feat: add Fallback function

Signed-off-by: Chen Su <ghosind@gmail.com>

Author: ghosind

fallback.go

@@ -0,0 +1,43 @@
+package async
+
+import "context"
+
+// Fallback tries to run the functions in order until one function does not panic or return an
+// error. It returns nil if one function succeeds, or returns the last error if all functions fail.
+//
+//	err := async.Fallback(func() error {
+//	  return errors.New("first error")
+//	}, func() error {
+//	  return errors.New("second error")
+//	}, func() error {
+//	  return nil
+//	}, func() error {
+//	  return errors.New("third error")
+//	})
+//	// err: <nil>
+func Fallback(fn AsyncFn, fallbacks ...AsyncFn) error {
+	return fallback(context.Background(), fn, fallbacks...)
+}
+
+// FallbackWithContext tries to run the functions in order with the specified context until one
+// function does not panic or return an error. It returns nil if one function succeeds, or returns
+// the last error if all functions fail.
+func FallbackWithContext(ctx context.Context, fn AsyncFn, fallbacks ...AsyncFn) error {
+	return fallback(ctx, fn, fallbacks...)
+}
+
+// fallback runs the functions in order until one function does not panic or return an error.
+func fallback(parent context.Context, fn AsyncFn, fallbacks ...AsyncFn) error {
+	funcs := append([]AsyncFn{fn}, fallbacks...)
+	ctx := getContext(parent)
+	var err error
+
+	for _, fn := range funcs {
+		_, err = invokeAsyncFn(fn, ctx, nil)
+		if err == nil {
+			return nil
+		}
+	}
+
+	return err
+}

fallback_test.go

@@ -0,0 +1,94 @@
+package async_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/ghosind/go-assert"
+	"github.com/ghosind/go-async"
+)
+
+func TestFallback(t *testing.T) {
+	a := assert.New(t)
+	runCount := 0
+
+	err := async.Fallback(
+		func() error {
+			runCount++
+			return errors.New("first error")
+		},
+		func() error {
+			runCount++
+			return nil
+		},
+		func() error {
+			runCount++
+			return errors.New("last error")
+		},
+	)
+	a.NilNow(err)
+	a.EqualNow(runCount, 2)
+}
+
+func TestFallbackAllFailed(t *testing.T) {
+	a := assert.New(t)
+	runCount := 0
+	finalErr := errors.New("third error")
+
+	err := async.Fallback(
+		func() error {
+			runCount++
+			return errors.New("first error")
+		},
+		func() error {
+			runCount++
+			return errors.New("second error")
+		},
+		func() error {
+			runCount++
+			return finalErr
+		},
+	)
+	a.IsErrorNow(err, finalErr)
+	a.EqualNow(runCount, 3)
+}
+
+func TestFallbackWithContext(t *testing.T) {
+	a := assert.New(t)
+	runCount := 0
+
+	err := async.FallbackWithContext(
+		context.Background(),
+		func() error {
+			runCount++
+			return errors.New("first error")
+		},
+		func() error {
+			runCount++
+			return nil
+		},
+		func() error {
+			runCount++
+			return errors.New("last error")
+		},
+	)
+	a.NilNow(err)
+	a.EqualNow(runCount, 2)
+}
+
+func ExampleFallback() {
+	err := async.Fallback(func() error {
+		return errors.New("first error")
+	}, func() error {
+		return errors.New("second error")
+	}, func() error {
+		return nil
+	}, func() error {
+		return errors.New("third error")
+	})
+	if err != nil {
+		// handle the error
+	}
+	// err: <nil>
+}