Why does gorm db.First() panic with "invalid memory address or nil pointer dereference"? [duplicate]

Question

I can't figure out if I've done something silly or if I've found a bug in gorm. While I'm very well aware of what "invalid memory address or nil pointer dereference" means, I am completely mystified as to why it appears here.

In short, I call db.First() and I receive a panic for no obvious reason.

The relevant bits of my code:

package main

import (
    "fmt"
    "github.com/gorilla/mux"
    "github.com/jinzhu/gorm"
    "net/http"
    "os"
)

type message struct {
    gorm.Model
    Title string
    Body  string `sql:"size:0"` // blob
}

var db = gorm.DB{} // garbage

func messageHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    m := message{}
    query := db.First(&m, vars["id"])
    if query.Error != nil {
        if query.Error == gorm.RecordNotFound {
            notFoundHandler(w, r)
            return
        } else {
            fmt.Fprintf(os.Stderr, "database query failed: %v", query.Error)
            internalServerErrorHandler(w, r)
            return
        }
    }

    // actually do something useful
}

func main() {
    db, err := gorm.Open("sqlite3", "/tmp/gorm.db")
    // ...
}

db is opened in main() in the package, and is stored as a package variable. This doesn't seem very clean, but it appears to work...

The panic:

2015/07/16 20:56:12 http: panic serving [::1]:37326: runtime error: invalid memory address or nil pointer dereference
goroutine 26 [running]:
net/http.func·011()
        /usr/lib/golang/src/net/http/server.go:1130 +0xbb
github.com/jinzhu/gorm.(*DB).First(0xd28720, 0x79f220, 0xc2080b2600, 0xc2080ef220, 0x1, 0x1, 0xd)
        /home/error/go/src/github.com/jinzhu/gorm/main.go:200 +0x154
main.messageHandler(0x7f4f2e785bd8, 0xc208051c20, 0xc208035790)
        /home/error/go/src/myproject/messages.go:28 +0x2c1
net/http.HandlerFunc.ServeHTTP(0x9c3948, 0x7f4f2e785bd8, 0xc208051c20, 0xc208035790)
        /usr/lib/golang/src/net/http/server.go:1265 +0x41
github.com/gorilla/mux.(*Router).ServeHTTP(0xc2080d9630, 0x7f4f2e785bd8, 0xc208051c20, 0xc208035790)
        /home/error/go/src/github.com/gorilla/mux/mux.go:98 +0x297
net/http.serverHandler.ServeHTTP(0xc2080890e0, 0x7f4f2e785bd8, 0xc208051c20, 0xc208035790)
        /usr/lib/golang/src/net/http/server.go:1703 +0x19a
net/http.(*conn).serve(0xc208051b80)
        /usr/lib/golang/src/net/http/server.go:1204 +0xb57
created by net/http.(*Server).Serve
        /usr/lib/golang/src/net/http/server.go:1751 +0x35e

...where line 28 of my code is query := db.First(&m, vars["id"])

I reviewed the noted line in gorm and the First() function, but this also isn't terribly obvious.

    return newScope.Set("gorm:order_by_primary_key", "ASC").
        inlineCondition(where...).callCallbacks(s.parent.callback.queries).db

In order to figure out what might be going on, I made the following changes to my code:

First attempt: Is it complaining about being passed a string? Let's give it an integer instead. After all, the example uses an integer.

    id, _ := strconv.Atoi(vars["id"])
    query := db.First(&m, id)

Panic again, at exactly the same place.

Second attempt: Did I create my variable m the wrong way? Maybe it really needs to be allocated with new first.

    m := new(message)
    query := db.First(m, vars["id"])

Panic again, at exactly the same place.

Third attempt: I simply hardcoded the ID to be looked up, just in case gorilla/mux was misbehaving.

    m := message{}
    query := db.First(&m, 3)

Panic again, at exactly the same place.

Finally, I tested with an empty database table, with a populated table requesting an ID that exists, and with a populated table requesting an ID that does not exist. In all three cases I receive the same panic.

The most interesting part of all is that apparently net/http is recovering the panic, and then my notFoundHandler() runs and I see its template output in the browser.

I am currently using the mattn/go-sqlite3 driver.

My environment is Fedora 22 x86_64 with cgo 1.4.2 as provided in Fedora RPM packages.

$ go version
go version go1.4.2 linux/amd64

$ go env
GOARCH="amd64"
GOBIN=""
GOCHAR="6"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/error/go"
GORACE=""
GOROOT="/usr/lib/golang"
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

What's going on? Where is this panic coming from? How do I fix it?

Answer 1

You're shadowing your global db variable:

var db = gorm.DB{} // garbage

Your initialisation in main() should be changed to:

var err error
// Note the assignment and not initialise + assign operator
db, err = gorm.Open("sqlite3", "/tmp/gorm.db")

Otherwise, db is nil and results in the panic.