Fix race condition in DoesNotHave

[jarifibrahim]

There is a race condition in the t.bf variable if table.LoadBloomsOnOpen is set to false.
This issue was also seen in hypermodeinc/dgraph#4689 (comment)

==================
WARNING: DATA RACE
Read at 0x00c00028eed8 by goroutine 20:
  github.com/dgraph-io/badger/v2/table.(*Table).DoesNotHave()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/table/table.go:515 +0x3f8
  github.com/dgraph-io/badger/v2.(*IteratorOptions).pickTables()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/iterator.go:407 +0x29e
  github.com/dgraph-io/badger/v2.(*levelHandler).appendIterators()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/level_handler.go:308 +0x38e
  github.com/dgraph-io/badger/v2.(*levelsController).appendIterators()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/levels.go:1025 +0xcc
  github.com/dgraph-io/badger/v2.(*Txn).NewIterator()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/iterator.go:468 +0x4d3
  github.com/dgraph-io/badger/v2.(*Txn).NewKeyIterator()
      /home/aman/gocode/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/iterator.go:489 +0xfb
  github.com/dgraph-io/dgraph/posting.getNew()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/posting/mvcc.go:345 +0x20c
  github.com/dgraph-io/dgraph/posting.(*LocalCache).getInternal()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/posting/lists.go:208 +0x4d6
  github.com/dgraph-io/dgraph/worker.(*queryState).handleUidPostings.func1()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/posting/lists.go:241 +0x416
  github.com/dgraph-io/dgraph/worker.(*queryState).handleUidPostings.func2()
      /home/aman/gocode/src/github.com/dgraph-io/dgraph/worker/task.go:811 +0x47

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This change is 

[mangalaman93]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ashish-goswami and @manishrjain)

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @jarifibrahim)

---

table/table.go, line 385 at r1 (raw file):

	if t.opt.LoadBloomsOnOpen {
		// We don't need to acquire lock here because this will be called in sequential manner.

Just add the lock here as well.

[jarifibrahim]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @mangalaman93, and @manishrjain)

---

table/table.go, line 385 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Just add the lock here as well.

Done.