Use method is introduced(with tests)

singleflight/singleflight.go

@@ -6,7 +6,10 @@
 // mechanism.
 package singleflight // import "golang.org/x/sync/singleflight"
 
-import "sync"
+import (
+	"sync"
+	"sync/atomic"
+)
 
 // call is an in-flight or completed singleflight.Do call
 type call struct {
@@ -49,6 +52,39 @@ type Result struct {
 // original to complete and receives the same results.
 // The return value shared indicates whether v was given to multiple callers.
 func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
+	c := g.doNoChan(key, fn)
+	return c.val, c.err, c.dups > 0
+}
+
+// 'Use' calls 'new' at most once at a time, then invokes 'fn' with the resulting values.
+// The 'dispose' argument invokes after the last call to fn has returned.
+//
+// `Use` is designed for scenario when 'new' generates a temporary resource, which has to be cleaned up after last 'fn' is done using it
+//  Notes:
+//   'dispose' is called at most once, after last fn been completed. 'dispose' will NOT get called if/when 'new' returns an error
+//   'fn' is called on each goroutine with values returned by 'new', regardless of whether or not 'new' returned an error
+//   results of 'new' are passed to 'fn'.
+//
+// Return: 'Use' propagates return value from 'fn'
+func (g *Group) Use(
+	key string,
+	new func() (interface{}, error),
+	fn func(interface{}, error) error,
+	dispose func(interface{}),
+) error {
+	c := g.doNoChan(key, new)
+	if c.err == nil && dispose != nil {
+		defer func() {
+			if atomic.AddInt64(&c.dups, -1) == -1 {
+				dispose(c.val)
+			}
+		}()
+	}
+
+	return fn(c.val, c.err)
+}
+
+func (g *Group) doNoChan(key string, fn func() (interface{}, error)) *call {
 	g.mu.Lock()
 	if g.m == nil {
 		g.m = make(map[string]*call)
@@ -57,15 +93,15 @@ func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, e
 		c.dups++
 		g.mu.Unlock()
 		c.wg.Wait()
-		return c.val, c.err, true
+		return c
 	}
 	c := new(call)
 	c.wg.Add(1)
 	g.m[key] = c
 	g.mu.Unlock()
 
 	g.doCall(c, key, fn)
-	return c.val, c.err, c.dups > 0
+	return c
 }
 
 // DoChan is like Do but returns a channel that will receive the

singleflight/singleflight_test.go

@@ -7,12 +7,130 @@ package singleflight
 import (
 	"errors"
 	"fmt"
+	"io/ioutil"
+	"os"
 	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
 )
 
+func testConcurrentHelper(t *testing.T, inGoroutine func(routineIndex, goroutineCount int)) {
+	var wg, wgGoroutines sync.WaitGroup
+	const callers = 4
+	//ref := make([]RefCounter, callers)
+	wgGoroutines.Add(callers)
+	for i := 0; i < callers; i++ {
+		wg.Add(1)
+		go func(index int) {
+			defer wg.Done()
+
+			wgGoroutines.Done()
+			wgGoroutines.Wait() // ensure that all goroutines started and reached this point
+
+			inGoroutine(i, callers)
+		}(i)
+	}
+	wg.Wait()
+
+}
+
+func TestUse(t *testing.T) {
+	var g Group
+	var newCount, handleCount, disposeCount int64
+
+	testConcurrentHelper(
+		t,
+		func(index, goroutineCount int) {
+			g.Use(
+				"key",
+				// 'new' is a slow function that creates a temp resource
+				func() (interface{}, error) {
+					time.Sleep(200 * time.Millisecond) // let more goroutines enter Do
+					atomic.AddInt64(&newCount, 1)
+					return "bar", nil
+				},
+				// 'fn' to be called by each goroutine
+				func(s interface{}, e error) error {
+					// handle s
+					if newCount != 1 {
+						t.Errorf("goroutine %v: newCount(%v) expected to be set prior to this function getting called", index, newCount)
+					}
+					atomic.AddInt64(&handleCount, 1)
+					if disposeCount > 0 {
+						t.Errorf("goroutine %v: disposeCount(%v) should not be incremented until all fn are completed", index, disposeCount)
+					}
+					return e
+				},
+				// 'dispose' - to be called once at the end
+				func(s interface{}) {
+					// cleaning up "bar"
+					atomic.AddInt64(&disposeCount, 1)
+					if handleCount != int64(goroutineCount) {
+						t.Errorf("dispose is expected to be called when all %v fn been completed, but %v have been completed instead", goroutineCount, handleCount)
+					}
+				},
+			)
+		},
+	)
+
+	if newCount != 1 {
+		t.Errorf("new expected to be called exactly once, was called %v", newCount)
+	}
+	if disposeCount != 1 {
+		t.Errorf("dispose expected to be called exactly once, was called %v", disposeCount)
+	}
+}
+
+func TestUseWithResource(t *testing.T) {
+	// use this "global" var for checkes after that testConcurrentHelper call
+	var tempFileName string
+
+	var g Group
+	testConcurrentHelper(
+		t,
+		func(_, _ int) {
+			g.Use(
+				"key",
+				// 'new' is a slow function that creates a temp resource
+				func() (interface{}, error) {
+					time.Sleep(200 * time.Millisecond) // let more goroutines enter Do
+					f, e := ioutil.TempFile("", "pat")
+					if e != nil {
+						return nil, e
+					}
+					defer f.Close()
+					tempFileName = f.Name()
+
+					// fill temp file with sequence of n.Write(...) calls
+
+					return f.Name(), e
+				},
+				// 'fn' to be called by each goroutine
+				func(s interface{}, e error) error {
+					// handle s
+					if e != nil {
+						// send alternative payload
+					}
+					if e == nil {
+						/*tempFileName*/ _ = s.(string)
+						// send Content of tempFileName to HTTPWriter
+					}
+					return e
+				},
+				// 'dispose' - to be called once at the end
+				func(s interface{}) {
+					// cleaning up "bar"
+					os.Remove(s.(string))
+				},
+			)
+		},
+	)
+	if _, e := os.Stat(tempFileName); !os.IsNotExist(e) {
+		t.Errorf("test has created a temp file '%v', but failed to cleaned it", tempFileName)
+	}
+}
+
 func TestDo(t *testing.T) {
 	var g Group
 	v, err, _ := g.Do("key", func() (interface{}, error) {