bloblru: add test for GetOrCompute

internal/bloblru/cache_test.go

@@ -1,11 +1,14 @@
 package bloblru
 
 import (
+	"context"
+	"fmt"
 	"math/rand"
 	"testing"
 
 	"github.com/restic/restic/internal/restic"
 	rtest "github.com/restic/restic/internal/test"
+	"golang.org/x/sync/errgroup"
 )
 
 func TestCache(t *testing.T) {
@@ -52,6 +55,70 @@ func TestCache(t *testing.T) {
 	rtest.Equals(t, cacheSize, c.free)
 }
 
+func TestCacheGetOrCompute(t *testing.T) {
+	var id1, id2 restic.ID
+	id1[0] = 1
+	id2[0] = 2
+
+	const (
+		kiB       = 1 << 10
+		cacheSize = 64*kiB + 3*overhead
+	)
+
+	c := New(cacheSize)
+
+	e := fmt.Errorf("broken")
+	_, err := c.GetOrCompute(id1, func() ([]byte, error) {
+		return nil, e
+	})
+	rtest.Equals(t, e, err, "expected error was not returned")
+
+	// fill buffer
+	data1 := make([]byte, 10*kiB)
+	blob, err := c.GetOrCompute(id1, func() ([]byte, error) {
+		return data1, nil
+	})
+	rtest.OK(t, err)
+	rtest.Equals(t, &data1[0], &blob[0], "wrong buffer returend")
+
+	// now the buffer should be returned without calling the compute function
+	blob, err = c.GetOrCompute(id1, func() ([]byte, error) {
+		return nil, e
+	})
+	rtest.OK(t, err)
+	rtest.Equals(t, &data1[0], &blob[0], "wrong buffer returend")
+
+	// check concurrency
+	wg, _ := errgroup.WithContext(context.TODO())
+	wait := make(chan struct{})
+	calls := make(chan struct{}, 10)
+
+	// start a bunch of blocking goroutines
+	for i := 0; i < 10; i++ {
+		wg.Go(func() error {
+			buf, err := c.GetOrCompute(id2, func() ([]byte, error) {
+				// block to ensure that multiple requests are waiting in parallel
+				<-wait
+				calls <- struct{}{}
+				return make([]byte, 42), nil
+			})
+			if len(buf) != 42 {
+				return fmt.Errorf("wrong buffer")
+			}
+			return err
+		})
+	}
+
+	close(wait)
+	rtest.OK(t, wg.Wait())
+	close(calls)
+	count := 0
+	for range calls {
+		count++
+	}
+	rtest.Equals(t, 1, count, "expected exactly one call of the compute function")
+}
+
 func BenchmarkAdd(b *testing.B) {
 	const (
 		MiB    = 1 << 20