How to get the stacktrace of a panic (and store as a variable)

Question

As we all know, panics produce a stacktrace to stdout (Playground link).:

panic: runtime error: index out of range
goroutine 1 [running]:
main.main()
    /tmp/sa         


        

Answer 1

Create a log file to add the stacktrace to the file for stdout or stderr. This will add the data including the time with line of the error inside a file.

package main

import (
    "log"
    "os"
    "runtime/debug"
)

func main() {

    defer func() {
        if r := recover(); r != nil {
            log.Println(string(debug.Stack()))
        }
    }()

    //create your file with desired read/write permissions
    f, err := os.OpenFile("filename", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0644)
    if err != nil {
        log.Println(err)
    }

    //set output of logs to f
    log.SetOutput(f)
    var mySlice []int
    j := mySlice[0]

    log.Println("Hello, playground %d", j)

    //defer to close when you're done with it, not because you think it's idiomatic!
    f.Close()
}

Working example on Go playground

Answer 2

Like @Volker mentioned above, and what was posted as a comment, we can use the runtime/debug package.

package main

import (
    "fmt"
    "runtime/debug"
)

func main() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("stacktrace from panic: \n" + string(debug.Stack()))
        }
    }()

    var mySlice []int
    j := mySlice[0]

    fmt.Printf("Hello, playground %d", j)
}

prints

stacktrace from panic: 
goroutine 1 [running]:
runtime/debug.Stack(0x1042ff18, 0x98b2, 0xf0ba0, 0x17d048)
    /usr/local/go/src/runtime/debug/stack.go:24 +0xc0
main.main.func1()
    /tmp/sandbox973508195/main.go:11 +0x60
panic(0xf0ba0, 0x17d048)
    /usr/local/go/src/runtime/panic.go:502 +0x2c0
main.main()
    /tmp/sandbox973508195/main.go:16 +0x60

Playground link.