Query returns inconsistent results after upgrading to dgraph:v1.1.0

[VarunPeddina]

What version of Dgraph are you using?

1.1.0

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

schema

firstName: string @index(trigram) .
lastName: string @index(trigram) .

mutate

{
  set {
    _:luke <firstName> "Luke" .
    _:luke <lastName> "Skywalker" .
    _:leia <firstName> "Princess" .
    _:leia <lastName> "Leia" .
    _:han <firstName> "Han" .
    _:han <lastName> "Solo" .
    _:har <firstName> "Harrison" .
    _:har <lastName> "Ford" .
    _:ss <firstName> "Steven" .
    _:ss <lastName> "Spielberg" .
  }
}

query

{
  response(func: has(firstName)) @filter(regexp(firstName, /.*han/i) OR  regexp(lastName, /.*han/i)) {
    uid
    firstName
    lastName
  }
}

Expected behaviour and actual result.

{
  "data": {
    "response": [
      {
        "uid": "0xa1237",
        "firstName": "Han",
        "lastName": "Solo"
      }
    ]
  }
}

Keep running the same query again and again and you will see the following result intermittently

{
  "data": {
    "response": []
  }
}

Can see the desired results if we filter only on either firstName or lastName predicate, behaves like some sort of race condition.
Consistently reproducible irrespective of the transaction used NewTxn / NewReadOnlyTxn

ref: https://discuss.dgraph.io/t/dgraph-search-query-returns-inconsistent-results-after-upgrade-to-dgraph-v1-1-0/5092

[MichelDiz]

Yeah, that feels like a bug.
If you run the query below it works as needed

{
  response(func: has(lastName)) @filter(
    has(lastName) AND
    regexp(firstName, /.*han/i) OR  
    regexp(lastName, /.*han/i)
  ) {
    uid
    firstName
    lastName
  }
}

[VarunPeddina]

@MichelDiz Now it's returning the following error in ratel if you keep running a few times

Error Name: TypeError

Message: Failed to fetch

URL: undefined

and the following error in alpha

panic: runtime error: index out of range

goroutine 14474 [running]:
github.com/dgraph-io/dgraph/worker.(*queryState).handleUidPostings.func1(0x0, 0x5, 0x0, 0x9d9dc6)
	/tmp/go/src/github.com/dgraph-io/dgraph/worker/task.go:614 +0x15cc
github.com/dgraph-io/dgraph/worker.(*queryState).handleUidPostings.func2(0xc000d52a80, 0xc000c254a0, 0x0, 0x5)
	/tmp/go/src/github.com/dgraph-io/dgraph/worker/task.go:691 +0x3a
created by github.com/dgraph-io/dgraph/worker.(*queryState).handleUidPostings
	/tmp/go/src/github.com/dgraph-io/dgraph/worker/task.go:690 +0x3b3
[Decoder]: Using assembly version of decoder

It's not crashing if we don't use has.
This is happenning even if the predicates has just these 5 records.

[mangalaman93]

Result of git bisect -

git bisect start
# bad: [4f15d4d3f63d53ef50de041e4cf7f237863a65c4] Periodically run GC in all dgraph commands. (#4032)
git bisect bad 4f15d4d3f63d53ef50de041e4cf7f237863a65c4
# bad: [7bb2f525cc231ee52cf48a45b305d2d30fb01abd] Un-export reindex functions that no longer need to be exported. (#2937)
git bisect bad 7bb2f525cc231ee52cf48a45b305d2d30fb01abd
# good: [20e2c36fa5c6cc711ab7c9f2e1023c7cf062bfe6] Add before and after dgraph MD5s to make install.
git bisect good 20e2c36fa5c6cc711ab7c9f2e1023c7cf062bfe6
# good: [20e2c36fa5c6cc711ab7c9f2e1023c7cf062bfe6] Add before and after dgraph MD5s to make install.
git bisect good 20e2c36fa5c6cc711ab7c9f2e1023c7cf062bfe6
# bad: [6d3ac389d4870e0fc650cdf04ee2d1fe91a9e8bd] Add minweight and maxweight arguments to k-shortest path algorithms. (#2915)
git bisect bad 6d3ac389d4870e0fc650cdf04ee2d1fe91a9e8bd
# bad: [a4756c206adc19b5ba640373407b30ef3a01f030] RegExp filter work without filter (#2913)
git bisect bad a4756c206adc19b5ba640373407b30ef3a01f030
# good: [c4cae75a841cbc64bafc30dd26c01b0312ed0c3c] don't create ACL predicates when the ACL feature is not turned on (#2924)
git bisect good c4cae75a841cbc64bafc30dd26c01b0312ed0c3c
# first bad commit: [a4756c206adc19b5ba640373407b30ef3a01f030] RegExp filter work without filter (#2913)

[mangalaman93]

This is caused by a race condition -

==================
WARNING: DATA RACE
Read at 0x00c0003f38c0 by goroutine 246:
  github.com/dgraph-io/dgraph/algo.IntersectWith()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/algo/uidlist.go:138 +0x52
  github.com/dgraph-io/dgraph/worker.(*queryState).handleRegexFunction()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:1038 +0xec7
  github.com/dgraph-io/dgraph/worker.(*queryState).helpProcessTask()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:860 +0x679
  github.com/dgraph-io/dgraph/worker.processTask()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:764 +0x649
  github.com/dgraph-io/dgraph/worker.ProcessTaskOverNetwork()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:147 +0x5be
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/query/query.go:1959 +0x3839

Previous write at 0x00c0003f38c0 by goroutine 271:
  github.com/dgraph-io/dgraph/algo.IntersectWith()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/algo/uidlist.go:160 +0x214
  github.com/dgraph-io/dgraph/worker.(*queryState).handleRegexFunction()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:1038 +0xec7
  github.com/dgraph-io/dgraph/worker.(*queryState).helpProcessTask()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:860 +0x679
  github.com/dgraph-io/dgraph/worker.processTask()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:764 +0x649
  github.com/dgraph-io/dgraph/worker.ProcessTaskOverNetwork()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:147 +0x5be
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/query/query.go:1959 +0x3839

Goroutine 246 (running) created at:
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/query/query.go:2014 +0x1d6b

Goroutine 271 (finished) created at:
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/query/query.go:2014 +0x1d6b
==================