Panic and Recover

Remarks#

This article assumes knowledge of https://stackoverflow.com/documentation/go/2795/defer/9429/defer-basics#t=201607290732570770784

For ordinary error handling, read the topic on error handling

Panic

A panic halts normal execution flow and exits the current function. Any deferred calls will then be executed before control is passed to the next higher function on the stack. Each stack’s function will exit and run deferred calls until the panic is handled using a deferred recover(), or until the panic reaches main() and terminates the program. If this occurs, the argument provided to panic and a stack trace will be printed to stderr.

package main

import "fmt"

func foo() {
    defer fmt.Println("Exiting foo")
    panic("bar")
}

func main() {
    defer fmt.Println("Exiting main")
    foo()
}

Output:

Exiting foo
Exiting main
panic: bar


goroutine 1 [running]:
panic(0x128360, 0x1040a130)
    /usr/local/go/src/runtime/panic.go:481 +0x700
main.foo()
    /tmp/sandbox550159908/main.go:7 +0x160
main.main()
    /tmp/sandbox550159908/main.go:12 +0x120

It is important to note that panic will accept any type as its parameter.

Recover

Recover as the name implies, can attempt to recover from a panic. The recover must be attempted in a deferred statement as normal execution flow has been halted. The recover statement must appear directly within the deferred function enclosure. Recover statements in functions called by deferred function calls will not be honored. The recover() call will return the argument provided to the initial panic, if the program is currently panicking. If the program is not currently panicking, recover() will return nil.

package main

import "fmt"

func foo() {
    panic("bar")
}

func bar() {
    defer func() {
        if msg := recover(); msg != nil {
            fmt.Printf("Recovered with message %s\n", msg)
        }
    }()
    foo()
    fmt.Println("Never gets executed")
}

func main() {
    fmt.Println("Entering main")
    bar()
    fmt.Println("Exiting main the normal way")
}

Output:

Entering main
Recovered with message bar
Exiting main the normal way