Return empty list of loop devices when losetup doesn't print anything

pkg/util/losetup/losetup.go

@@ -32,6 +32,11 @@ func ListLoopDevices(ctx context.Context, executor exec.Interface) ([]*LoopDevic
 		return nil, fmt.Errorf("failed to run losetup with args: %v: %w, stdout: %q, stderr: %q", args, err, stdout, stderr.String())
 	}
 
+	// losetup may return nothing, when there are no loop devices.
+	if stdout.Len() == 0 {
+		return []*LoopDevice{}, nil
+	}
+
 	output := &losetupOutput{}
 	err = json.Unmarshal(stdout.Bytes(), output)
 	if err != nil {

pkg/util/losetup/losetup_test.go

@@ -0,0 +1,109 @@
+// Copyright (c) 2024 ScyllaDB.
+
+package losetup
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/scylladb/scylla-operator/pkg/util/exectest"
+	testingexec "k8s.io/utils/exec/testing"
+)
+
+func TestListLoopDevices(t *testing.T) {
+	t.Parallel()
+
+	tt := []struct {
+		name                string
+		commands            []exectest.Command
+		expectedLoopDevices []*LoopDevice
+		expectedError       error
+	}{
+		{
+			name: "losetup fails with random error",
+			commands: []exectest.Command{
+				{
+					Cmd:    "losetup",
+					Args:   []string{"--all", "--list", "--json"},
+					Stdout: []byte("stdout output"),
+					Stderr: []byte("stderr error"),
+					Err:    testingexec.FakeExitError{Status: 666},
+				},
+			},
+			expectedLoopDevices: nil,
+			expectedError:       fmt.Errorf(`failed to run losetup with args: [--all --list --json]: %w, stdout: "stdout output", stderr: "stderr error"`, testingexec.FakeExitError{Status: 666}),
+		},
+		{
+			name: "losetup returns empty list when losetup doesn't output anything",
+			commands: []exectest.Command{
+				{
+					Cmd:    "losetup",
+					Args:   []string{"--all", "--list", "--json"},
+					Stdout: []byte(""),
+					Stderr: nil,
+					Err:    nil,
+				},
+			},
+			expectedLoopDevices: []*LoopDevice{},
+			expectedError:       nil,
+		},
+		{
+			name: "losetup returns empty list when losetup returns empty array of loop devices",
+			commands: []exectest.Command{
+				{
+					Cmd:    "losetup",
+					Args:   []string{"--all", "--list", "--json"},
+					Stdout: []byte(fmt.Sprintf(`{"loopdevices":[]}`)),
+					Stderr: nil,
+					Err:    nil,
+				},
+			},
+			expectedLoopDevices: []*LoopDevice{},
+			expectedError:       nil,
+		},
+		{
+			name: "losetup returns loop devices from json output",
+			commands: []exectest.Command{
+				{
+					Cmd:    "losetup",
+					Args:   []string{"--all", "--list", "--json"},
+					Stdout: []byte(fmt.Sprintf(`{"loopdevices":[{"name":"/dev/loop0","back-file":"/mnt/disk0.img"},{"name":"/dev/loop1","back-file":"/mnt/disk1.img"}]}`)),
+					Stderr: nil,
+					Err:    nil,
+				},
+			},
+			expectedLoopDevices: []*LoopDevice{
+				{
+					Name:        "/dev/loop0",
+					BackingFile: "/mnt/disk0.img",
+				},
+				{
+					Name:        "/dev/loop1",
+					BackingFile: "/mnt/disk1.img",
+				},
+			},
+			expectedError: nil,
+		},
+	}
+
+	for i := range tt {
+		tc := tt[i]
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+
+			executor := exectest.NewFakeExec(tc.commands...)
+			loopDevices, err := ListLoopDevices(ctx, executor)
+			if !reflect.DeepEqual(err, tc.expectedError) {
+				t.Fatalf("expected %v error, got %v", tc.expectedError, err)
+			}
+			if !reflect.DeepEqual(loopDevices, tc.expectedLoopDevices) {
+				t.Fatalf("expected %v, got %v", tc.expectedLoopDevices, loopDevices)
+			}
+		})
+	}
+}