Fail loudly on a touch read, stop the workers before their sockets close, and file down the small edges

fb.go

@@ -170,23 +170,24 @@ func findDisplay(fd int) (connID, crtcID uint32, err error) {
 }
 
 // The card number is not stable across machines, so the card driving a
-// connected display is the one we want.
-func openCard() (int, error) {
+// connected display is the one we want, and the display it found comes back
+// with it.
+func openCard() (fd int, connID, crtcID uint32, err error) {
 	paths, err := filepath.Glob("/dev/dri/card*")
 	if err != nil {
-		return -1, err
+		return -1, 0, 0, err
 	}
 	for _, p := range paths {
 		fd, err := unix.Open(p, unix.O_RDWR|unix.O_CLOEXEC, 0)
 		if err != nil {
 			continue
 		}
-		if _, _, err := findDisplay(fd); err == nil {
-			return fd, nil
+		if connID, crtcID, err := findDisplay(fd); err == nil {
+			return fd, connID, crtcID, nil
 		}
 		unix.Close(fd)
 	}
-	return -1, fmt.Errorf("no drm device with a connected display")
+	return -1, 0, 0, fmt.Errorf("no drm device with a connected display")
 }
 
 func (fb *framebuffer) addScanout(i int) error {
@@ -223,7 +224,7 @@ func (fb *framebuffer) addScanout(i int) error {
 }
 
 func openFramebuffer() (*framebuffer, error) {
-	fd, err := openCard()
+	fd, connID, crtcID, err := openCard()
 	if err != nil {
 		return nil, err
 	}
@@ -236,11 +237,6 @@ func openFramebuffer() (*framebuffer, error) {
 		return nil, fmt.Errorf("take drm master: %w", err)
 	}
 
-	connID, crtcID, err := findDisplay(fd)
-	if err != nil {
-		fb.close()
-		return nil, err
-	}
 	mode, err := preferredMode(fd, connID)
 	if err != nil {
 		fb.close()