panic: reflect: call of reflect.Value.Type on zero Value

[hocine]

I'm getting this error and I'm not sure what's causing it or how to debug it.

panic: reflect: call of reflect.Value.Type on zero Value

I'm getting it when doing a Get or Put with a the entity type Service:

type Service struct {
    Date      time.Time
    Patient   int
    Dentist   *datastore.Key
    Procedure *datastore.Key
    Insurance *datastore.Key
    Created   time.Time
    Updated   time.Time
    Key       *datastore.Key
    Version   int
}

And, this is the code that's failing with cachestore. They both work fine with datastore.Get/Put, but fail with cachestore.

GETTING:

var (
    c     = appengine.NewContext(r)
    query = datastore.NewQuery("Service").Order("-Date").KeysOnly()
    keys  []*datastore.Key
)

if k != nil {
    s := fmt.Sprintf("%s =", k.Kind())
    query = query.Filter(s, k)
}

keys, err := query.GetAll(c, nil)
if err != nil {
    Log(r, 5, "There was an error querying the datastore for %v: %v", query, err)
    return nil
}

for _, k := range keys {
    x := new(Service)
    if err := cachestore.Get(c, k, x); err != nil {
        Log(r, 2, "Couldn't find service: %v", err)
    } else {
        Log(r, 1, "Service found: %v", x)

        if x.Key == nil {
            x.Key = k
        }
        services = append(services, *x)
    }
}
Log(r, 1, "Found %v services: %v", len(services), services)

PUTTING:

var (
    c    = appengine.NewContext(r)
    serv = new(Service)
    err  error
)

date := r.FormValue("date")
if serv.Date, err = validatedate(date); err != nil {
    Log(r, 1, "Date '%s' is not valid: %v", date, err)
    return
}
Log(r, 1, "String is valid: %v", date)

patient := r.FormValue("patient")
if serv.Patient, err = validatepatientid(patient); err != nil {
    Log(r, 1, "Patient ID '%s' is not valid: %v", patient, err)
    return
}
Log(r, 1, "Patient ID is valid: %v", patient)

dentist := r.FormValue("dentist")
if serv.Dentist, err = validatedentistid(c, dentist); err != nil {
    Log(r, 1, "Dentist ID '%s' is not valid: %v", dentist, err)
    return
}
Log(r, 1, "Dentist ID is valid: %v", dentist)

procedure := r.FormValue("procedure")
if serv.Procedure, err = validateprocedureid(c, procedure); err != nil {
    Log(r, 1, "Procedure ID '%s' is not valid: %v", procedure, err)
    return
}
Log(r, 1, "Procedure ID is valid: %v", procedure)

insurance := r.FormValue("insurance")
if serv.Insurance, err = validateinsuranceid(c, insurance); err != nil {
    Log(r, 1, "Insurance ID '%s' is not valid: %v", insurance, err)
    return
}
Log(r, 1, "Insurance ID is valid: %v", insurance)

k := datastore.NewIncompleteKey(c, "Service", nil)

serv.Created = time.Now()
serv.Updated = time.Now()
serv.Version = API_VERSION

if serv.Key, err = cachestore.Put(c, k, serv); err != nil {
    Log(r, 5, "An error occurred while adding the %v procedure to the datastore: %v", k, err)
    return
}
Log(r, 1, "Placed service in the datastore at %v: %v", serv.Key, serv)

[oschmid]

I'll take a look at it when I get home tonight. Thanks for reporting the issue.

[hocine]

No problem, thanks for responding so quickly! I'm sure I'm doing something wrong because it's working fine in my other app. I just don't know what it could be.

[oschmid]

That panic happens when the reflect.Value.Type() method is called on an zero value (i.e. uninitialized struct, nil, 0, empty string, etc). cachestore calls this method in checkMultiArg() (appengine.go : 54) on the reflected value of the destination (when decoding) or source (when encoding). This would be a *Service in your case. In which case I know exactly what's wrong, and it's my fault: cachestore doesn't work with pointer destinations or sources. I'll see what I can do to get that working. In the mean time, try Get/Pull with a Service rather than a *Service, that should work.

[hocine]

Oh, I thought it wanted pointers to empty structs. From the docs on cachestore.Get: ....it is recommended to pass either a pointer to a zero valued struct....

[oschmid]

Oops, that should say "either a pointer OR a zero valued struct" and since I haven't figured out how to get gob to nicely marshal/unmarshal with pointers (even though it should do that automatically according to the docs) it only works with zero valued structs.

[oschmid]

Updated the docs and the readme so people know about the problem.