Why my db connection couldn't be accessed from handler? - database

Question

I have code like this:
package main
import (
"database/sql"
"flag"
"fmt"
"log"
"net/http"
"os"
_ "github.com/go-sql-driver/mysql"
)
// Default constant for configuration
const DefaultHTTPAddr = ":8080"
const DefaultDSN = "root:root#tcp(127.0.0.1:3306)/librarian"
// Parameters
var (
httpAddr string
dsn string
db *sql.DB
)
// init initializes this package.
func init() {
flag.StringVar(&httpAddr, "addr", DefaultHTTPAddr, "Set the HTTP bind address")
flag.StringVar(&dsn, "dsn", DefaultDSN, "Set the Data Source Name")
flag.Usage = func() {
fmt.Fprintf(os.Stderr, "Usage: %s [options]\n", os.Args[0])
flag.PrintDefaults()
}
}
type Book struct {
id int
title string
author string
}
func main() {
flag.Parse()
var err error
db, err := sql.Open("mysql", DefaultDSN)
if err != nil {
log.Fatal(err)
}
if err = db.Ping(); err != nil {
log.Fatal(err)
}
// handler
http.HandleFunc("/", homepage)
http.HandleFunc("/books", booksIndex)
log.Println("httpd started successfully")
http.ListenAndServe(httpAddr, nil)
}
func booksIndex(w http.ResponseWriter, r *http.Request) {
if r.Method != "GET" {
http.Error(w, http.StatusText(405), 405)
return
}
rows, err := db.Query("SELECT * FROM books")
if err != nil {
log.Fatal(err)
}
defer rows.Close()
books := make([]*Book, 0)
for rows.Next() {
bk := new(Book)
err = rows.Scan(&bk.id, &bk.title, &bk.author)
if err != nil {
log.Fatal(err)
}
books = append(books, bk)
}
if err = rows.Err(); err != nil {
log.Fatal(err)
}
for _, bk := range books {
fmt.Fprintf(w, "%v, %v, %v\n", bk.id, bk.title, bk.author)
}
}
func homepage(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Welcome!")
}
Everytime I try to access /books it always panic.
$ curl localhost:8080/books
curl: (52) Empty reply from server
Like this one:
2016/05/31 11:56:38 http: panic serving 127.0.0.1:51711: runtime error: invalid memory address or nil pointer dereference
goroutine 6 [running]:
net/http.(*conn).serve.func1(0xc820074100)
/usr/local/opt/go/libexec/src/net/http/server.go:1389 +0xc1
panic(0x357b40, 0xc82000a150)
/usr/local/opt/go/libexec/src/runtime/panic.go:443 +0x4e9
database/sql.(*DB).conn(0x0, 0xc820010b01, 0xc8200de000, 0x0, 0x0)
/usr/local/opt/go/libexec/src/database/sql/sql.go:778 +0xac9
database/sql.(*DB).query(0x0, 0x4022c0, 0x13, 0x0, 0x0, 0x0, 0x3c4601, 0x6, 0x0, 0x0)
/usr/local/opt/go/libexec/src/database/sql/sql.go:1073 +0x46
database/sql.(*DB).Query(0x0, 0x4022c0, 0x13, 0x0, 0x0, 0x0, 0xc820016280, 0x0, 0x0)
/usr/local/opt/go/libexec/src/database/sql/sql.go:1061 +0xa3
main.booksIndex(0x6979c0, 0xc82006da00, 0xc8200e2000)
/Users/rahmatawaludin/gocode/src/github.com/rahmatawaludin/librarian/main.go:68 +0xd9
net/http.HandlerFunc.ServeHTTP(0x4666f8, 0x6979c0, 0xc82006da00, 0xc8200e2000)
/usr/local/opt/go/libexec/src/net/http/server.go:1618 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc820010ba0, 0x6979c0, 0xc82006da00, 0xc8200e2000)
/usr/local/opt/go/libexec/src/net/http/server.go:1910 +0x17d
net/http.serverHandler.ServeHTTP(0xc820074080, 0x6979c0, 0xc82006da00, 0xc8200e2000)
/usr/local/opt/go/libexec/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc820074100)
/usr/local/opt/go/libexec/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
/usr/local/opt/go/libexec/src/net/http/server.go:2137 +0x44e
I thought I could access db from booksIndex as I set in to be global variable. When I move the db initialization to booksIndex the errors doesn't show up.
What parts is wrong in my code?
Also, I'm new in Golang. If you have any suggestion on how to organize my code, please tell me. Thanks.. :)

Answer 1

Your db variable in function is shadowing global variable.
When you do this:
db,err:=
It assigns it to a new local variable db.
This is because it's not from same block. According to the standard:
Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier
in the same block (or the parameter lists if the block is the function
body) with the same type, and at least one of the non-blank variables
is new. As a consequence, redeclaration can only appear in a
multi-variable short declaration. Redeclaration does not introduce a
new variable; it just assigns a new value to the original.
So global variable remains nil pointer. And when it is access, you get nil pointer dereference
Change it to d and then assign it later to db. Or which is more correct(as given in comment by rahmat):=
d , err =
I would recommend you to have separate files for handler, models etc. And look into https://github.com/mattermost/platform to how to organise the code.