Alpha panics with invalid UID 0 in Dgraph

[danielmai]

https://discuss.dgraph.io/t/dgraph-cluster-crashes-when-recreating-index/6353

What version of Dgraph are you using?

v20.03.0

Have you tried reproducing the issue with the latest release?

Yes.

What is the hardware spec (RAM, OS)?

N/A (macOS)

Steps to reproduce the issue (command/config used to run Dgraph).

Alpha can panic when the dataset includes UID 0. UID 0 isn't a valid UID in Dgraph. Based on the discussion, there are two issues here:

1. Dgraph is exporting the dataset with UID <0x0>.
2. Dgraph accepts a write with the UID <0x0>.

Normally, when Dgraph processes a mutation with <0x0>, an error is returned saying UID has to be greater than one:

$ curl -s -H 'Content-Type: application/rdf' localhost:8080/mutate -d '{ set { <0x0> <name> "" . } }' | jq .
{
  "errors": [
    {
      "message": "UID has to be greater than one",
      "extensions": {
        "code": "ErrorInvalidRequest"
      }
    }
  ],
  "data": null
}

This error also shows up for regular mutations and with live loader.

$ dgraph live -f g01.rdf.gz -s g01.schema.gz
...
Processing data file "g01.rdf.gz"
Error while mutating: UID has to be greater than one s.Code Unknown
Error while mutating: UID has to be greater than one s.Code Unknown
Error while mutating: UID has to be greater than one s.Code Unknown
Error while mutating: UID has to be greater than one s.Code Unknown

Bulk loader does not return this error and writes UIDs <0x0> to the p directory.

Steps to reproduce:

1. Download the g01.rdf.gz and g01.schema.gz RDF and schema data set from Discuss.

2. Start a Dgraph Zero

3. Run bulk loader with the dataset

   dgraph bulk -f g01.rdf.gz -s g01.schema.gz
   

4. Run Alpha with the bulk load p directory.

5. Remove the hash index and add it back:

   $ curl localhost:8080/alter -d 'user.userName: string .'
   $ curl localhost:8080/alter -d 'user.userName: string @index(hash) .'
   

6. Then, this panic shows up in Alpha, and restarting Alpha will show the same panic:

   2020/04/17 16:40:49 Assert failed
   github.com/dgraph-io/dgraph/x.AssertTrue
   	/ext-go/1/src/github.com/dgraph-io/dgraph/x/error.go:94
   github.com/dgraph-io/dgraph/posting.(*Txn).addIndexMutations
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/index.go:94
   github.com/dgraph-io/dgraph/posting.rebuildTokIndex.func1.1
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/index.go:921
   github.com/dgraph-io/dgraph/posting.(*List).iterate
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/list.go:677
   github.com/dgraph-io/dgraph/posting.(*List).Iterate
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/list.go:556
   github.com/dgraph-io/dgraph/posting.rebuildTokIndex.func1
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/index.go:912
   github.com/dgraph-io/dgraph/posting.(*rebuilder).Run.func1
   	/ext-go/1/src/github.com/dgraph-io/dgraph/posting/index.go:604
   github.com/dgraph-io/badger/v2.(*Stream).produceKVs.func1
   	/go/pkg/mod/github.com/dgraph-io/badger/[email protected]/stream.go:191
   github.com/dgraph-io/badger/v2.(*Stream).produceKVs
   	/go/pkg/mod/github.com/dgraph-io/badger/[email protected]/stream.go:234
   github.com/dgraph-io/badger/v2.(*Stream).Orchestrate.func1
   	/go/pkg/mod/github.com/dgraph-io/badger/[email protected]/stream.go:337
   runtime.goexit
   	/usr/local/go/src/runtime/asm_amd64.s:1373
   

Expected behaviour

The Alpha should not export data with UID 0, and bulk loader should write successfully write data with UID 0.