Add WithFailFast() (#118)

In sourcegraph/conc#104, a "FailFast" option is
likely going to be added to iterators that, on error, stops running
additional tasks and returns the error that caused the first failure.

"FailFast" seems is a pretty standard description for this behavior, and
combining two common options into a single option with a more
discoverable name seems like a nice thing to do before 1.0.

Author: tanaka0722-dev

pool/context_pool.go

@@ -81,6 +81,16 @@ func (p *ContextPool) WithCancelOnError() *ContextPool {
 	return p
 }
 
+// WithFailFast is an alias for the combination of WithFirstError and
+// WithCancelOnError. By default, the errors from all tasks are returned and
+// the pool's context is not canceled until the parent context is canceled.
+func (p *ContextPool) WithFailFast() *ContextPool {
+	p.panicIfInitialized()
+	p.WithFirstError()
+	p.WithCancelOnError()
+	return p
+}
+
 // WithMaxGoroutines limits the number of goroutines in a pool.
 // Defaults to unlimited. Panics if n < 1.
 func (p *ContextPool) WithMaxGoroutines(n int) *ContextPool {

pool/context_pool_test.go

@@ -187,9 +187,9 @@ func TestContextPool(t *testing.T) {
 		require.NotErrorIs(t, err, err2)
 	})
 
-	t.Run("WithFirstError and WithCancelOnError", func(t *testing.T) {
+	t.Run("WithFailFast", func(t *testing.T) {
 		t.Parallel()
-		p := New().WithContext(bgctx).WithFirstError().WithCancelOnError()
+		p := New().WithContext(bgctx).WithFailFast()
 		p.Go(func(ctx context.Context) error {
 			return err1
 		})

pool/result_context_pool.go

@@ -62,6 +62,15 @@ func (p *ResultContextPool[T]) WithCancelOnError() *ResultContextPool[T] {
 	return p
 }
 
+// WithFailFast is an alias for the combination of WithFirstError and
+// WithCancelOnError. By default, the errors from all tasks are returned and
+// the pool's context is not canceled until the parent context is canceled.
+func (p *ResultContextPool[T]) WithFailFast() *ResultContextPool[T] {
+	p.panicIfInitialized()
+	p.contextPool.WithFailFast()
+	return p
+}
+
 // WithMaxGoroutines limits the number of goroutines in a pool.
 // Defaults to unlimited. Panics if n < 1.
 func (p *ResultContextPool[T]) WithMaxGoroutines(n int) *ResultContextPool[T] {

pool/result_context_pool_test.go

@@ -101,6 +101,22 @@ func TestResultContextPool(t *testing.T) {
 		require.ErrorIs(t, err, err1)
 	})
 
+	t.Run("WithFailFast", func(t *testing.T) {
+		t.Parallel()
+		p := NewWithResults[int]().WithContext(context.Background()).WithFailFast()
+		p.Go(func(ctx context.Context) (int, error) {
+			return 0, err1
+		})
+		p.Go(func(ctx context.Context) (int, error) {
+			<-ctx.Done()
+			return 1, ctx.Err()
+		})
+		results, err := p.Wait()
+		require.ErrorIs(t, err, err1)
+		require.NotErrorIs(t, err, context.Canceled)
+		require.Empty(t, results)
+	})
+
 	t.Run("WithCancelOnError and panic", func(t *testing.T) {
 		t.Parallel()
 		p := NewWithResults[int]().