internal/uevent: enumerate objects via sysfs

This is not a great way to implement cold boot, but I already have the implementation lying around.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/uevent/sysfs.go

@@ -0,0 +1,87 @@
+package uevent
+
+import (
+	"bytes"
+	"errors"
+	"io/fs"
+	"log"
+	"path/filepath"
+	"unsafe"
+)
+
+// Enumerate scans sysfs and emits [Synthetic] events. It returns the first
+// error it encounters.
+//
+// The specified filesystem must present the sysfs root.
+func Enumerate(
+	sysfs fs.FS,
+	handleWalkErr func(error) error,
+	events chan<- *Message,
+) error {
+	if handleWalkErr == nil {
+		handleWalkErr = func(err error) error {
+			if errors.Is(err, fs.ErrNotExist) {
+				log.Println("enumerate", err)
+				return nil
+			}
+			return err
+		}
+	}
+
+	return fs.WalkDir(sysfs, "devices", func(
+		path string,
+		d fs.DirEntry,
+		err error,
+	) error {
+		if err != nil {
+			return handleWalkErr(err)
+		}
+
+		if d.IsDir() || d.Name() != "uevent" {
+			return nil
+		}
+
+		msg := Message{
+			Action: Synthetic,
+
+			// cleans path, appears to be compatible with kernel behaviour
+			DevPath: filepath.Dir(path),
+		}
+
+		var target string
+		if target, err = fs.ReadLink(
+			sysfs,
+			filepath.Join(msg.DevPath, "subsystem"),
+		); err != nil {
+			if err = handleWalkErr(err); err != nil {
+				return err
+			}
+		} else {
+			msg.Env = append(msg.Env, "SUBSYSTEM="+filepath.Base(target))
+		}
+
+		// read entire file: slicing does not copy
+		var env []byte
+		if env, err = fs.ReadFile(sysfs, path); err != nil {
+			return handleWalkErr(err)
+		}
+
+		for _, s := range bytes.Split(env, []byte{'\n'}) {
+			if len(s) == 0 {
+				continue
+			}
+			msg.Env = append(msg.Env, unsafe.String(unsafe.SliceData(s), len(s)))
+		}
+
+		if len(msg.Env) == 0 {
+			// this implies absent subsystem, its error is already handled
+			return nil
+		}
+
+		if msg.DevPath != "" && msg.DevPath[0] != '/' {
+			msg.DevPath = "/" + msg.DevPath
+		}
+		events <- &msg
+		return nil
+	})
+}