client checks kernel module in /sys/module for WSL2 bridge networking

.changelog/17306.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+client: check kernel module in `/sys/module` to help with WSL2 bridge networking
+```

client/fingerprint/bridge_linux.go

@@ -56,6 +56,14 @@ func (f *BridgeFingerprint) detect(module string) error {
 	// accumulate errors from every place we might find the module
 	var errs error
 
+	// Check if the module is in /sys/modules
+	sysfsModulePath := fmt.Sprintf("/sys/module/%s", module)
+	if err := f.findDir(sysfsModulePath); err != nil {
+		errs = multierror.Append(errs, err)
+	} else {
+		return nil
+	}
+
 	// check if the module has been dynamically loaded
 	dynamicPath := "/proc/modules"
 	if err := f.searchFile(module, dynamicPath, f.regexp(dynamicModuleRe, module)); err != nil {
@@ -89,6 +97,14 @@ func (f *BridgeFingerprint) detect(module string) error {
 	return errs
 }
 
+func (f *BridgeFingerprint) findDir(dirname string) error {
+	if _, err := os.Stat(dirname); err != nil {
+		return fmt.Errorf("failed to find %s: %v", dirname, err)
+	} else {
+		return nil
+	}
+}
+
 func (f *BridgeFingerprint) searchFile(module, filename string, re *regexp.Regexp) error {
 	file, err := os.Open(filename)
 	if err != nil {

client/fingerprint/bridge_linux_test.go

@@ -19,11 +19,11 @@ func TestBridgeFingerprint_detect(t *testing.T) {
 	ci.Parallel(t)
 
 	f := &BridgeFingerprint{logger: testlog.HCLogger(t)}
-	require.NoError(t, f.detect("ip_tables"))
+	require.NoError(t, f.detect("kernel")) // kernel should be there.
 
 	err := f.detect("nonexistentmodule")
 	require.Error(t, err)
-	require.Contains(t, err.Error(), "3 errors occurred")
+	require.Contains(t, err.Error(), "4 errors occurred")
 }
 
 func writeFile(t *testing.T, prefix, content string) string {