rebuild freelist when opening with FreelistSync after NoFreelistSync

Writes pgidNoFreelist to the meta freelist page to detect when freelists
haven't been synced down.

Fixes #5

db.go

@@ -24,6 +24,8 @@ const version = 2
 // Represents a marker value to indicate that a file is a Bolt DB.
 const magic uint32 = 0xED0CDAED
 
+const pgidNoFreelist pgid = 0xffffffffffffffff
+
 // IgnoreNoSync specifies whether the NoSync field of a DB is ignored when
 // syncing changes to a file.  This is required as some operating systems,
 // such as OpenBSD, do not have a unified buffer cache (UBC) and writes
@@ -239,14 +241,29 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 		return nil, err
 	}
 
-	if db.NoFreelistSync {
-		db.freelist = newFreelist()
+	db.freelist = newFreelist()
+	noFreeList := db.meta().freelist == pgidNoFreelist
+	if noFreeList {
+		// Reconstruct free list by scanning the DB.
 		db.freelist.readIDs(db.freepages())
 	} else {
-		// Read in the freelist.
-		db.freelist = newFreelist()
+		// Read free list from freelist page.
 		db.freelist.read(db.page(db.meta().freelist))
 	}
+	db.stats.FreePageN = len(db.freelist.ids)
+
+	// Flush freelist when transitioning from no sync to sync so
+	// NoFreelistSync unaware boltdb can open the db later.
+	if !db.NoFreelistSync && noFreeList && ((mode & 0222) != 0) {
+		tx, err := db.Begin(true)
+		if tx != nil {
+			err = tx.Commit()
+		}
+		if err != nil {
+			_ = db.close()
+			return nil, err
+		}
+	}
 
 	// Mark the database as opened and return.
 	return db, nil
@@ -1065,7 +1082,8 @@ func (m *meta) copy(dest *meta) {
 func (m *meta) write(p *page) {
 	if m.root.root >= m.pgid {
 		panic(fmt.Sprintf("root bucket pgid (%d) above high water mark (%d)", m.root.root, m.pgid))
-	} else if m.freelist >= m.pgid {
+	} else if m.freelist >= m.pgid && m.freelist != pgidNoFreelist {
+		// TODO: reject pgidNoFreeList if !NoFreelistSync
 		panic(fmt.Sprintf("freelist pgid (%d) above high water mark (%d)", m.freelist, m.pgid))
 	}
 

db_test.go

@@ -422,6 +422,74 @@ func TestDB_Open_InitialMmapSize(t *testing.T) {
 	}
 }
 
+// TestOpen_RecoverFreeList tests opening the DB with free-list
+// write-out after no free list sync will recover the free list
+// and write it out.
+func TestOpen_RecoverFreeList(t *testing.T) {
+	db := MustOpenWithOption(&bolt.Options{NoFreelistSync: true})
+	defer db.MustClose()
+
+	// Write some pages.
+	tx, err := db.Begin(true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wbuf := make([]byte, 8192)
+	for i := 0; i < 100; i++ {
+		s := fmt.Sprintf("%d", i)
+		b, err := tx.CreateBucket([]byte(s))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err = b.Put([]byte(s), wbuf); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := tx.Commit(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Generate free pages.
+	if tx, err = db.Begin(true); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 50; i++ {
+		s := fmt.Sprintf("%d", i)
+		b := tx.Bucket([]byte(s))
+		if b == nil {
+			t.Fatal(err)
+		}
+		if err := b.Delete([]byte(s)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := tx.Commit(); err != nil {
+		t.Fatal(err)
+	}
+	if err := db.DB.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Record freelist count from opening with NoFreelistSync.
+	db.MustReopen()
+	freepages := db.Stats().FreePageN
+	if freepages == 0 {
+		t.Fatalf("no free pages on NoFreelistSync reopen")
+	}
+	if err := db.DB.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check free page count is reconstructed when opened with freelist sync.
+	db.o = &bolt.Options{}
+	db.MustReopen()
+	// One less free page for syncing the free list on open.
+	freepages--
+	if fp := db.Stats().FreePageN; fp < freepages {
+		t.Fatalf("closed with %d free pages, opened with %d", freepages, fp)
+	}
+}
+
 // Ensure that a database cannot open a transaction when it's not open.
 func TestDB_Begin_ErrDatabaseNotOpen(t *testing.T) {
 	var db bolt.DB

tx.go

@@ -174,6 +174,8 @@ func (tx *Tx) Commit() error {
 		if err != nil {
 			return err
 		}
+	} else {
+		tx.meta.freelist = pgidNoFreelist
 	}
 
 	// Write dirty pages to disk.
@@ -223,7 +225,9 @@ func (tx *Tx) commitFreelist() error {
 
 	// Free the freelist and allocate new pages for it. This will overestimate
 	// the size of the freelist but not underestimate the size (which would be bad).
-	tx.db.freelist.free(tx.meta.txid, tx.db.page(tx.meta.freelist))
+	if tx.meta.freelist != pgidNoFreelist {
+		tx.db.freelist.free(tx.meta.txid, tx.db.page(tx.meta.freelist))
+	}
 	p, err := tx.allocate((tx.db.freelist.size() / tx.db.pageSize) + 1)
 	if err != nil {
 		tx.rollback()