Understanding a Golang panic

Question

I have some Golang code that is, very intermittently (once every few hours), resulting in a panic and I need some guidance understanding how to find the cause. The code is this (with line numbers):

audio-process.go:

    var MyChannel chan<- interface{}
    var newDatagramList = list.New()

    func operateAudioProcessing() {
        var channel = make(chan interface{})
        MyChannel = channel
        newDatagramList.Init()
        ...
415     go func() {
416        for cmd := range channel {
417            switch msg := cmd.(type) {
418                 case *MyThing:
419                 {
420                     newDatagramList.PushBack(msg)
421                 }
422             }
423         }
424     }()
425 }

...and the panic occurs when newDatagramList.PushBack() is called at line 420. The code sending into this channel is:

audio-in.go:

    thing := new(MyThing)
    ...
    MyChannel <- thing

...and, to complete the picture, there is a separate timed go func() which processes newDatagramList as follows:

go func() {
    var next *list.Element
    for _ = range processTicker.C {
        for newElement := newDatagramList.Front(); newElement != nil; newElement = next {
            next = newElement.Next();
            myProcessingFunction(newElement.Value.(*MyThing))
            newDatagramList.Remove(newElement)
        }
}

The panic output is:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x5702d1]

goroutine 12 [running]:
panic(0x76e520, 0xc82000e100)
/usr/lib/go-1.6/src/runtime/panic.go:481 +0x3e6
container/list.(*List).PushBack(0xc820054e40, 0x6c4f80, 0xc827294e20, 0xc8273e0b01)
/usr/lib/go-1.6/src/container/list/list.go:139 +0x1c1
main.operateAudioProcessing.func2(0xc8200164e0, 0xc82000f310, 0xc820024078, 0x240)
/home/rob/gocode/src/audio-process.go:420 +0x58b
created by main.operateAudioProcessing
/home/rob/gocode/src/audio-process.go:442 +0x5ba

Which thing is the panic telling me is at fault here? There was no complaint about the allocation and send into the channel in the first place, so I don't see how that can be wrong. newDatagramList is clearly initialised (and the channel has been running and receiving messages for some time).

How do I go about determining what's making me go bang?

Answer 1

The panic stack:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x5702d1]

goroutine 12 [running]:
panic(0x76e520, 0xc82000e100)
/usr/lib/go-1.6/src/runtime/panic.go:481 +0x3e6
container/list.(*List).PushBack(0xc820054e40, 0x6c4f80, 0xc827294e20, 0xc8273e0b01)
/usr/lib/go-1.6/src/container/list/list.go:139 +0x1c1
main.operateAudioProcessing.func2(0xc8200164e0, 0xc82000f310, 0xc820024078, 0x240)
/home/rob/gocode/src/audio-process.go:420 +0x58b
created by main.operateAudioProcessing
/home/rob/gocode/src/audio-process.go:442 +0x5ba

Is saying that an "invalid memory address or nil pointer dereference" is occurring at the top listed frame

 container/list.(*List).PushBack(0xc820054e40, 0x6c4f80, 0xc827294e20, 0xc8273e0b01)

On

 /usr/lib/go-1.6/src/container/list/list.go:139 +0x1c1

Which looks to be from godoc

// PushBack inserts a new element e with value v at the back of list l and returns e.
func (l *List) PushBack(v interface{}) *Element {
    l.lazyInit()
    return l.insertValue(v, l.root.prev)
}

---

This is cool since it's in this frame it looks to be erroring on l.root.prev

The datatype that is being built on container/list is completely unsynchronized and there are many interleaving of go routines that could result in an in-between state where list element pointers are being added removed at the same time.

For example remove has many different operations:

func (l *List) remove(e *Element) *Element {
    e.prev.next = e.next
    e.next.prev = e.prev
    e.next = nil // avoid memory leaks
    e.prev = nil // avoid memory leaks
    e.list = nil
    l.len--
    return e
}

---

If this was executed synchronously there wouldn't be an issue but because multiple go routines are operating on it.

Suppose we have an element E, with a pointer to a previous and a next, and two goroutines acting on it

PREV - E - NEXT  

GOROUTINE1                                           GOROUTINE2

READ - E.PREV returns element (E2) with `NEXT` -> E

                                                    REMOVE(E) is called
                                                    e.next = nil // avoid memory leaks

E2.NEXT.NEXT access occurs now Nil!!! 
resulting in panic