- Prevent double-free failure when the same dataset is closed multiple times

In some cases, copying dataset object, either by passing by value or in
some other pattern can lead to code that will close the same dataset
multiple times (e.g. defer). Or otherwise, code with complicated
handling of dataset cleanup.
Fixes: #30, Closes: #31

a_test.go

@@ -34,6 +34,7 @@ func Test(t *testing.T) {
 	zfsTestDatasetSetProperty(t)
 	zfsTestDatasetHoldRelease(t)
 
+	zfsTestDoubleFreeOnDestroy(t)
 	zfsTestDatasetDestroy(t)
 
 	zpoolTestPoolDestroy(t)

destroy_test.go

@@ -60,4 +60,5 @@ func TestDataset_DestroyPromote(t *testing.T) {
 	t.Log("Destroy promote completed with success")
 	d.Close()
 	zpoolTestPoolDestroy(t)
+	cleanupVDisks()
 }

zfs.go

@@ -13,6 +13,7 @@ import (
 	"path"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 	"unsafe"
 )
@@ -49,6 +50,7 @@ type HoldTag struct {
 // Dataset - ZFS dataset object
 type Dataset struct {
 	list       C.dataset_list_ptr
+	closeOnce  *sync.Once
 	Type       DatasetType
 	Properties map[Prop]Property
 	Children   []Dataset
@@ -58,7 +60,7 @@ func (d *Dataset) openChildren() (err error) {
 	d.Children = make([]Dataset, 0, 5)
 	list := C.dataset_list_children(d.list)
 	for list != nil {
-		dataset := Dataset{list: list}
+		dataset := Dataset{list: list, closeOnce: new(sync.Once)}
 		dataset.Type = DatasetType(C.dataset_type(list))
 		dataset.Properties = make(map[Prop]Property)
 		err = dataset.ReloadProperties()
@@ -79,16 +81,20 @@ func (d *Dataset) openChildren() (err error) {
 // DatasetOpenAll recursive get handles to all available datasets on system
 // (file-systems, volumes or snapshots).
 func DatasetOpenAll() (datasets []Dataset, err error) {
-	var dataset Dataset
-	dataset.list = C.dataset_list_root()
-	for dataset.list != nil {
-		dataset.Type = DatasetType(C.dataset_type(dataset.list))
+	list := C.dataset_list_root()
+	for list != nil {
+		dataset := Dataset{
+			list:      list,
+			closeOnce: new(sync.Once),
+			Type:      DatasetType(C.dataset_type(list)),
+		}
+		dataset.Type = DatasetType(C.dataset_type(list))
 		err = dataset.ReloadProperties()
 		if err != nil {
 			return
 		}
 		datasets = append(datasets, dataset)
-		dataset.list = C.dataset_next(dataset.list)
+		list = C.dataset_next(list)
 	}
 	for ci := range datasets {
 		if err = datasets[ci].openChildren(); err != nil {
@@ -130,6 +136,7 @@ func DatasetOpenSingle(path string) (d Dataset, err error) {
 		err = fmt.Errorf("%s - %s", err.Error(), path)
 		return
 	}
+	d.closeOnce = new(sync.Once)
 	d.Type = DatasetType(C.dataset_type(d.list))
 	err = d.ReloadProperties()
 	if err != nil {
@@ -182,11 +189,13 @@ func DatasetCreate(path string, dtype DatasetType,
 // Close close dataset and all its recursive children datasets (close handle
 // and cleanup dataset object/s from memory)
 func (d *Dataset) Close() {
-	// path, _ := d.Path()
-	Global.Mtx.Lock()
-	C.dataset_list_close(d.list)
+	// if dataset was ever open
+	if d.closeOnce != nil {
+		d.closeOnce.Do(func() {
+			C.dataset_list_close(d.list)
+		})
+	}
 	d.list = nil
-	Global.Mtx.Unlock()
 	for _, cd := range d.Children {
 		cd.Close()
 	}

zfs_test.go

@@ -338,3 +338,35 @@ func ExampleDatasetOpenAll() {
 	}
 
 }
+
+func CopyAndDestroy(d *zfs.Dataset) (err error) {
+	if err = d.Destroy(false); err != nil {
+		return
+	}
+	d.Close()
+	return
+}
+
+func zfsTestDoubleFreeOnDestroy(t *testing.T) {
+	TSTDestroyPath := TSTPoolName + "/DESTROY"
+	println("TEST Doble Free On Destroy( ", TSTVolumePath, " ) ... ")
+	props := make(map[zfs.Prop]zfs.Property)
+	d, err := zfs.DatasetCreate(TSTDestroyPath, zfs.DatasetTypeFilesystem, props)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	d.Close()
+
+	d, err = zfs.DatasetOpen(TSTDestroyPath)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer d.Close()
+	if err = CopyAndDestroy(&d); err != nil {
+		t.Error(err)
+		return
+	}
+	print("PASS\n\n")
+}

zpool_test.go

@@ -51,12 +51,18 @@ func createTestpoolVdisks() (err error) {
 	return
 }
 
+func removeVDisk(path string) {
+	if err := os.Remove(path); err != nil {
+		println("Error: ", err.Error())
+	}
+}
+
 // Cleanup sparse files used for tests
 func cleanupVDisks() {
 	// try cleanup
-	os.Remove(s1path)
-	os.Remove(s2path)
-	os.Remove(s3path)
+	removeVDisk(s1path)
+	removeVDisk(s2path)
+	removeVDisk(s3path)
 }
 
 /* ------------------------------------------------------------------------- */