While developing a Validation API function, there were cases where evaluating a complex expression caused a panic for invalid expressions. When a panic occurs, the recover() function keeps the server from dying, but for a Validation API, we need to return a response to the client indicating that the expression is invalid.

1.panic() and recover() Functions

Let's look at how to return a value from a recover() function. These are the Go built-in functions we'll use today.

• panic()
  • when panic() is executed, it stops immediately
  • it runs all defer functions of the function and then terminates
• recover()
  • just as Java handles exceptions with a try ~ catch block, the recover() function plays a similar role
  • it prevents the program where a panic occurred from terminating and recovers it
  • it must be used together with defer

func main() {
	f()
	fmt.Println("Returned normally from f.")
}

func f() {
	defer func() {
		if r := recover(); r != nil {
			fmt.Println("Recovered in f", r)
		}
	}()
	fmt.Println("Calling g.")
	g(0)
	fmt.Println("Returned normally from g.")
}

func g(i int) {
	if i > 3 {
		fmt.Println("Panicking!")
		panic(fmt.Sprintf("%v", i))
	}
	defer fmt.Println("Defer in g", i)
	fmt.Println("Printing in g", i)
	g(i + 1)
}

You can see that a panic occurs in the g() function and the defer functions run in the order they were pushed onto the call stack. Then, when the defer function defined in f() runs, it calls recover(), restoring the panicking state back to normal and running the rest of the f() function's code.

$ go run main.go
Printing in g 0
Printing in g 1
Printing in g 2
Printing in g 3
Panicking!
Defer in g 3
Defer in g 2
Defer in g 1
Defer in g 0
Recovered in f 4
Returned normally from f.

2.Returning a Value from a recover() Function

To return a value from a recover() function, give a name to the return value of MyFunc() and assign a value to that named variable; the value will then be returned after the recover() function runs.

func main() {
	myFunc, err := MyFunc()
	fmt.Println("myFunc:", myFunc)
	fmt.Println("err:", err)
}

type Response struct {
	Message string
}

func MyFunc() (resp Response, err error) {
	defer func() {
		if r := recover(); r != nil {
			err = errors.New(fmt.Sprint(r))
			resp = Response{
				Message: "failure",
			}
		}
	}()
	panic("test")
	return Response{Message: "success"}, nil
}

Running it as a unit test confirms that the return value is returned as Response{Message:"failure"}.

func TestMyFunc(t *testing.T) {
	resp, err := MyFunc()
	assert.Error(t, err)
	assert.Equal(t, Response{
		Message: "failure",
	}, resp)
}

3.Printing a Stack Trace in recover()

To recover after a panic and print a stack trace for easier debugging, use the PrintStack() function included in the Debug package.

func MyFunc() (resp Response, err error) {
	defer func() {
		if r := recover(); r != nil {
			debug.PrintStack()
			err = errors.New(fmt.Sprint(r))
			resp = Response{
				Message: "Failure",
			}
		}
	}()
	panic("test")
	return Response{Message: "Success"}, nil
}

The debug.PrintStack() function prints a stack trace like the one below.

goroutine 1 [running]:
runtime/debug.Stack()
        /opt/homebrew/opt/go/libexec/src/runtime/debug/stack.go:24 +0x68
runtime/debug.PrintStack()
        /opt/homebrew/opt/go/libexec/src/runtime/debug/stack.go:16 +0x20
main.MyFunc.func1()
        /Users/user/GolandProjects/tutorials-go/go-recover/return-value/main.go:22 +0x48
panic({0x104a5b6e0, 0x104a6bc68})
        /opt/homebrew/opt/go/libexec/src/runtime/panic.go:838 +0x204
main.MyFunc()
        /Users/user/GolandProjects/tutorials-go/go-recover/return-value/main.go:29 +0x74
main.main()
        /Users/user/GolandProjects/tutorials-go/go-recover/return-value/main.go:10 +0x20
myFunc: {Failure}
err: test

4.References

• http://golang.site/go/article/20-Go-defer%EC%99%80-panic
• https://github.com/kenshin579/tutorials-go/pull/287
• https://stackoverflow.com/questions/68554968/why-does-go-panic-recover-to-return-value-with-local-variable-not-work
• https://stackoverflow.com/questions/19934641/go-returning-from-defer
• https://stackoverflow.com/questions/33167282/how-to-return-a-value-in-a-go-function-that-panics
• https://golangbot.com/panic-and-recover/