Golang Panic and Recover

Panic

Panics are similar to C++ and Java exceptions, but are only intended for run-time errors, such as following a nil pointer or attempting to index an array out of bounds.

A panic stops the normal execution of a goroutine:

• When a program panics, it immediately starts to unwind the call stack.
• This continues until the program crashes and prints a stack trace,
• or until the built-in recover function is called.

Recover

The built-in recover function can be used to regain control of a panicking goroutine and resume normal execution.

• A call to recover stops the unwinding and returns the argument passed to panic.
• If the goroutine is not panicking, recover returns nil.

Because the only code that runs while unwinding is inside deferred functions, recover is only useful inside such functions.

func main() {
    n := foo()
    fmt.Println("main received", n)
}

func foo() int {
    defer func() {
        if err := recover(); err != nil {
            fmt.Println(err)
        }
    }()
    m := 1
    panic("foo: fail")
    m = 2
    return m
}

Above codes print below lines to the console.

foo: fail
main received 0

Since the panic occurred before foo returned a value, n still has its initial zero value.

To return a value during a panic, you must use a named return value.

func main() {
    n := foo()
    fmt.Println("main received", n)
}

func foo() (m int) {
    defer func() {
        if err := recover(); err != nil {
            fmt.Println(err)
            m = 2
        }
    }()
    m := 1
    panic("foo: fail")
    m = 3
    return m
}

foo: fail
main received 2