# it.Next() blowing up db during key iteration

**URL:** <https://discuss.dgraph.io/t/it-next-blowing-up-db-during-key-iteration/12927>\
**Category:** Badger\
**Created:** [February 24, 2021, 5:39pm UTC](https://discuss.dgraph.io/t/it-next-blowing-up-db-during-key-iteration/12927 "2021-02-24T17:39:46Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![kmulvey](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@kmulvey](https://discuss.dgraph.io/u/kmulvey)\
**Post date:** [February 24, 2021, 5:39pm UTC](https://discuss.dgraph.io/t/it-next-blowing-up-db-during-key-iteration/12927/1 "2021-02-24T17:39:46Z")

</div>

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

```
$ go version
```

1.16

### What operating system are you using?

Alpine Linux: 4.19.0-13-amd64 #1 SMP Debian 4.19.160-2 (2020-11-28) x86\_64 Linux

for what its worth this is running in k8s

### What version of Badger are you using?

tried v3.2011.1 & master (ec5b35e677c9c89bd6347bba5bf3f63ea063f723)

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

yes (ec5b35e677c9c89bd6347bba5bf3f63ea063f723)

### Steps to Reproduce the issue

This seems to be similar (same as?) this [fix(iterator): fix iterator when data does not exist in read only mode by NamanJain8 · Pull Request #1670 · dgraph-io/badger · GitHub](https://github.com/dgraph-io/badger/pull/1670)

I dont fully understand what causes it, but this is our code:

```
var keys [][]byte
var err = store.db.View(func(txn *badger.Txn) error {
	opts := badger.DefaultIteratorOptions
	opts.PrefetchValues = false
	it := txn.NewIterator(opts)
	defer it.Close()
	for it.Rewind(); it.Valid(); it.Next() {
		var key []byte
		key = it.Item().KeyCopy(key)
		keys = append(keys, key)
	}
	return nil
})

```

We also tried db.Update() and got the same results.

### What Badger options were set?

badger.DefaultOptions(dir).WithCompression(options.None).WithLogger(dbLogger)

### What did you do?

Iterate over the keys

### What did you expect to see?

The DB not crash

### What did you see instead?

```auto
unexpected fault address 0x7f470435556e
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0x7f470435556e pc=0x99beaa]
goroutine 5420 [running]:
runtime.throw(0x11d2d33, 0x5)
        /usr/local/go/src/runtime/panic.go:1116 +0x72 fp=0xc00e18d4a8 sp=0xc00e18d478 pc=0x437e12
runtime.sigpanic()
        /usr/local/go/src/runtime/signal_unix.go:749 +0x405 fp=0xc00e18d4d8 sp=0xc00e18d4a8 pc=0x44e3e5
encoding/binary.bigEndian.Uint32(...)
        /usr/local/go/src/encoding/binary/binary.go:113
github.com/dgraph-io/badger/v3/y.BytesToU32(...)
        /build/vendor/github.com/dgraph-io/badger/v3/y/y.go:273
github.com/dgraph-io/badger/v3/table.(*Table).block(0xc040903500, 0x285, 0x101, 0x0, 0x0, 0x0)
        /build/vendor/github.com/dgraph-io/badger/v3/table/table.go:587 +0x2ea fp=0xc0017216d8 sp=0xc0017214e0 pc=0x99be8a
github.com/dgraph-io/badger/v3/table.(*Iterator).next(0xc00f2e2380)
        /build/vendor/github.com/dgraph-io/badger/v3/table/iterator.go:331 +0xd0 fp=0xc001721728 sp=0xc0017216d8 pc=0x9976f0
github.com/dgraph-io/badger/v3/table.(*Iterator).next(0xc00f2e2380)
        /build/vendor/github.com/dgraph-io/badger/v3/table/iterator.go:348 +0x1ea fp=0xc001721778 sp=0xc001721728 pc=0x99780a
github.com/dgraph-io/badger/v3/table.(*Iterator).Next(0xc00f2e2380)
        /build/vendor/github.com/dgraph-io/badger/v3/table/iterator.go:406 +0x3e fp=0xc001721790 sp=0xc001721778 pc=0x997c7e
github.com/dgraph-io/badger/v3/table.(*ConcatIterator).Next(0xc0014f1900)
        /build/vendor/github.com/dgraph-io/badger/v3/table/iterator.go:529 +0x33 fp=0xc0017217f0 sp=0xc001721790 pc=0x998673
github.com/dgraph-io/badger/v3/table.(*node).next(0xc0014f60f0)
        /build/vendor/github.com/dgraph-io/badger/v3/table/merge_iterator.go:81 +0x3d fp=0xc001721808 sp=0xc0017217f0 pc=0x998cdd
github.com/dgraph-io/badger/v3/table.(*MergeIterator).Next(0xc0014f60b0)
        /build/vendor/github.com/dgraph-io/badger/v3/table/merge_iterator.go:157 +0x38 fp=0xc001721870 sp=0xc001721808 pc=0x9990d8
github.com/dgraph-io/badger/v3.(*Iterator).parseItem(0xc0014f6160, 0xc00f895401)
        /build/vendor/github.com/dgraph-io/badger/v3/iterator.go:690 +0x464 fp=0xc001721958 sp=0xc001721870 pc=0x9c1d84
github.com/dgraph-io/badger/v3.(*Iterator).Next(0xc0014f6160)
        /build/vendor/github.com/dgraph-io/badger/v3/iterator.go:594 +0x133 fp=0xc001721978 sp=0xc001721958 pc=0x9c1773

```

---

<div class="post-metadata">

**Author:** ![Naman](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/naman/32/3498_2.png) [@Naman](https://discuss.dgraph.io/u/Naman)\
**Post date:** [February 24, 2021, 6:39pm UTC](https://discuss.dgraph.io/t/it-next-blowing-up-db-during-key-iteration/12927/2 "2021-02-24T18:39:20Z")

</div>

Hey @kmulvey, thanks for reaching out. It is not related to the PR you mentioned.  
I hope you are not running the code over a DB created over older version.  
Can you share the full code to reproduce it on our end? If you can’t share publicly, you can mail me at naman[at][dgraph.io](http://dgraph.io)  
Thanks.