fix db.grow is unusable when NoFreelistSync is on

db.go

@@ -1120,7 +1120,7 @@ func (db *DB) grow(sz int) error {
 
 	// If the data is smaller than the alloc size then only allocate what's needed.
 	// Once it goes over the allocation size then allocate in chunks.
-	if db.datasz < db.AllocSize {
+	if db.datasz <= db.AllocSize {
 		sz = db.datasz
 	} else {
 		sz += db.AllocSize

tx.go

@@ -156,6 +156,8 @@ func (tx *Tx) Commit() error {
 		tx.stats.IncRebalanceTime(time.Since(startTime))
 	}
 
+	opgid := tx.meta.pgid
+
 	// spill data onto dirty pages.
 	startTime = time.Now()
 	if err := tx.root.spill(); err != nil {
@@ -181,6 +183,14 @@ func (tx *Tx) Commit() error {
 		tx.meta.freelist = pgidNoFreelist
 	}
 
+	// If the high water mark has moved up then attempt to grow the database.
+	if tx.meta.pgid > opgid {
+		if err := tx.db.grow(int(tx.meta.pgid+1) * tx.db.pageSize); err != nil {
+			tx.rollback()
+			return err
+		}
+	}
+
 	// Write dirty pages to disk.
 	startTime = time.Now()
 	if err := tx.write(); err != nil {
@@ -225,7 +235,6 @@ func (tx *Tx) Commit() error {
 func (tx *Tx) commitFreelist() error {
 	// Allocate new pages for the new free list. This will overestimate
 	// the size of the freelist but not underestimate the size (which would be bad).
-	opgid := tx.meta.pgid
 	p, err := tx.allocate((tx.db.freelist.size() / tx.db.pageSize) + 1)
 	if err != nil {
 		tx.rollback()
@@ -236,13 +245,6 @@ func (tx *Tx) commitFreelist() error {
 		return err
 	}
 	tx.meta.freelist = p.id
-	// If the high water mark has moved up then attempt to grow the database.
-	if tx.meta.pgid > opgid {
-		if err := tx.db.grow(int(tx.meta.pgid+1) * tx.db.pageSize); err != nil {
-			tx.rollback()
-			return err
-		}
-	}
 
 	return nil
 }

tx_test.go

@@ -6,10 +6,12 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"runtime"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	bolt "go.etcd.io/bbolt"
 	"go.etcd.io/bbolt/internal/btesting"
@@ -1009,3 +1011,45 @@ func TestTxStats_Sub(t *testing.T) {
 	assert.Equal(t, int64(10001), diff.GetWrite())
 	assert.Equal(t, 10009*time.Second, diff.GetWriteTime())
 }
+
+// TestTx_TruncateBeforeWrite ensures the file is truncated ahead whether we sync freelist or not.
+func TestTx_TruncateBeforeWrite(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		return
+	}
+	for _, isSyncFreelist := range []bool{false, true} {
+		t.Run(fmt.Sprintf("isSyncFreelist:%v", isSyncFreelist), func(t *testing.T) {
+			// Open the database.
+			db := btesting.MustCreateDBWithOption(t, &bolt.Options{
+				NoFreelistSync: isSyncFreelist,
+			})
+
+			bigvalue := make([]byte, db.AllocSize/100)
+			count := 0
+			for {
+				count++
+				tx, err := db.Begin(true)
+				require.NoError(t, err)
+				b, err := tx.CreateBucketIfNotExists([]byte("bucket"))
+				require.NoError(t, err)
+				err = b.Put([]byte{byte(count)}, bigvalue)
+				require.NoError(t, err)
+				err = tx.Commit()
+				require.NoError(t, err)
+
+				size := fileSize(db.Path())
+
+				if size > int64(db.AllocSize) && size < int64(db.AllocSize)*2 {
+					// db.grow expands the file aggresively, that double the size while smaller than db.AllocSize,
+					// or increase with a step of db.AllocSize if larger, by which we can test if db.grow has run.
+					t.Fatalf("db.grow doesn't run when file size changes. file size: %d", size)
+				}
+				if size > int64(db.AllocSize) {
+					break
+				}
+			}
+			db.MustClose()
+			db.MustDeleteFile()
+		})
+	}
+}