panic: runtime error: invalid memory address using sync.WaitGroup

[Rhymond]

We get panic when we use xray with sync.WaitGroup. As you can see in a error below, context becomes nil and func getTraceHeaderFromContext throws panic as there is no required value in it.

we use algnhsa server adapter and mux.router for routing (it shouldn't cause an issue, but I added it to code example below) and we pass http.Request context to GetObjectWithContext function.

It looks like we're getting into some sort of race condition as it works 50% of time and another 50% we're getting a panic.

var s3c *s3.S3

func main() {
    s3c := s3.New(session.New())
    xray.AWS(s3c.Client)

    router := mux.NewRouter()
    router.Path("/").Methods("GET").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        results := getObjects(r.Context(), []string{"1", "2", "3"})
        return
    })
    handler := xray.Handler(xray.NewFixedSegmentNamer("app"), router), err
    algnhsa.ListenAndServe(handler, nil)
}

func getOjbects(ctx context.Context, ids []string) []getResult {
    var wg sync.WaitGroup
    md := make([]getResult, len(ids))

    wg.Add(len(ids))
    for i, id := range ids {
        go func(key string, index int) {
            defer wg.Done()
            input := &s3.GetObjectInput{
                Bucket: aws.String(s.BucketName),
                Key:    aws.String(key),
            }
            result, err := s3c.GetObjectWithContext(ctx, input)
            md[index] = getResult{
                p:     result,
                err:   err,
            }
        }(id, i)
    }

    wg.Wait()
    // We're not reaching this point.
    return
}

Error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xac7ff6]
goroutine 129 [running]:
github.com/aws/aws-xray-sdk-go/xray.getTraceHeaderFromContext(0x0, 0x0, 0x403e28)
/go/src/github.com/aws/aws-xray-sdk-go/xray/lambda.go:36 +0x26
github.com/aws/aws-xray-sdk-go/xray.BeginSubsegment(0x0, 0x0, 0xd39c1b, 0x3, 0x0, 0x0, 0x0)
/go/src/github.com/aws/aws-xray-sdk-go/xray/segment.go:167 +0x91
github.com/aws/aws-xray-sdk-go/xray.(*HTTPSubsegments).TLSHandshakeStart(0xc0001a8880)
/go/src/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:101 +0x8d
github.com/aws/aws-xray-sdk-go/xray.NewClientTrace.func6()
/go/src/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:226 +0x2a
net/http.(*persistConn).addTLS.func2(0xc0001a8900, 0xc0007a5c00, 0xc0008a4f00, 0xc000779620)
/usr/local/go/src/net/http/transport.go:1169 +0xa6
created by net/http.(*persistConn).addTLS
/usr/local/go/src/net/http/transport.go:1167 +0x17e
2019/04/17 09:39:42 unexpected EOF
2019/04/17 09:39:42 unexpected EOF
END RequestId: d9f5ac74-ea74-43ab-a1e2-89cfb5d29131

[t-yuki]

I've fixed it: #105

[luluzhao]

@t-yuki Thank you so much for submitting that PR. I already merged it. @Rhymond I will make sure we included the fix for next release.

[Rhymond]

Thanks both!