Mysql: *sql.DB invalid memory address or nil pointer dereference

We need the following information:

• Your Go-MySQL-Driver version (or git SHA)
• Your Go version (run go version in your console)

go version go1.1.2 darwin/amd64
Go-MySQL-Driver is latest version

I think the panic shouldn't occur with v1.0.3, which is a bugfix release we released yesterday (go get-able since a few minutes).

But I'm investigating further, the stacktrace looks really strange to me. The panic is caused by a nil-connection in stmt.Close (variable mc) where a connection should never be nil with the above code.

v1.0.3 can at least prevent the nil-pointer panic, since it checks mc == nil first.

Can you reproduce this error?

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x41 pc=0x5304bc]
goroutine 25466 [running]:
github.com/go-sql-driver/mysql.(*mysqlConn).writeCommandPacketUint32(0x0, 0x305e92619, 0xc22b2fe650, 0x2b8b3c6167c0)
    /Users/jinke/golang/src/github.com/go-sql-driver/mysql/packets.go:327 +0x1c
github.com/go-sql-driver/mysql.(*mysqlStmt).Close(0xc22b87a6f0, 0x0, 0x0)
    /Users/jinke/golang/src/github.com/go-sql-driver/mysql/statement.go:24 +0x46
database/sql.(*driverConn).finalClose(0xc202a42780, 0xc2014fd1c0, 0x2210b5a80)
    /usr/local/go/src/pkg/database/sql/sql.go:Jump {0 autohome 25 44 258 1977 hkyjh 12.68 394 397 2011-9-27  http://k.autohome.com.cn/spec/8288/hkyjh 2013-11-02 14:21:38 6}
285 +0x87
database/sql.func·002(0xc2000ca200, 0xc2000aebd0)
    /usr/local/go/src/pkg/database/sql/sql.go:372 +0x2c
database/sql.(*driverConn).Close(0xc202a42780, 0x32, 0x9)
    /usr/local/go/src/pkg/database/sql/sql.go:278 +0x174
database/sql.(*DB).putConn(0xc2000ca1e0, 0xc202a42780, 0x0, 0x0)
    /usr/local/go/src/pkg/database/sql/sql.go:598 +0x2e2
database/sql.func·004()
    /usr/local/go/src/pkg/database/sql/sql.go:664 +0x41
database/sql.(*DB).exec(0xc2000ca1e0, 0x738330, 0x40, 0x2b8b3c616cb8, 0x2, ...)
    /usr/local/go/src/pkg/database/sql/sql.go:661 +0xb2
database/sql.(*DB).Exec(0xc2000ca1e0, 0x738330, 0x40, 0x2b8b3c616cb8, 0x2, ...)
    /usr/local/go/src/pkg/database/sql/sql.go:650 +0x98
cds_spider/newCar/koubei.AddOk(0x6dee10, 0x8, 0x1, 0x0, 0x0, ...)
    /Users/jinke/golang/src/cds_spider/newCar/koubei/log.go:49 +0x568
cds_spider/newCar/koubei.(*KouBei).Save(0xc20a0a74d0, 0x0, 0x0, 0x0)
    /Users/jinke/golang/src/cds_spider/newCar/koubei/koubei.go:29 +0x3b7
main.func·006()
    /Users/jinke/golang/src/cds_spider/newCar/koubei/main/autohome.go:241 +0x37
created by main.func·007

/Users/jinke/golang/src/cds_spider/newCar/koubei/log.go

func AddOk(source string, count int) error {
    row := dbpool.DB.QueryRow("SELECT * FROM cheduoshao_spider_status.koubei WHERE source=? AND spider_date=?", source, gcode.Date())
    var (
        id, quantity         int
        sources, spider_date string
    )

    row.Scan(&id, &sources, &quantity, &spider_date)

    if id == 0 {
        stmt, err := dbpool.DB.Prepare("INSERT cheduoshao_spider_status.koubei SET source=?, quantity=?, spider_date=?")
        if err != nil {
            return err
        }
        defer stmt.Close()

