SQL query results to go inside a nested struct, using Golang - mysql

Question

Below is the whole code of the program. It is a service that forwards requests. Currently working. What I am trying to do is get rid of the yml file that is currently storing all configs and move them to db. I don't want to mess with the code much, so my idea was to simply store the db data in the same structs.
// Config contains configuration for this service
type Instance struct {
User string `json:"user"`
Password string `json:"password"`
InstanceId string `json:"instance_id"`
InstanceType string `json:"instance_type"`
InstanceMode string `json:"instance_mode"`
ClientId string `json:"client_id"`
ClientSecret string `json:"client_secret"`
PublicKey string `json:"pubkey"`
Apis Api `json:"apis"`
}
// API struct
type Api struct {
Name string `json:"name"`
Url string `json:"url"`
Version string `json:"version"`
IsBhdGw bool `json:"isBhdGw"`
Key string `json:"key"`
}
// API struc
type InfoResponse struct {
InstanceId string `json:"instance_id"`
InstanceType string `json:"instance_type"`
InstanceMode string `json:"instance_mode"`
ClientId string `json:"client_id"`
ClientSecret string `json:"client_secret"`
PublicKey string `json:"pubkey"`
Apis Api `json:"apis"`
}
type Settings struct {
Port int `json:"port"`
User string `json:"user"`
Secret string `json:"secret"`
Mode string `json:"mode"`
}
type Instances struct {
Instances []Instance
}
var payloadLength int = 3
var instances Instances
var settings Settings
var db *sql.DB
func fetchInstances() []Instance {
rows, err := db.Query("SELECT Instances.InstanceId, Instances.User, Instances.Password, Instances.InstanceType, Instances.InstanceMode, Instances.ClientId, Instances.ClientSecret, Instances.PublicKey, Api.Name, Api.Url, Api.Version, Api.IsBhdGw, Api.Key FROM Instances, Api")
if err != nil {
panic(err.Error())
}
defer rows.Close()
instances := []Instance{}
for rows.Next() {
var instance Instance
var apis Api
err := rows.Scan(&instance.InstanceId, &instance.User, &instance.Password,
&instance.InstanceType, &instance.InstanceMode, &instance.ClientId, &instance.ClientSecret, &instance.PublicKey,
&apis.Name, &apis.Url, &apis.Version, &apis.IsBhdGw, &apis.Key)
if err != nil {
panic(err.Error())
}
instance.Apis = apis
instances = append(instances, instance)
}
return instances
}
When I run this I get a "panic: runtime error: invalid memory address or nil pointer dereference"

Answer 1

You are attempting to connect to an uninitiated database connection. This is due to variable shadowing.
Although you've defined a global db variable:
var db *sql.DB
Your main() method creates a new one when it connects:
db, err := sql.Open(dbDriver, dbUser+":"+dbPass+"#tcp(db:3306)/"+dbName)
So then when you call fetchInstances, which uses the global variable, the global one is still unset.
The easiest solution is to change your code in main() as follows:
var err error
db, err = sql.Open(dbDriver, dbUser+":"+dbPass+"#tcp(db:3306)/"+dbName)
But the better solution is to never use a global variable. Use the locally-defined variable in main, then pass that to fetchInstances.
As a side note: ALWAYS, ALWAYS check your errors. Twice in your code, you fail to check db connection errors, as in:
db, err := sql.Open(dbDriver, dbUser+":"+dbPass+"#tcp(db:3306)/"+dbName)
err = db.Ping()
if err != nil {
fmt.Println(err)
os.Exit(1)
}
If the connection fails, your db.Ping() call will likely panic. Instead, you should do:
db, err := sql.Open(dbDriver, dbUser+":"+dbPass+"#tcp(db:3306)/"+dbName)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
err = db.Ping()
if err != nil {
fmt.Println(err)
os.Exit(1)
}