Fix race condition in DoesNotHave (#1287)

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)

table/table.go

@@ -96,6 +96,7 @@ type Table struct {
 
 	fd        *os.File // Own fd.
 	tableSize int      // Initialized in OpenTable, using fd.Stat().
+	bfLock    sync.Mutex
 
 	blockIndex []*pb.BlockOffset
 	ref        int32    // For file garbage collection. Atomic.
@@ -381,7 +382,9 @@ func (t *Table) readIndex() error {
 	t.blockIndex = index.Offsets
 
 	if t.opt.LoadBloomsOnOpen {
+		t.bfLock.Lock()
 		t.bf, _ = t.readBloomFilter()
+		t.bfLock.Unlock()
 	}
 
 	return nil
@@ -511,11 +514,13 @@ func (t *Table) DoesNotHave(hash uint64) bool {
 
 	// Return fast if the cache is absent.
 	if t.opt.BfCache == nil {
+		t.bfLock.Lock()
 		// Load bloomfilter into memory if the cache is absent.
 		if t.bf == nil {
 			y.AssertTrue(!t.opt.LoadBloomsOnOpen)
 			t.bf, _ = t.readBloomFilter()
 		}
+		t.bfLock.Unlock()
 		return !t.bf.Has(hash)
 	}
 

table/table_test.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -932,3 +933,22 @@ func TestOpenKVSize(t *testing.T) {
 	var entrySize uint64 = 15 /* DiffKey len */ + 4 /* Header Size */ + 4 /* Encoded vp */
 	require.Equal(t, entrySize, table.EstimatedSize())
 }
+
+// Run this test with command "go test -race -run TestDoesNotHaveRace"
+func TestDoesNotHaveRace(t *testing.T) {
+	opts := getTestTableOptions()
+	f := buildTestTable(t, "key", 10000, opts)
+	table, err := OpenTable(f, opts)
+	require.NoError(t, err)
+	defer table.DecrRef()
+
+	var wg sync.WaitGroup
+	wg.Add(5)
+	for i := 0; i < 5; i++ {
+		go func() {
+			require.True(t, table.DoesNotHave(uint64(1237882)))
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}