test: check concurrent WriteTo operations aren't corrupted

Reliably triggers consistency check failures on ramdisk without freelist
free fix.

db_test.go

@@ -9,6 +9,7 @@ import (
 	"hash/fnv"
 	"io/ioutil"
 	"log"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -588,6 +589,65 @@ func TestDB_BeginRW(t *testing.T) {
 	}
 }
 
+// TestDB_Concurrent_WriteTo checks that issuing WriteTo operations concurrently
+// with commits does not produce corrupted db files.
+func TestDB_Concurrent_WriteTo(t *testing.T) {
+	o := &bolt.Options{NoFreelistSync: false}
+	db := MustOpenWithOption(o)
+	defer db.MustClose()
+
+	var wg sync.WaitGroup
+	wtxs, rtxs := 5, 5
+	wg.Add(wtxs * rtxs)
+	f := func(tx *bolt.Tx) {
+		defer wg.Done()
+		f, err := ioutil.TempFile("", "bolt-")
+		if err != nil {
+			panic(err)
+		}
+		time.Sleep(time.Duration(rand.Intn(20)+1) * time.Millisecond)
+		tx.WriteTo(f)
+		tx.Rollback()
+		f.Close()
+		snap := &DB{nil, f.Name(), o}
+		snap.MustReopen()
+		defer snap.MustClose()
+		snap.MustCheck()
+	}
+
+	tx1, err := db.Begin(true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tx1.CreateBucket([]byte("abc")); err != nil {
+		t.Fatal(err)
+	}
+	if err := tx1.Commit(); err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 0; i < wtxs; i++ {
+		tx, err := db.Begin(true)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := tx.Bucket([]byte("abc")).Put([]byte{0}, []byte{0}); err != nil {
+			t.Fatal(err)
+		}
+		for j := 0; j < rtxs; j++ {
+			rtx, rerr := db.Begin(false)
+			if rerr != nil {
+				t.Fatal(rerr)
+			}
+			go f(rtx)
+		}
+		if err := tx.Commit(); err != nil {
+			t.Fatal(err)
+		}
+	}
+	wg.Wait()
+}
+
 // Ensure that opening a transaction while the DB is closed returns an error.
 func TestDB_BeginRW_Closed(t *testing.T) {
 	var db bolt.DB