Revert "Set readTs in write batch to latest done readTs so that we ca…

…n discard older versions of keys during compactions."

This reverts commit 6b796b3.

Note:

We're seeing an Assert failure due to this change in Dgraph.

```
zero1     | 2019/05/03 23:06:22 Name: badger.PendingReads doneUntil: 4. Index: 2
zero1     | github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y.AssertTruef
zero1     | 	/home/mrjn/go/src/github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y/error.go:62
zero1     | github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y.(*WaterMark).process.func1
zero1     | 	/home/mrjn/go/src/github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y/watermark.go:175
zero1     | github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y.(*WaterMark).process
zero1     | 	/home/mrjn/go/src/github.com/dgraph-io/dgraph/vendor/github.com/dgraph-io/badger/y/watermark.go:225
zero1     | runtime.goexit
zero1     | 	/usr/lib/go-1.10/src/runtime/asm_amd64.s:2361
```

batch.go

@@ -36,14 +36,7 @@ type WriteBatch struct {
 // creating and committing transactions. Due to the nature of SSI guaratees provided by Badger,
 // blind writes can never encounter transaction conflicts (ErrConflict).
 func (db *DB) NewWriteBatch() *WriteBatch {
-	txn := db.newTransaction(true, true)
-	// If we let it stay at zero, compactions would not allow older key versions to be deleted,
-	// because the read timestamps of pending txns, would be zero. Therefore, we set it to the
-	// maximum read timestamp that's done. This allows compactions to discard key versions below
-	// this read timestamp, while also not blocking on pending txns to finish before starting this
-	// one.
-	txn.readTs = db.orc.readMark.DoneUntil()
-	return &WriteBatch{db: db, txn: txn}
+	return &WriteBatch{db: db, txn: db.newTransaction(true, true)}
 }
 
 // Cancel function must be called if there's a chance that Flush might not get
@@ -135,8 +128,7 @@ func (wb *WriteBatch) commit() error {
 	wb.wg.Add(1)
 	wb.txn.CommitWith(wb.callback)
 	wb.txn = wb.db.newTransaction(true, true)
-	// See comment about readTs in NewWriteBatch.
-	wb.txn.readTs = wb.db.orc.readMark.DoneUntil()
+	wb.txn.readTs = 0 // We're not reading anything.
 	return wb.err
 }
 

batch_test.go

@@ -18,8 +18,6 @@ package badger
 
 import (
 	"fmt"
-	"io/ioutil"
-	"os"
 	"testing"
 	"time"
 
@@ -69,59 +67,3 @@ func TestWriteBatch(t *testing.T) {
 		require.NoError(t, err)
 	})
 }
-
-func TestWriteBatchCompaction(t *testing.T) {
-	dir, err := ioutil.TempDir(".", "badger-test")
-	require.NoError(t, err)
-	defer os.RemoveAll(dir)
-
-	opts := DefaultOptions
-	opts.ValueDir = dir
-	opts.Dir = dir
-
-	db, err := Open(opts)
-	require.NoError(t, err)
-
-	wb := db.NewWriteBatch()
-	entries := 10000
-	for i := 0; i < entries; i++ {
-		require.Nil(t, wb.Set([]byte(fmt.Sprintf("foo%d", i)), []byte("bar"), 0))
-	}
-	require.Nil(t, wb.Flush())
-
-	wb = db.NewWriteBatch()
-	// Delete 50% of the entries
-	for i := 0; i < entries/2; i++ {
-		require.Nil(t, wb.Delete([]byte(fmt.Sprintf("foo%d", i))))
-	}
-	require.Nil(t, wb.Flush())
-
-	// It is necessary that we call db.Update(..) before compaction so that the db.orc.readMark
-	// value is incremented. The transactions in WriteBatch call do not increment the
-	// db.orc.readMark value and hence compaction wouldn't discard any entries added by write batch
-	// if we do not increment the db.orc.readMark value
-	require.Nil(t, db.Update(func(txn *Txn) error {
-		txn.Set([]byte("key1"), []byte("val1"))
-		return nil
-	}))
-
-	// Close DB to force compaction
-	require.Nil(t, db.Close())
-
-	db, err = Open(opts)
-	require.NoError(t, err)
-	defer db.Close()
-
-	iopt := DefaultIteratorOptions
-	iopt.AllVersions = true
-	txn := db.NewTransaction(false)
-	defer txn.Discard()
-	it := txn.NewIterator(iopt)
-	defer it.Close()
-	countAfterCompaction := 0
-	for it.Rewind(); it.Valid(); it.Next() {
-		countAfterCompaction++
-	}
-	// We have deleted 50% of the keys
-	require.Less(t, countAfterCompaction, entries+entries/2)
-}