        _, err = stmt.Exec(source, 1, gcode.Date())
        if err != nil {
            return err
        }
    } else { 
        dbpool.DB.Exec("UPDATE main.koubei SET quantity=? WHERE id=?", quantity+1, id)
    }

    return nil
}

dbpool.DB.Exec("UPDATE main.koubei SET quantity=? WHERE id=?", quantity+1, id) IS ERROR

Seem to have not found such a situation before, my system is OS X 10.9

Check if row.Scan(&id, &sources, &quantity, &spider_date) returns an error.
Did you try v1.0.3? Like I said, it should at least avoid the panics.

I tried a few times more, I just delete the old version, go get it again
I changed to:

err := row.Scan(&id, &sources, &quantity, &spider_date)
    if err != nil {
        return err
    }

I think I was able to reproduce this bug. Once again, this bug is related to https://codereview.appspot.com/14920046 / Go issue 5718 (or #98, #142).
stmt.Exec ignores errors by the driver and puts bad connections back to the pool.

v1.0.3 includes #143, which at least avoids the panics. v1.1, which will be released in a few hours, will include #151, which improves this a bit more.

I'd suggest to either manually patch your Go version with https://codereview.appspot.com/14920046 or avoid stmt.Exec until the next stable release with this fix included is released. That would probably be Go 1.3 in June 2014...

And a note to your code above:
If you use a stmt only once, there is no reason to manually prepare it. Just use db.Exec, which is by the way not bugged like stmt.Exec.

Here is how I would modify your code above:

func AddOk(source string, count int) error {
    var (
        id, quantity         int
        sources, spider_date string
    )
    if err := dbpool.DB.QueryRow("SELECT * FROM cheduoshao_spider_status.koubei WHERE source=? AND spider_date=?", source, gcode.Date()).Scan(&id, &sources, &quantity, &spider_date); err != nil {
        return err
    }

    if id == 0 {
        if _, err =: dbpool.DB.Exec("INSERT cheduoshao_spider_status.koubei SET source=?, quantity=?, spider_date=?", source, 1, gcode.Date()); err != nil {
            return err
        }
    } else { 
        if _, err =: dbpool.DB.Exec("UPDATE main.koubei SET quantity=? WHERE id=?", quantity+1, id); err != nil {
            return err
        }
    }

    return nil
}

Ok,Thanks!

I'm having too this problem, go version is go1.7.4 linux/amd64 and go-sql-driver is the latest. I'm connecting to a RDS instance of AWS.

