zfs: fix batch destroy panic if all snaps are undestroyable

See https://github.com/zrepl/zrepl/pull/259#issuecomment-585334023

panic: runtime error: index out of range [0] with length 0
goroutine 14 [running]:
github.com/zrepl/zrepl/zfs.tryBatch(0xd6aa20, 0xc0000b8018, 0xc00025e0c0, 0x0, 0x6, 0xd61d80, 0x1280df8, 0xd58920, 0xc000132000)
        zrepl/zfs/versions_destroy.go:129 +0x302
github.com/zrepl/zrepl/zfs.doDestroyBatchedRec(0xd6aa20, 0xc0000b8018, 0xc000578a80, 0x6, 0x6, 0xd61d80, 0x1280df8)
        zrepl/zfs/versions_destroy.go:184 +0x4a5
github.com/zrepl/zrepl/zfs.doDestroyBatched(0xd6aa20, 0xc0000b8018, 0xc000222780, 0x6, 0x8, 0xd61d80, 0x1280df8)
        zrepl/zfs/versions_destroy.go:95 +0xc7
github.com/zrepl/zrepl/zfs.doDestroy(0xd6aa20, 0xc0000b8018, 0xc0005788d0, 0x6, 0x6, 0xd61d80, 0x1280df8)
        zrepl/zfs/versions_destroy.go:82 +0x362
github.com/zrepl/zrepl/zfs.ZFSDestroyFilesystemVersions(...)
        zrepl/zfs/versions_destroy.go:41
github.com/zrepl/zrepl/endpoint.doDestroySnapshots(0xd6aaa0, 0xc0004412c0, 0xc00057ca00, 0xc0005785a0, 0x6, 0x6, 0xb68940, 0xc5df01, 0xc000150a80)
        zrepl/endpoint/endpoint.go:785 +0x388
github.com/zrepl/zrepl/endpoint.(*Receiver).DestroySnapshots(0xc000127500, 0xd6aaa0, 0xc0004412c0, 0xc0002ca280, 0xc000150880, 0xd73ca0, 0xc00057c960)
        zrepl/endpoint/endpoint.go:751 +0xdb
github.com/zrepl/zrepl/daemon/pruner.doOneAttemptExec(0xc000429980, 0xc000429958, 0xc0001cb180)
        zrepl/daemon/pruner/pruner.go:531 +0x51f
github.com/zrepl/zrepl/daemon/pruner.doOneAttempt(0xc000429980, 0xc000429958)
        zrepl/daemon/pruner/pruner.go:486 +0x1064
github.com/zrepl/zrepl/daemon/pruner.(*Pruner).prune(0xc00011e280, 0xd6aaa0, 0xc0004412c0, 0x7f4906fff7e8, 0xc000127500, 0x7f4906fff738, 0xc0001324e0, 0xc000064420, 0x1, 0x1, ...)
        zrepl/daemon/pruner/pruner.go:214 +0x53
github.com/zrepl/zrepl/daemon/pruner.(*Pruner).Prune(...)
        zrepl/daemon/pruner/pruner.go:200
github.com/zrepl/zrepl/daemon/job.(*ActiveSide).do(0xc000268000, 0xd6a9e0, 0xc0002223c0)
        zrepl/daemon/job/active.go:482 +0x906
github.com/zrepl/zrepl/daemon/job.(*ActiveSide).Run(0xc000268000, 0xd6aaa0, 0xc000127080)
        zrepl/daemon/job/active.go:404 +0x289
github.com/zrepl/zrepl/daemon.(*jobs).start.func1(0xc000032200, 0xd73ca0, 0xc00000f2e0, 0xd6efa0, 0xc000268000, 0xd6aaa0, 0xc000126c90)
        zrepl/daemon/daemon.go:220 +0x121
created by github.com/zrepl/zrepl/daemon.(*jobs).start
        zrepl/daemon/daemon.go:216 +0x52e

zfs/versions_destroy.go

@@ -126,6 +126,10 @@ func buildBatches(reqs []*DestroySnapOp) [][]*DestroySnapOp {
 
