# Panic when concurrently closing DB while some goroutines doing reads

**URL:** <https://discuss.dgraph.io/t/panic-when-concurrently-closing-db-while-some-goroutines-doing-reads/14250>\
**Category:** Badger\
**Tags:** status:accepted, kind:bug, ticket:created\
**Created:** [May 20, 2021, 8:47am UTC](https://discuss.dgraph.io/t/panic-when-concurrently-closing-db-while-some-goroutines-doing-reads/14250 "2021-05-20T08:47:12Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Marco](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/marco/32/8249_2.png) [@Marco](https://discuss.dgraph.io/u/Marco)\
**Post date:** [May 20, 2021, 8:47am UTC](https://discuss.dgraph.io/t/panic-when-concurrently-closing-db-while-some-goroutines-doing-reads/14250/1 "2021-05-20T08:47:12Z")

</div>

### What version of Go are you using (`go version`)?

```
go version go1.16.3 darwin/amd64
```

### What operating system are you using?

macOS 11.3  
Ubuntu 20.04.2.0 LTS

### What version of Badger are you using?

`v3.2011.1`

### Does this issue reproduce with the latest master?

yes

### Steps to Reproduce the issue

```auto
package main

import (
	"log"
	"os"
	"sync"
	"time"

	"github.com/dgraph-io/badger/v3"
)

func main() {
	dir, err := os.MkdirTemp("", "badger")
	if err != nil {
		log.Fatalln(err)
	}
	defer os.RemoveAll(dir)

	db, err := badger.Open(badger.DefaultOptions(dir))
	if err != nil {
		log.Fatalln(err)
	}

	key := []byte("key")
	err = db.Update(func(txn *badger.Txn) error {
		return txn.Set(key, []byte("value"))
	})
	if err != nil {
		log.Fatalln(err)
	}

	var wg sync.WaitGroup
	for i := 0; i < 10; i++ {
		i := i
		wg.Add(1)
		go func() {
			defer wg.Done()
			for {
				err := db.View(func(txn *badger.Txn) error {
					_, err := txn.Get(key)
					if err == badger.ErrKeyNotFound {
						return nil
					}
					return err
				})
				if err != nil {
					log.Println(i, ": ", err)
					return
				}
			}
		}()
	}

	time.Sleep(time.Second * 1)
	err = db.Close()
	if err != nil {
		log.Fatalln(err)
	}

	wg.Wait()
}

```

### What Badger options were set?

`badger.DefaultOptions(dir)`

### What did you do?

Close the database while concurrently reading a key from multiple goroutines. See the above code…

### What did you expect to see?

Reading goroutines to fail with an `error` (`ErrDBClosed`) when database is closed. I don’t see any info about this on `close()` method.

### What did you see instead?

A panic:

```auto
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4375254]

goroutine 75 [running]:
github.com/dgraph-io/badger/v3.(*memTable).IncrRef(...)
        /Users/marco/.go/pkg/mod/github.com/dgraph-io/badger/v3@v3.0.0-20210519143323-3b210404e360/memtable.go:231
github.com/dgraph-io/badger/v3.(*DB).getMemTables(0xc000128900, 0x0, 0x0, 0x0, 0x0)
        /Users/marco/.go/pkg/mod/github.com/dgraph-io/badger/v3@v3.0.0-20210519143323-3b210404e360/db.go:701 +0x2d4
github.com/dgraph-io/badger/v3.(*DB).get(0xc000128900, 0xc0074729d0, 0xb, 0xb, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /Users/marco/.go/pkg/mod/github.com/dgraph-io/badger/v3@v3.0.0-20210519143323-3b210404e360/db.go:735 +0x92
github.com/dgraph-io/badger/v3.(*Txn).Get(0xc00747ac00, 0xc0004b1dbc, 0x3, 0x3, 0x43ade79, 0xc0000689c0, 0x1)
        /Users/marco/.go/pkg/mod/github.com/dgraph-io/badger/v3@v3.0.0-20210519143323-3b210404e360/txn.go:498 +0x14d
main.main.func2.1(0xc00747ac00, 0x0, 0xc00747ac00)
        /Users/marco/projects/playground/main.go:40 +0x4a
github.com/dgraph-io/badger/v3.(*DB).View(0xc000128900, 0xc000178f40, 0x0, 0x0)
        /Users/marco/.go/pkg/mod/github.com/dgraph-io/badger/v3@v3.0.0-20210519143323-3b210404e360/txn.go:828 +0x95
main.main.func2(0xc000020160, 0xc000128900, 0xc0004b1dbc, 0x3, 0x3, 0x8)
        /Users/marco/projects/playground/main.go:39 +0xbe
created by main.main
        /Users/marco/projects/playground/main.go:36 +0x365

```