runtime error: invalid memory address or nil pointer dereference
/usr/local/go/src/runtime/panic.go:458 (0x42c653)
    gopanic: reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
/usr/local/go/src/runtime/panic.go:62 (0x42b1ad)
    panicmem: panic(memoryError)
/usr/local/go/src/runtime/sigpanic_unix.go:24 (0x441054)
    sigpanic: panicmem()
/usr/local/go/src/database/sql/sql.go:781 (0x557d0a)
    (*DB).conn: db.mu.Lock()
/usr/local/go/src/database/sql/sql.go:1074 (0x559fbb)
    (*DB).query: ci, err := db.conn(strategy)
/usr/local/go/src/database/sql/sql.go:1062 (0x559dc0)
    (*DB).Query: rows, err = db.query(query, args, cachedOrNewConn)
/home/danilo/Go/src/github.com/DaniloMeritocracy/autocomplete/autocomplete.go:192 (0x402e27)
    _getJSON: rows, err := db.Query(sqlString)
/home/danilo/Go/src/github.com/DaniloMeritocracy/autocomplete/autocomplete.go:130 (0x402b54)
    school: res, err := _getJSON("SELECT * FROM university WHERE LOWER(name) LIKE '" + strings.Replace(term, "'", "\\'", -1) + "%' LIMIT 15")
/home/danilo/Go/src/github.com/DaniloMeritocracy/autocomplete/autocomplete.go:56 (0x403985)
    main.func1: results = school(term)
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/context.go:97 (0x45d53a)
    (*Context).Next: c.handlers[c.index](c)
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/recovery.go:45 (0x46d31a)
    RecoveryWithWriter.func1: c.Next()
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/context.go:97 (0x45d53a)
    (*Context).Next: c.handlers[c.index](c)
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/logger.go:72 (0x46c40f)
    LoggerWithWriter.func1: c.Next()
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/context.go:97 (0x45d53a)
    (*Context).Next: c.handlers[c.index](c)
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/gin.go:284 (0x4638ce)
    (*Engine).handleHTTPRequest: context.Next()
/home/danilo/Go/src/gopkg.in/gin-gonic/gin.v1/gin.go:265 (0x4631b0)
    (*Engine).ServeHTTP: engine.handleHTTPRequest(c)
/usr/local/go/src/net/http/server.go:2202 (0x4e1fdd)
    serverHandler.ServeHTTP: handler.ServeHTTP(rw, req)
/usr/local/go/src/net/http/server.go:1579 (0x4de947)
    (*conn).serve: serverHandler{c.server}.ServeHTTP(w, w.req)
/usr/local/go/src/runtime/asm_amd64.s:2086 (0x45c0f1)
    goexit: BYTE    $0x90   // NOP

This is the function _getJSON, that returns a JSON-Encoded string of the results:

func _getJSON(sqlString string) (string, error) {
    rows, err := db.Query(sqlString)
    if err != nil {
        return "", err
    }
    defer rows.Close()
    columns, err := rows.Columns()
    if err != nil {
        return "", err
    }
    count := len(columns)
    tableData := make([]map[string]interface{}, 0)
    values := make([]interface{}, count)
    valuePtrs := make([]interface{}, count)
    for rows.Next() {
        for i := 0; i < count; i++ {
            valuePtrs[i] = &values[i]
        }
        rows.Scan(valuePtrs...)
        entry := make(map[string]interface{})
        for i, col := range columns {
            var v interface{}
            val := values[i]
            b, ok := val.([]byte)
            if ok {
                v = string(b)
            } else {
                v = val
            }
            entry[col] = v
        }
        tableData = append(tableData, entry)
    }
    jsonData, err := json.Marshal(tableData)
    if err != nil {
        return "", err
    }
    //fmt.Println(string(jsonData))
    return string(jsonData), nil
}

@DaniloPolani Your db is nil.

Why should it be nil? I have a db var outside the functions var db *sql.DB and in the main func I opened the connection with

db, err := sql.Open("mysql", "<USERNAME>:<PASSWORD>@tcp(<HOST>:3306)/<DB_NAME>")
    if err != nil {
        panic(err.Error())
    }
    defer db.Close()

Then it is called in _getJSON(). Credentials are OK. Maybe it's an AWS problem? I'll try with localhost.

db, err := sql.Open("mysql", "<USERNAME>:<PASSWORD>@tcp(<HOST>:3306)/<DB_NAME>")

Then, db is local variable of main, not global variable _getJSON() using.

Without you provide complete reproducible code, no one can point out
where your code have race condition.
go build -race is your friend.

@methane, you were right, I had to change db, err := [...] with db, err = [...] declaring also error. My bad, distraction error.

Just ran into same issue. Surprised that this was issue.
Solution by @danilopolani works.

@danilopolani @methane you guys saved my time

@danilopolani You are right, thank you very much for your help, I have been searching for this problem for a long time, thank you

running into same issue.. how do i use @danilopolani solution?

Thanks @methane @danilopolani I made the same mistake, luckily I found this discussion.

OMG, I could have never imagined the solution was about variables scoping.
Thank you very much, guys.