internal/uevent: synthetic events for coldboot

This causes the kernel to regenerate events that happened before earlyinit started.

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

internal/uevent/coldboot.go

@@ -0,0 +1,71 @@
+package uevent
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"log"
+	"os"
+	"path/filepath"
+)
+
+// synthAdd is prepared bytes written to uevent to cause a synthetic add event
+// to be emitted during coldboot.
+var synthAdd = []byte(KOBJ_ADD.String())
+
+// Coldboot writes "add" to every uevent file that it finds in /sys/devices.
+// This causes the kernel to regenerate the uevents for these paths.
+//
+// The specified pathname must present the sysfs root.
+//
+// Note that while [AOSP documentation] claims to also scan /sys/class and
+// /sys/block, this is no longer the case, and the documentation was not updated
+// when this changed.
+//
+// [AOSP documentation]: https://android.googlesource.com/platform/system/core/+/master/init/README.ueventd.md
+func Coldboot(
+	ctx context.Context,
+	pathname string,
+	visited chan<- string,
+	handleWalkErr func(error) error,
+) error {
+	if handleWalkErr == nil {
+		handleWalkErr = func(err error) error {
+			if errors.Is(err, fs.ErrNotExist) {
+				log.Println("coldboot", err)
+				return nil
+			}
+			return err
+		}
+	}
+
+	return filepath.WalkDir(filepath.Join(pathname, "devices"), func(
+		path string,
+		d fs.DirEntry,
+		err error,
+	) error {
+		if err != nil {
+			return handleWalkErr(err)
+		}
+		if err = ctx.Err(); err != nil {
+			return err
+		}
+
+		if d.IsDir() || d.Name() != "uevent" {
+			return nil
+		}
+		if err = os.WriteFile(path, synthAdd, 0); err != nil {
+			return handleWalkErr(err)
+		}
+
+		select {
+		case visited <- path:
+			break
+
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+
+		return nil
+	})
+}