Cancelable reads (#120)

This commit implements cancelable reads, which allows Bubble Tea programs to run in succession in a single application. It also makes sure all goroutines terminate before `Program.Start()` returns.

Closes #24.

cancelreader_linux.go

@@ -0,0 +1,152 @@
+//go:build linux
+// +build linux
+
+// nolint:revive
+package tea
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// newCancelReader returns a reader and a cancel function. If the input reader
+// is an *os.File, the cancel function can be used to interrupt a blocking call
+// read call. In this case, the cancel function returns true if the call was
+// cancelled successfully. If the input reader is not a *os.File, the cancel
+// function does nothing and always returns false. The linux implementation is
+// based on the epoll mechanism.
+func newCancelReader(reader io.Reader) (cancelReader, error) {
+	file, ok := reader.(*os.File)
+	if !ok {
+		return newFallbackCancelReader(reader)
+	}
+
+	epoll, err := unix.EpollCreate1(0)
+	if err != nil {
+		return nil, fmt.Errorf("create epoll: %w", err)
+	}
+
+	r := &epollCancelReader{
+		file:  file,
+		epoll: epoll,
+	}
+
+	r.cancelSignalReader, r.cancelSignalWriter, err = os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+
+	err = unix.EpollCtl(epoll, unix.EPOLL_CTL_ADD, int(file.Fd()), &unix.EpollEvent{
+		Events: unix.EPOLLIN,
+		Fd:     int32(file.Fd()),
+	})
+	if err != nil {
+		return nil, fmt.Errorf("add reader to epoll interrest list")
+	}
+
+	err = unix.EpollCtl(epoll, unix.EPOLL_CTL_ADD, int(r.cancelSignalReader.Fd()), &unix.EpollEvent{
+		Events: unix.EPOLLIN,
+		Fd:     int32(r.cancelSignalReader.Fd()),
+	})
+	if err != nil {
+		return nil, fmt.Errorf("add reader to epoll interrest list")
+	}
+
+	return r, nil
+}
+
+type epollCancelReader struct {
+	file               *os.File
+	cancelSignalReader *os.File
+	cancelSignalWriter *os.File
+	cancelMixin
+	epoll int
+}
+
+func (r *epollCancelReader) Read(data []byte) (int, error) {
+	if r.isCancelled() {
+		return 0, errCanceled
+	}
+
+	err := r.wait()
+	if err != nil {
+		if errors.Is(err, errCanceled) {
+			// remove signal from pipe
+			var b [1]byte
+			_, readErr := r.cancelSignalReader.Read(b[:])
+			if readErr != nil {
+				return 0, fmt.Errorf("reading cancel signal: %w", readErr)
+			}
+		}
+
+		return 0, err
+	}
+
+	return r.file.Read(data)
+}
+
+func (r *epollCancelReader) Cancel() bool {
+	r.setCancelled()
+
+	// send cancel signal
+	_, err := r.cancelSignalWriter.Write([]byte{'c'})
+	return err == nil
+}
+
+func (r *epollCancelReader) Close() error {
+	var errMsgs []string
+
+	// close kqueue
+	err := unix.Close(r.epoll)
+	if err != nil {
+		errMsgs = append(errMsgs, fmt.Sprintf("closing epoll: %v", err))
+	}
+
+	// close pipe
+	err = r.cancelSignalWriter.Close()
+	if err != nil {
+		errMsgs = append(errMsgs, fmt.Sprintf("closing cancel signal writer: %v", err))
+	}
+
+	err = r.cancelSignalReader.Close()
+	if err != nil {
+		errMsgs = append(errMsgs, fmt.Sprintf("closing cancel signal reader: %v", err))
+	}
+
+	if len(errMsgs) > 0 {
+		return fmt.Errorf(strings.Join(errMsgs, ", "))
+	}
+
+	return nil
+}
+
+func (r *epollCancelReader) wait() error {
+	events := make([]unix.EpollEvent, 1)
+
+	for {
+		_, err := unix.EpollWait(r.epoll, events, -1)
+		if errors.Is(err, unix.EINTR) {
+			continue // try again if the syscall was interrupted
+		}
+
+		if err != nil {
+			return fmt.Errorf("kevent: %w", err)
+		}
+
+		break
+	}
+
+	switch events[0].Fd {
+	case int32(r.file.Fd()):
+		return nil
+	case int32(r.cancelSignalReader.Fd()):
+		return errCanceled
+	}
+
+	return fmt.Errorf("unknown error")
+}