Minor improvements for implicit errgroup initialization

Should be coverage friendly

Author: egorse

internal/errgroup/errgroup.go

@@ -30,21 +30,15 @@ func WithContext(ctx context.Context) (*Group, context.Context) {
 // Go runs the provided f function in a dedicated goroutine and waits for its
 // completion or for the parent context cancellation.
 func (g *Group) Go(f func() error) {
-	if g.grp == nil {
-		g.grp = &errgroup.Group{}
-	}
-	g.grp.Go(g.wrap(f))
+	g.getErrGroup().Go(g.wrap(f))
 }
 
 // Wait blocks until all function calls from the Go method have returned, then
 // returns the first non-nil error (if any) from them.
 // If the error group was created via WithContext then the Wait returns error
 // of cancelled parent context prior any functions calls complete.
 func (g *Group) Wait() error {
-	if g.grp == nil {
-		g.grp = &errgroup.Group{}
-	}
-	return g.grp.Wait()
+	return g.getErrGroup().Wait()
 }
 
 // SetLimit limits the number of active goroutines in this group to at most n.
@@ -55,21 +49,15 @@ func (g *Group) Wait() error {
 //
 // The limit must not be modified while any goroutines in the group are active.
 func (g *Group) SetLimit(n int) {
-	if g.grp == nil {
-		g.grp = &errgroup.Group{}
-	}
-	g.grp.SetLimit(n)
+	g.getErrGroup().SetLimit(n)
 }
 
 // TryGo calls the given function in a new goroutine only if the number of
 // active goroutines in the group is currently below the configured limit.
 //
 // The return value reports whether the goroutine was started.
 func (g *Group) TryGo(f func() error) bool {
-	if g.grp == nil {
-		g.grp = &errgroup.Group{}
-	}
-	return g.grp.TryGo(g.wrap(f))
+	return g.getErrGroup().TryGo(g.wrap(f))
 }
 
 func (g *Group) wrap(f func() error) func() error {
@@ -106,3 +94,14 @@ func (g *Group) wrap(f func() error) func() error {
 		}
 	}
 }
+
+// The getErrGroup returns actual x/sync/errgroup.Group.
+// If the group is not allocated it would implicitly allocate it.
+// Thats allows the internal/errgroup.Group be fully
+// compatible to x/sync/errgroup.Group
+func (g *Group) getErrGroup() *errgroup.Group {
+	if g.grp == nil {
+		g.grp = &errgroup.Group{}
+	}
+	return g.grp
+}