net_class: catch syscall.EINVAL and better handle read errors

prometheus#483 introduced a bug
that terminates attribute reading when the returned error is
syscall.Errno, which is what util.SysReadFile() will typically
return. Handle that case specifically.

While doing that pull the error checking into
ParseNetClassAttribute() for clarity and to allow external
callers to handle errors correctly.

Signed-off-by: Dan Williams <dcbw@redhat.com>

sysfs/net_class.go

@@ -117,14 +117,35 @@ func (fs FS) NetClass() (NetClass, error) {
 	return netClass, nil
 }
 
+// isFatalError returns true if the error should terminate attribute reading
+func isFatalError(err error) bool {
+	var errno syscall.Errno
+
+	if os.IsNotExist(err) {
+		return false
+	} else if os.IsPermission(err) {
+		return false
+	} else if err.Error() == "operation not supported" {
+		return false
+	} else if errors.Is(err, os.ErrInvalid) {
+		return false
+	} else if errors.As(err, &errno) && (errno == syscall.EINVAL) {
+		return false
+	}
+	return true
+}
+
 // ParseNetClassAttribute parses a given file in /sys/class/net/<iface>
 // and sets the value in a given NetClassIface object if the value was readable.
-// It returns an error if the file cannot be read.
+// It returns an error if the file cannot be read and the error is fatal.
 func ParseNetClassAttribute(devicePath, attrName string, interfaceClass *NetClassIface) error {
 	attrPath := filepath.Join(devicePath, attrName)
 	value, err := util.SysReadFile(attrPath)
 	if err != nil {
-		return fmt.Errorf("failed to read file %q: %w", attrPath, err)
+		if isFatalError(err) {
+			return fmt.Errorf("failed to read file %q: %w", attrPath, err)
+		}
+		return nil
 	}
 
 	vp := util.NewValueParser(value)
@@ -202,12 +223,8 @@ func parseNetClassIface(devicePath string) (*NetClassIface, error) {
 		if !f.Type().IsRegular() {
 			continue
 		}
-		err := ParseNetClassAttribute(devicePath, f.Name(), &interfaceClass)
-		if err != nil {
-			// Return fatal errors to caller
-			if !os.IsNotExist(err) && !os.IsPermission(err) && err.Error() != "operation not supported" && !errors.Is(err, os.ErrInvalid) {
-				return nil, err
-			}
+		if err := ParseNetClassAttribute(devicePath, f.Name(), &interfaceClass); err != nil {
+			return nil, err
 		}
 	}