internal/uevent: consume kernel-originated events

These are not possible to cover outside integration vm. Extreme care is required when dealing with this method, so keep it simple.

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

internal/uevent/uevent.go

@@ -4,6 +4,9 @@
 package uevent
 
 import (
+	"context"
+	"errors"
+	"strconv"
 	"sync/atomic"
 	"syscall"
 
@@ -48,3 +51,55 @@ func Dial() (*Conn, error) {
 	}
 	return &Conn{conn: c}, err
 }
+
+var (
+	// ErrBadSocket is returned by [Conn.Consume] for a reply from a
+	// syscall.Sockaddr with unexpected concrete type.
+	ErrBadSocket = errors.New("unexpected socket address")
+)
+
+// BadPortError is returned by [Conn.Consume] upon receiving a message that did
+// not come from the kernel.
+type BadPortError syscall.SockaddrNetlink
+
+var _ Recoverable = new(BadPortError)
+
+func (*BadPortError) recoverable() {}
+func (e *BadPortError) Error() string {
+	return "unexpected message from port id " + strconv.Itoa(int(e.Pid)) +
+		" on NETLINK_KOBJECT_UEVENT"
+}
+
+// Consume continuously receives and parses events from the kernel. It returns
+// the first error it encounters.
+//
+// Callers must not restart event processing after a non-nil error that does not
+// satisfy [Recoverable] is returned.
+func (c *Conn) Consume(ctx context.Context, events chan<- *Message) error {
+	if err := c.enterExcl(); err != nil {
+		return err
+	}
+	defer c.exitExcl()
+
+	for {
+		data, from, err := c.conn.Recvfrom(ctx, 0)
+		if err != nil {
+			return err
+		}
+
+		// lib/kobject_uevent.c:
+		// set portid 0 to inform userspace message comes from kernel
+		if v, ok := from.(*syscall.SockaddrNetlink); !ok {
+			return ErrBadSocket
+		} else if v.Pid != 0 {
+			return (*BadPortError)(v)
+
+		}
+
+		var msg Message
+		if err = msg.UnmarshalBinary(data); err != nil {
+			return err
+		}
+		events <- &msg
+	}
+}

internal/uevent/uevent_test.go

@@ -1,9 +1,14 @@
 package uevent_test
 
 import (
+	"context"
 	"encoding"
+	"os"
 	"reflect"
+	"sync"
+	"syscall"
 	"testing"
+	"time"
 
 	"hakurei.app/internal/uevent"
 )
@@ -108,6 +113,92 @@ func adeB[V any, S interface {
 	}
 }
 
+func TestDialConsume(t *testing.T) {
+	t.Parallel()
+
+	c, err := uevent.Dial()
+	if err != nil {
+		t.Fatalf("Dial: error = %v", err)
+	}
+	t.Cleanup(func() {
+		if closeErr := c.Close(); closeErr != nil {
+			t.Fatal(err)
+		}
+	})
+
+	// check kernel-assigned port id
+	c0, err0 := uevent.Dial()
+	if err0 != nil {
+		t.Fatalf("Dial: error = %v", err)
+	}
+	t.Cleanup(func() {
+		if closeErr := c0.Close(); closeErr != nil {
+			t.Fatal(closeErr)
+		}
+	})
+
+	var wg sync.WaitGroup
+	done := make(chan struct{})
+	events := make(chan *uevent.Message, 1<<10)
+	go func() {
+		defer close(done)
+		for msg := range events {
+			t.Log(msg)
+		}
+	}()
+	t.Cleanup(func() {
+		wg.Wait()
+		close(events)
+		<-done
+	})
+
+	ctx, cancel := context.WithCancel(t.Context())
+	defer cancel()
+
+	wg.Go(func() {
+		if err = c.Consume(ctx, events); err != context.Canceled {
+			panic(err)
+		}
+	})
+	wg.Go(func() {
+		if err0 = c0.Consume(ctx, events); err0 != context.Canceled {
+			panic(err0)
+		}
+	})
+
+	if testing.Verbose() {
+		if d, perr := time.ParseDuration(os.Getenv(
+			"ROSA_UEVENT_TEST_DURATION",
+		)); perr != nil {
+			t.Logf("skipping long test: error = %v", perr)
+		} else {
+			time.Sleep(d)
+		}
+	}
+	cancel()
+	wg.Wait()
+
+	ctx, cancel = context.WithCancel(t.Context())
+	defer cancel()
+
+	var errs [2]error
+	exclExit := make(chan struct{})
+	wg.Go(func() {
+		defer func() { exclExit <- struct{}{} }()
+		errs[0] = c.Consume(ctx, events)
+	})
+	wg.Go(func() {
+		defer func() { exclExit <- struct{}{} }()
+		errs[1] = c.Consume(ctx, events)
+	})
+	<-exclExit
+	cancel()
+	<-exclExit
+	if errs[0] != syscall.EAGAIN && errs[1] != syscall.EAGAIN {
+		t.Fatalf("enterExcl: err0 = %v, err1 = %v", errs[0], errs[1])
+	}
+}
+
 func TestErrors(t *testing.T) {
 	t.Parallel()
 
@@ -138,6 +229,10 @@ func TestErrors(t *testing.T) {
 			Data: "\x00",
 			Kind: 0xbad,
 		}, `section "" is invalid`},
+
+		{"BadPortError", &uevent.BadPortError{
+			Pid: 1,
+		}, "unexpected message from port id 1 on NETLINK_KOBJECT_UEVENT"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {