What is causing my HTTP server to fail with "exit status -1073741819"? - http

Question

As an exercise I created a small HTTP server that generates random game mechanics, similar to this one. I wrote it on a Windows 7 (32-bit) system and it works flawlessly. However, when I run it on my home machine, Windows 7 (64-bit), it always fails with the same message: exit status -1073741819. I haven't managed to find anything on the web which references that status code, so I don't know how important it is.
Here's code for the server, with redundancy abridged:
package main
import (
"fmt"
"math/rand"
"time"
"net/http"
"html/template"
)
// Info about a game mechanic
type MechanicInfo struct { Name, Desc string }
// Print a mechanic as a string
func (m MechanicInfo) String() string {
return fmt.Sprintf("%s: %s", m.Name, m.Desc)
}
// A possible game mechanic
var (
UnkillableObjects = &MechanicInfo{"Avoiding Unkillable Objects",
"There are objects that the player cannot touch. These are different from normal enemies because they cannot be destroyed or moved."}
//...
Race = &MechanicInfo{"Race",
"The player must reach a place before the opponent does. Like \"Timed\" except the enemy as a \"timer\" can be slowed down by the player's actions, or there may be multiple enemies being raced against."}
)
// Slice containing all game mechanics
var GameMechanics []*MechanicInfo
// Pseudorandom number generator
var prng *rand.Rand
// Get a random mechanic
func RandMechanic() *MechanicInfo {
i := prng.Intn(len(GameMechanics))
return GameMechanics[i]
}
// Initialize the package
func init() {
prng = rand.New(rand.NewSource(time.Now().Unix()))
GameMechanics = make([]*MechanicInfo, 34)
GameMechanics[0] = UnkillableObjects
//...
GameMechanics[33] = Race
}
// serving
var index = template.Must(template.ParseFiles(
"templates/_base.html",
"templates/index.html",
))
func randMechHandler(w http.ResponseWriter, req *http.Request) {
mechanics := [3]*MechanicInfo{RandMechanic(), RandMechanic(), RandMechanic()}
if err := index.Execute(w, mechanics); err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
func main() {
http.HandleFunc("/", randMechHandler)
if err := http.ListenAndServe(":80", nil); err != nil {
panic(err)
}
}
In addition, the unabridged code, the _base.html template, and the index.html template.
What could be causing this issue? Is there a process for debugging a cryptic exit status like this?

Answer 1

When I ran it, I got the following two errors:
template: content:6: nil pointer evaluating *main.MechanicInfo.Name
http: multiple response.WriteHeader calls
The former was in the web browser, the latter in the console window where I launched your server.
The nil pointer problem is because your abridged program leaves GameMechanics[1:32] set to nil.
The second error is interesting. The only place in your program that any methods on your http.ResponseWriter get called is inside of index.Execute, which is not your code -- meaning maybe there is something wrong happening in html/template. I'm testing this with Go 1.0.2.
I put _base.html at the top of index.html and then changed index to this:
var index = template.Must(template.ParseFiles("templates/index.html"))
and the http.WriteHeaders warning went away.
Not really an answer, but a direction you could explore.
As a bonus, here's the more "Go way" of writing your program. Note that I simplified the use of the PRNG (you don't need to instantiate unless you want several going in parallel) and simplified the structure initializer:
package main
import (
"fmt"
"html/template"
"math/rand"
"net/http"
)
// Info about a game mechanic
type MechanicInfo struct{ Name, Desc string }
// Print a mechanic as a string
func (m MechanicInfo) String() string {
return fmt.Sprintf("%s: %s", m.Name, m.Desc)
}
// The game mechanics
var GameMechanics = [...]*MechanicInfo{
{"Avoiding Unkillable Objects",
"There are objects that the player cannot touch. These are different from normal enemies because they cannot be destroyed or moved."},
{"Race",
"The player must reach a place before the opponent does. Like \"Timed\" except the enemy as a \"timer\" can be slowed down by the player's actions, or there may be multiple enemies being raced against."},
}
// Get a random mechanic
func RandMechanic() *MechanicInfo {
i := rand.Intn(len(GameMechanics))
return GameMechanics[i]
}
var index = template.Must(template.ParseFiles("templates/index.html"))
func randMechHandler(w http.ResponseWriter, req *http.Request) {
mechanics := [3]*MechanicInfo{RandMechanic(), RandMechanic(), RandMechanic()}
if err := index.Execute(w, mechanics); err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
func main() {
http.HandleFunc("/", randMechHandler)
if err := http.ListenAndServe(":80", nil); err != nil {
panic(err)
}
}