 }
 
-func (d *DefaultRefreshable) unsubscribe(consumerFnPtr *func(interface{})) {
-	d.Lock()
-	defer d.Unlock()
-
-	matchIdx := -1
-	for idx, currSub := range d.subscribers {
-		if currSub == consumerFnPtr {
-			matchIdx = idx
-			break
+func (d *defaultRefreshable[T]) unsubscribe(consumerFnPtr *func(T)) UnsubscribeFunc {
+	return func() {
+		d.mux.Lock()
+		defer d.mux.Unlock()
+
+		matchIdx := -1
+		for idx, currSub := range d.subscribers {
+			if currSub == consumerFnPtr {
+				matchIdx = idx
+				break
+			}
+		}
+		if matchIdx != -1 {
+			d.subscribers = append(d.subscribers[:matchIdx], d.subscribers[matchIdx+1:]...)
 		}
 	}
-	if matchIdx != -1 {
-		d.subscribers = append(d.subscribers[:matchIdx], d.subscribers[matchIdx+1:]...)
-	}
-}
 
-func (d *DefaultRefreshable) Map(mapFn func(interface{}) interface{}) Refreshable {
-	newRefreshable := NewDefaultRefreshable(mapFn(d.Current()))
-	d.Subscribe(func(updatedVal interface{}) {
-		_ = newRefreshable.Update(mapFn(updatedVal))
-	})
-	return newRefreshable
 }