internal/uevent: exclusive socket access

This is a much simplified mutex, since blocking is not required.

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

internal/uevent/uevent.go

@@ -4,6 +4,7 @@
 package uevent
 
 import (
+	"sync/atomic"
 	"syscall"
 
 	"hakurei.app/internal/netlink"
@@ -17,7 +18,23 @@ type (
 )
 
 // Conn represents a NETLINK_KOBJECT_UEVENT socket.
-type Conn struct{ conn *netlink.Conn }
+type Conn struct {
+	conn *netlink.Conn
+
+	// Whether currently between a call to enterExcl and exitExcl.
+	excl atomic.Bool
+}
+
+// enterExcl must be called entering a critical section that interacts with conn.
+func (c *Conn) enterExcl() error {
+	if !c.excl.CompareAndSwap(false, true) {
+		return syscall.EAGAIN
+	}
+	return nil
+}
+
+// exitExcl must be called exiting a critical section that interacts with conn.
+func (c *Conn) exitExcl() { c.excl.Store(false) }
 
 // Close closes the underlying socket.
 func (c *Conn) Close() error { return c.conn.Close() }
@@ -29,5 +46,5 @@ func Dial() (*Conn, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &Conn{c}, err
+	return &Conn{conn: c}, err
 }