internal/pipewire: EPOLL_CTL_ADD instead of EPOLL_CTL_MOD

Implementation is no longer tied down by the limitations of SyscallConn.

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

internal/pipewire/pipewire.go

@@ -188,63 +188,57 @@ func (conn *unixConn) wantsEpoll() error {
 	if !conn.epoll {
 		conn.epoll = true
 		conn.epollFd, conn.epollErr = syscall.EpollCreate1(syscall.EPOLL_CLOEXEC)
+		if conn.epollErr == nil {
+			if conn.epollErr = syscall.EpollCtl(conn.epollFd, syscall.EPOLL_CTL_ADD, conn.fd, &syscall.EpollEvent{
+				Events: syscall.EPOLLERR | syscall.EPOLLHUP,
+				Fd:     int32(conn.fd),
+			}); conn.epollErr != nil {
+				_ = syscall.Close(conn.epollFd)
+			}
+		}
 	}
 	return conn.epollErr
 }
 
-// wait waits for a specific I/O event on fd and returns a function
-// that must be deferred by the caller regardless of error.
-func (conn *unixConn) wait(event uint32, errP *error) (cleanupFunc func()) {
+// wait waits for a specific I/O event on fd. Caller must arrange for wantsEpoll
+// to be called somewhere before wait is called.
+func (conn *unixConn) wait(event uint32) (err error) {
 	if conn.timeout == 0 {
-		return func() {}
+		return nil
 	}
 	deadline := time.Now().Add(conn.timeout)
 	conn.timeout = 0
 
-	if *errP = syscall.EpollCtl(conn.epollFd, syscall.EPOLL_CTL_ADD, conn.fd, &syscall.EpollEvent{
+	if err = syscall.EpollCtl(conn.epollFd, syscall.EPOLL_CTL_MOD, conn.fd, &syscall.EpollEvent{
 		Events: event | syscall.EPOLLERR | syscall.EPOLLHUP,
 		Fd:     int32(conn.fd),
-	}); *errP != nil {
-		return func() {}
-	} else {
-		cleanupFunc = func() {
-			// fd is guaranteed to remain valid while f executes but not after f returns
-			if epDelErr := syscall.EpollCtl(conn.epollFd, syscall.EPOLL_CTL_DEL, conn.fd, nil); epDelErr != nil && *errP == nil {
-				*errP = epDelErr
-				return
-			}
-		}
+	}); err != nil {
+		return
 	}
 
 	for timeout := deadline.Sub(time.Now()); timeout > 0; timeout = deadline.Sub(time.Now()) {
-		if n, err := syscall.EpollWait(conn.epollFd, conn.epollBuf[:], int(timeout/time.Millisecond)); err != nil {
-			*errP = err
+		var n int
+		if n, err = syscall.EpollWait(conn.epollFd, conn.epollBuf[:], int(timeout/time.Millisecond)); err != nil {
 			return
-		} else {
-			switch n {
-			case 1: // only the socket fd is ever added
-				if conn.epollBuf[0].Fd != int32(conn.fd) { // unreachable
-					err = syscall.ENOTRECOVERABLE
-					break
-				}
-				if conn.epollBuf[0].Events&event == event ||
-					conn.epollBuf[0].Events&syscall.EPOLLERR|syscall.EPOLLHUP != 0 {
-					break
-				}
-				*errP = syscall.ETIME
-				continue
+		}
 
-			case 0: // timeout
-				err = syscall.ETIMEDOUT
-				break
-
-			default: // unreachable
-				err = syscall.ENOTRECOVERABLE
-				break
+		switch n {
+		case 1: // only the socket fd is ever added
+			if conn.epollBuf[0].Fd != int32(conn.fd) { // unreachable
+				return syscall.ENOTRECOVERABLE
 			}
+			if conn.epollBuf[0].Events&event == event ||
+				conn.epollBuf[0].Events&syscall.EPOLLERR|syscall.EPOLLHUP != 0 {
+				return nil
+			}
+			err = syscall.ETIME
+			continue
 
-			*errP = err
-			break
+		case 0: // timeout
+			return syscall.ETIMEDOUT
+
+		default: // unreachable
+			return syscall.ENOTRECOVERABLE
 		}
 	}
 	return
@@ -254,9 +248,7 @@ func (conn *unixConn) wait(event uint32, errP *error) (cleanupFunc func()) {
 func (conn *unixConn) Recvmsg(p, oob []byte, flags int) (n, oobn, recvflags int, err error) {
 	if err = conn.wantsEpoll(); err != nil {
 		return
-	}
-	defer conn.wait(syscall.EPOLLIN, &err)()
-	if err != nil {
+	} else if err = conn.wait(syscall.EPOLLIN); err != nil {
 		return
 	}
 
@@ -268,9 +260,7 @@ func (conn *unixConn) Recvmsg(p, oob []byte, flags int) (n, oobn, recvflags int,
 func (conn *unixConn) Sendmsg(p, oob []byte, flags int) (n int, err error) {
 	if err = conn.wantsEpoll(); err != nil {
 		return
-	}
-	defer conn.wait(syscall.EPOLLOUT, &err)()
-	if err != nil {
+	} else if err = conn.wait(syscall.EPOLLOUT); err != nil {
 		return
 	}