[BUG]: Empty slice is returned as nil

[stepanovdmitrii]

What version of Badger are you using?

a09e983

What version of Go are you using?

1.22.4

Have you tried reproducing the issue with the latest release?

Yes

What is the hardware spec (RAM, CPU, OS)?

Linux workstation 6.8.11-300.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Mon May 27 14:53:33 UTC 2024 x86_64 GNU/Linux

What steps will reproduce the bug?

Test:

func TestSetAndGetEmptySlice(t *testing.T) {
	dir := t.TempDir()
	db, err := Open(DefaultOptions(dir))
	require.NoError(t, err)
	defer func() {
		require.NoError(t, db.Close())
	}()

	key := []byte("key")
	value := []byte{}
	err = db.Update(func(txn *Txn) error {
		return txn.Set(key, value)
	})
	require.NoError(t, err)

	var item *Item
	err = db.View(func(txn *Txn) (dbErr error) {
		item, dbErr = txn.Get(key)
		return dbErr
	})
	require.NoError(t, err)
	require.NotNil(t, item)

	dbValue, err := item.ValueCopy(nil)
	require.NoError(t, err)
	require.Equal(t, value, dbValue)
}

Test result:

--- FAIL: TestSetAndGetEmptySlice (0.01s)
...
        	Error:      	Not equal: 
        	            	expected: []byte{}
        	            	actual  : []byte(nil)
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,3 +1,2 @@
        	            	-([]uint8) {
        	            	-}
        	            	+([]uint8) <nil>
        	            	 
        	Test:       	TestSetAndGetEmptySlice
FAIL
FAIL	github.com/dgraph-io/badger/v4	0.021s
FAIL

Expected behavior and actual result.

An empty slice should be returned.

Additional information

The similar test for bbolt passes:

func TestDB_SetAndGetEmptySlice(t *testing.T) {
	db := btesting.MustCreateDB(t)
	bucket := []byte("test bucket")
	key := []byte("key")
	value := []byte{}
	err := db.Update(func(tx *bolt.Tx) error {
		b, err := tx.CreateBucketIfNotExists(bucket)
		if err != nil {
			return err
		}
		return b.Put(key, value)
	})
	require.NoError(t, err)

	var dbValue []byte
	err = db.View(func(tx *bolt.Tx) error {
		b := tx.Bucket(bucket)
		dbValue = b.Get(key)
		return nil
	})
	require.NoError(t, err)

	require.Equal(t, value, dbValue)
}