loopback: fix race condition opening loopback device

the loopback device file could be already used/removed by another
process.  Since the process is inherently racy, just grab the next
available index and try again until it succeeds.

Closes: #2038

Signed-off-by: Giuseppe Scrivano <[email protected]>

pkg/loopback/attach_loopback.go

@@ -58,6 +58,13 @@ func openNextAvailableLoopback(index int, sparseName string, sparseFile *os.File
 		loopFile, err = os.OpenFile(target, os.O_RDWR, 0o644)
 		if err != nil {
 			if errors.Is(err, fs.ErrNotExist) {
+				// Another process could have taken the loopback device in the meantime.  So repeat
+				// the process with the next loopback device.
+				index, err = getNextFreeLoopbackIndex()
+				if err != nil {
+					logrus.Debugf("Error retrieving the next available loopback: %s", err)
+					return nil, err
+				}
 				continue
 			}
 			logrus.Errorf("Opening loopback device: %s", err)

pkg/loopback/attach_test.go

@@ -0,0 +1,49 @@
+//go:build linux && cgo
+// +build linux,cgo
+
+package loopback
+
+import (
+	"os"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	maxDevicesPerGoroutine = 1000
+	maxGoroutines          = 10
+)
+
+func TestAttachLoopbackDeviceRace(t *testing.T) {
+	createLoopbackDevice := func() {
+		// Create a file to use as a backing file
+		f, err := os.CreateTemp(t.TempDir(), "loopback-test")
+		require.NoError(t, err)
+		defer f.Close()
+
+		defer os.Remove(f.Name())
+
+		lp, err := AttachLoopDevice(f.Name())
+		assert.NoError(t, err)
+		assert.NotNil(t, lp, "loopback device file should not be nil")
+		if lp != nil {
+			lp.Close()
+		}
+	}
+
+	wg := sync.WaitGroup{}
+
+	for i := 0; i < maxGoroutines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < maxDevicesPerGoroutine; i++ {
+				createLoopbackDevice()
+			}
+		}()
+	}
+	wg.Wait()
+}