Fix a hang

Before this change, if select() returned EINTR we would hang.

This change:
* Makes us ignore EINTRs and try again
* Makes sure to not hang, even if the twin main loop would fail for some
  other reason.

build.sh

@@ -28,4 +28,4 @@ fi
 go build -trimpath -ldflags="-s -w -X main.versionString=${VERSION}" -o "${BINARY}"
 
 # Alternative build line, if you want to attach to the running process in the Go debugger:
-# go build -ldflags="-X main.versionString=${VERSION}" -gcflags='-N -l' -o "${BINARY}"
+# go build -ldflags="-X main.versionString=${VERSION}" -gcflags="all=-N -l" -o "${BINARY}"

twin/screen-setup-windows.go

@@ -55,6 +55,10 @@ func (r *interruptableReaderImpl) Interrupt() {
 	r.shutdownRequested.Store(true)
 }
 
+func (r *interruptableReaderImpl) Close() error {
+	return nil
+}
+
 func newInterruptableReader(base *os.File) (interruptableReader, error) {
 	return &interruptableReaderImpl{base: base}, nil
 }

twin/screen-setup.go

@@ -24,6 +24,19 @@ type interruptableReaderImpl struct {
 }
 
 func (r *interruptableReaderImpl) Read(p []byte) (n int, err error) {
+	for {
+		n, err = r.read(p)
+		if err == syscall.EINTR {
+			// Not really a problem, we can get this on window resizes for
+			// example, just try again.
+			continue
+		}
+
+		return
+	}
+}
+
+func (r *interruptableReaderImpl) read(p []byte) (n int, err error) {
 	// "This argument should be set to the highest-numbered file descriptor in
 	// any of the three sets, plus 1. The indicated file descriptors in each set
 	// are checked, up to this limit"
@@ -55,7 +68,7 @@ func (r *interruptableReaderImpl) Read(p []byte) (n int, err error) {
 		err = io.EOF
 
 		// Let Interrupt() know we're done
-		r.interruptionComplete <- struct{}{}
+		r.Close()
 
 		return
 	}
@@ -77,10 +90,18 @@ func (r *interruptableReaderImpl) Interrupt() {
 	<-r.interruptionComplete
 }
 
+func (r *interruptableReaderImpl) Close() error {
+	select {
+	case r.interruptionComplete <- struct{}{}:
+	default:
+	}
+	return nil
+}
+
 func newInterruptableReader(base *os.File) (interruptableReader, error) {
 	reader := interruptableReaderImpl{
 		base:                 base,
-		interruptionComplete: make(chan struct{}),
+		interruptionComplete: make(chan struct{}, 1),
 	}
 	pr, pw, err := os.Pipe()
 	if err != nil {

twin/screen.go

@@ -74,6 +74,12 @@ type interruptableReader interface {
 
 	// Interrupt unblocks the read call, either now or eventually.
 	Interrupt()
+
+	// Close() should be called after you are done with the interruptableReader.
+	//
+	// It will not close the underlying reader, but it will prevent Interrupt()
+	// from hanging if called after a failure in the screen mainLoop().
+	Close() error
 }
 
 type UnixScreen struct {
@@ -366,6 +372,7 @@ func (screen *UnixScreen) ShowCursorAt(column int, row int) {
 
 func (screen *UnixScreen) mainLoop() {
 	defer func() {
+		screen.ttyInReader.Close()
 		log.Debug("Twin screen main loop done")
 	}()