From 386b851495d42c4e02908838373a06d0a533e170 Mon Sep 17 00:00:00 2001
From: Anthony Romano <heyitsanthony@users.noreply.github.com>
Date: Thu, 16 Nov 2017 07:57:56 -0800
Subject: [PATCH 1/2] freelist: set alloc tx for freelist to prior txn

Was causing freelist corruption on tx.WriteTo
---
 freelist.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/freelist.go b/freelist.go
index 318cc3bd2..266f15429 100644
--- a/freelist.go
+++ b/freelist.go
@@ -132,9 +132,9 @@ func (f *freelist) free(txid txid, p *page) {
 	allocTxid, ok := f.allocs[p.id]
 	if ok {
 		delete(f.allocs, p.id)
-	} else if (p.flags & (freelistPageFlag | metaPageFlag)) != 0 {
-		// Safe to claim txid as allocating since these types are private to txid.
-		allocTxid = txid
+	} else if (p.flags & freelistPageFlag) != 0 {
+		// Freelist is always allocated by prior tx.
+		allocTxid = txid - 1
 	}
 
 	for id := p.id; id <= p.id+pgid(p.overflow); id++ {

From 41fefe7322263b61e5669a9bdd136570c15c0c69 Mon Sep 17 00:00:00 2001
From: Anthony Romano <heyitsanthony@users.noreply.github.com>
Date: Thu, 16 Nov 2017 07:58:34 -0800
Subject: [PATCH 2/2] test: check concurrent WriteTo operations aren't
 corrupted

Reliably triggers consistency check failures on ramdisk without freelist
free fix.
---
 db_test.go | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/db_test.go b/db_test.go
index 629150c9e..e3a58c3ca 100644
--- a/db_test.go
+++ b/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