ensure hashmap init clears maps

This reorders some statements in the hashmap initialization to ensure we
always start fresh, even when no pageids were passed to it.

fixes #791

Signed-off-by: Thomas Jungblut <tjungblu@redhat.com>

internal/freelist/freelist_test.go

@@ -8,6 +8,8 @@ import (
 	"testing"
 	"unsafe"
 
+	"github.com/stretchr/testify/require"
+
 	"go.etcd.io/bbolt/internal/common"
 )
 
@@ -179,6 +181,45 @@ func TestFreelist_releaseRange(t *testing.T) {
 	}
 }
 
+func TestFreeList_init(t *testing.T) {
+	buf := make([]byte, 4096)
+	f := newTestFreelist()
+	f.Init(common.Pgids{5, 6, 8})
+
+	p := common.LoadPage(buf)
+	f.Write(p)
+
+	f2 := newTestFreelist()
+	f2.Read(p)
+	require.Equal(t, common.Pgids{5, 6, 8}, f2.freePageIds())
+
+	// When initializing the freelist with an empty list of page ID,
+	// it should reset the freelist page IDs.
+	f2.Init([]common.Pgid{})
+	require.Equal(t, common.Pgids{}, f2.freePageIds())
+}
+
+func TestFreeList_reload(t *testing.T) {
+	buf := make([]byte, 4096)
+	f := newTestFreelist()
+	f.Init(common.Pgids{5, 6, 8})
+
+	p := common.LoadPage(buf)
+	f.Write(p)
+
+	f2 := newTestFreelist()
+	f2.Read(p)
+	require.Equal(t, common.Pgids{5, 6, 8}, f2.freePageIds())
+
+	f2.Free(common.Txid(5), common.NewPage(10, common.LeafPageFlag, 0, 2))
+
+	// reload shouldn't affect the pending list
+	f2.Reload(p)
+
+	require.Equal(t, common.Pgids{5, 6, 8}, f2.freePageIds())
+	require.Equal(t, []common.Pgid{10, 11, 12}, f2.pendingPageIds()[5].ids)
+}
+
 // Ensure that a freelist can deserialize from a freelist page.
 func TestFreelist_read(t *testing.T) {
 	// Create a page.

internal/freelist/hashmap.go

@@ -21,22 +21,22 @@ type hashMap struct {
 }
 
 func (f *hashMap) Init(pgids common.Pgids) {
+	// reset the counter when freelist init
+	f.freePagesCount = 0
+	f.freemaps = make(map[uint64]pidSet)
+	f.forwardMap = make(map[common.Pgid]uint64)
+	f.backwardMap = make(map[common.Pgid]uint64)
+
 	if len(pgids) == 0 {
 		return
 	}
 
-	size := uint64(1)
-	start := pgids[0]
-	// reset the counter when freelist init
-	f.freePagesCount = 0
-
 	if !sort.SliceIsSorted([]common.Pgid(pgids), func(i, j int) bool { return pgids[i] < pgids[j] }) {
 		panic("pgids not sorted")
 	}
 
-	f.freemaps = make(map[uint64]pidSet)
-	f.forwardMap = make(map[common.Pgid]uint64)
-	f.backwardMap = make(map[common.Pgid]uint64)
+	size := uint64(1)
+	start := pgids[0]
 
 	for i := 1; i < len(pgids); i++ {
 		// continuous page
@@ -117,7 +117,7 @@ func (f *hashMap) FreeCount() int {
 func (f *hashMap) freePageIds() common.Pgids {
 	count := f.FreeCount()
 	if count == 0 {
-		return nil
+		return common.Pgids{}
 	}
 
 	m := make([]common.Pgid, 0, count)