 // batch must be on same Filesystem, panics otherwise
 func tryBatch(ctx context.Context, batch []*DestroySnapOp, d destroyer) error {
+	if len(batch) == 0 {
+		return nil
+	}
+
 	batchFS := batch[0].Filesystem
 	batchNames := make([]string, len(batch))
 	for i := range batchNames {

zfs/versions_destroy_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"regexp"
 	"strings"
 	"syscall"
 	"testing"
@@ -19,7 +20,7 @@ type mockBatchDestroy struct {
 	mtx              chainlock.L
 	calls            []string
 	commaUnsupported bool
-	undestroyable    string
+	undestroyable    *regexp.Regexp
 	randomerror      string
 	e2biglen         int
 }
@@ -38,17 +39,38 @@ func (m *mockBatchDestroy) Destroy(args []string) error {
 		return &os.PathError{Err: syscall.E2BIG} // TestExcessiveArgumentsResultInE2BIG checks that this errors is produced
 	}
 	m.calls = append(m.calls, a)
+
+	var snapnames []string
 	if m.commaUnsupported {
+		snapnames = append(snapnames, a)
 		if strings.Contains(a, ",") {
 			return fmt.Errorf("unsupported syntax mock error")
 		}
+	} else {
+		snapnames = append(snapnames, strings.Split(a, ",")...)
 	}
+	fs, vt, firstsnapname, err := DecomposeVersionString(snapnames[0])
+	if err != nil {
+		panic(err)
+	}
+	if vt != Snapshot {
+		panic(vt)
+	}
+	snapnames[0] = firstsnapname
 
-	if m.undestroyable != "" && strings.Contains(a, m.undestroyable) {
+	var undestroyable []string
+	if m.undestroyable != nil {
+		for _, snap := range snapnames {
+			if m.undestroyable.MatchString(snap) {
+				undestroyable = append(undestroyable, snap)
+			}
+		}
+	}
+	if len(undestroyable) > 0 {
 		return &DestroySnapshotsError{
-			Filesystem:    "PLACEHOLDER",
+			Filesystem:    fs,
-			Undestroyable: []string{m.undestroyable},
+			Undestroyable: undestroyable,
-			Reason:        []string{"undestroyable reason"},
+			Reason:        []string{"undestroyable mock with regexp " + m.undestroyable.String()},
 		}
 	}
 	if m.randomerror != "" && strings.Contains(a, m.randomerror) {
@@ -82,7 +104,7 @@ func TestBatchDestroySnaps(t *testing.T) {
 		nilErrs()
 		mock := &mockBatchDestroy{
 			commaUnsupported: false,
-			undestroyable:    "undestroyable",
+			undestroyable:    regexp.MustCompile(`undestroyable`),
 			randomerror:      "randomerror",
 		}
 
@@ -118,11 +140,37 @@ func TestBatchDestroySnaps(t *testing.T) {
 		)
 	})
 
+	t.Run("all_undestroyable", func(t *testing.T) {
+
+		opsTemplate := []*DestroySnapOp{
+			&DestroySnapOp{"zroot/a", "foo", new(error)},
+			&DestroySnapOp{"zroot/a", "bar", new(error)},
+		}
+
+		mock := &mockBatchDestroy{
+			commaUnsupported: false,
+			undestroyable:    regexp.MustCompile(`.*`),
+		}
+
+		doDestroy(context.TODO(), opsTemplate, mock)
+
+		defer mock.mtx.Lock().Unlock()
+		assert.Equal(
+			t,
+			[]string{
+				"zroot/a@bar,foo", // reordered snaps in lexicographical order
+				"zroot/a@bar",
+				"zroot/a@foo",
+			},
+			mock.calls,
+		)
+	})
+
 	t.Run("comma_syntax_unsupported", func(t *testing.T) {
 		nilErrs()
 		mock := &mockBatchDestroy{
 			commaUnsupported: true,
-			undestroyable:    "undestroyable",
+			undestroyable:    regexp.MustCompile(`undestroyable`),
 			randomerror:      "randomerror",
 		}