sentinel error for group done so we can still cancel

collect.go

@@ -223,6 +223,16 @@ func FeedWithErrs[T any](executor Executor, receiver func(context.Context, T) er
 	return making
 }
 
+// groupError returns the error associated with a group's context; if the error
+// was errGroupDone, that doesn't count as an error and nil is returned instead.
+func groupError(ctx context.Context) error {
+	err := context.Cause(ctx)
+	if err == errGroupDone {
+		return nil
+	}
+	return err
+}
+
 var _ ErrGroupExecutor = &errGroup{}
 
 type errGroup struct {
@@ -242,7 +252,7 @@ func (eg *errGroup) Go(op func(context.Context) error) {
 func (eg *errGroup) Wait() error {
 	eg.g.Wait()
 	ctx, _ := eg.g.getContext()
-	return context.Cause(ctx)
+	return groupError(ctx)
 }
 
 func makePipeGroup[T any, R any](executor Executor) *pipeGroup[T, R] {
@@ -329,7 +339,7 @@ func (cg collectingGroup[T]) Go(op func(context.Context) (T, error)) {
 func (cg collectingGroup[T]) Wait() ([]T, error) {
 	cg.doWait()
 	ctx, _ := cg.g.getContext()
-	if err := context.Cause(ctx); err != nil {
+	if err := groupError(ctx); err != nil {
 		// We have an error; return it
 		return nil, err
 	}
@@ -358,7 +368,7 @@ func (fg feedingGroup[T]) Go(op func(context.Context) (T, error)) {
 func (fg feedingGroup[T]) Wait() error {
 	fg.doWait()
 	ctx, _ := fg.g.getContext()
-	return context.Cause(ctx)
+	return groupError(ctx)
 }
 
 var _ AllErrsExecutor = multiErrGroup{}
@@ -381,7 +391,7 @@ func (meg multiErrGroup) Wait() MultiError {
 	meg.doWait()
 	err := CombineErrors(*meg.res...)
 	ctx, _ := meg.g.getContext()
-	if cause := context.Cause(ctx); cause != nil {
+	if cause := groupError(ctx); cause != nil {
 		return CombineErrors(cause, err)
 	}
 	return err
@@ -415,7 +425,7 @@ func (ceg collectingMultiErrGroup[T]) Wait() ([]T, MultiError) {
 	ceg.doWait()
 	res, err := ceg.res.values, CombineErrors(ceg.res.errs...)
 	ctx, _ := ceg.g.getContext()
-	if cause := context.Cause(ctx); cause != nil {
+	if cause := groupError(ctx); cause != nil {
 		return res, CombineErrors(cause, err)
 	}
 	return res, err
@@ -439,7 +449,7 @@ func (feg feedingMultiErrGroup[T]) Wait() MultiError {
 	feg.doWait()
 	err := CombineErrors(*feg.res...)
 	ctx, _ := feg.g.getContext()
-	if cause := context.Cause(ctx); cause != nil {
+	if cause := groupError(ctx); cause != nil {
 		return CombineErrors(cause, err)
 	}
 	return err

group.go

@@ -15,7 +15,14 @@ const bufferSize = 8
 
 const misuseMessage = "parallel executor misuse: don't reuse executors"
 
-var errPanicked = errors.New("panicked")
+var (
+	errPanicked       = errors.New("panicked")
+	errGroupDone      = errors.New("executor done")
+	errGroupAbandoned = errors.New("executor abandoned")
+
+	// Contexts are canceled with this error when executors are awaited.
+	GroupDoneError = errGroupDone
+)
 
 // NOTE: If you want to really get crazy with it, it IS permissible and safe to
 // call Go(...) from multiple threads without additional synchronization, on
@@ -63,7 +70,7 @@ func Limited(ctx context.Context, maxGoroutines int) Executor {
 		gctx, cancel := context.WithCancelCause(ctx)
 		g := &runner{ctx: gctx, cancel: cancel}
 		runtime.SetFinalizer(g, func(doomed *runner) {
-			doomed.cancel(nil)
+			doomed.cancel(errGroupAbandoned)
 		})
 		return g
 	}
@@ -74,7 +81,7 @@ func Limited(ctx context.Context, maxGoroutines int) Executor {
 	}
 	runtime.SetFinalizer(making, func(doomed *limitedGroup) {
 		close(doomed.ops)
-		doomed.g.cancel(nil)
+		doomed.g.cancel(errGroupAbandoned)
 	})
 	return making
 }
@@ -102,7 +109,7 @@ func (n *runner) Wait() {
 	n.awaited.Store(true)
 	// We are ending the group's lifetime within this function call; defer the
 	// cancelation and unset our finalizer.
-	n.cancel(nil)
+	n.cancel(errGroupDone)
 	runtime.SetFinalizer(n, nil)
 }
 
@@ -113,7 +120,7 @@ func (n *runner) getContext() (context.Context, context.CancelCauseFunc) {
 func makeGroup(ctx context.Context, cancel context.CancelCauseFunc) *group {
 	g := &group{runner: runner{ctx: ctx, cancel: cancel}}
 	runtime.SetFinalizer(g, func(doomed *group) {
-		doomed.cancel(nil)
+		doomed.cancel(errGroupAbandoned)
 	})
 	return g
 }
@@ -161,7 +168,7 @@ func (g *group) Wait() {
 	g.awaited.Store(true)
 	// We are ending the group's lifetime within this function call; defer the
 	// cancelation and unset our finalizer.
-	defer g.cancel(nil)
+	defer g.cancel(errGroupDone)
 	runtime.SetFinalizer(g, nil)
 	g.wg.Wait()
 	g.checkPanic()