improve focus-send

Fall back to `dash` on macOS, because `bash` randomly drops serial
input, causing the tool to hang.

Flush the command buffer before sending the requested command. A
failed upload session can cause characters to remain in the command
buffer.

Redirect stdin instead of using a separate file descriptor. Also do
this before running `stty`. This allows the `stty` settings to
actually take effect on macOS, which seems to reset the termios
state of serial devices upon the last close of the device.

Tested on macOS 10.15 and Ubuntu 20.04.

Signed-off-by: Taylor Yu <tlyu@mit.edu>

bin/focus-send

@@ -17,14 +17,22 @@ set -e
 
 OS=$(uname -s)
 
+# igncr absorbs CR from Focus CRLF line endings
+# -echo is needed because raw doesn't turn it off on Linux
+STTY_ARGS="9600 raw igncr -echo"
+
 case ${OS} in
     Linux)
         DEVICE="${DEVICE:-/dev/ttyACM0}"
-        stty -F "${DEVICE}" 9600 raw -echo
         ;;
     Darwin)
+        # bash on macOS has a bug that randomly drops serial input
+        if [ -n "$BASH_VERSION" ] && [ -x /bin/dash ]; then
+            # Prevent loop in case someone exported it
+            export -n BASH_VERSION
+            exec /bin/dash "$0" "$@"
+        fi
         DEVICE="${DEVICE:-/dev/cu.usbmodemCkbio01E}"
-        stty -f "${DEVICE}" 9600 raw -echo
         ;;
     *)
         echo "Error Unknown OS : ${OS}" >&2
@@ -32,14 +40,24 @@ case ${OS} in
         ;;
 esac
 
+# Redirect prior to running stty, because macOS sometimes resets termios
+# state upon last close of a terminal device.
+exec < "${DEVICE}"
+# shellcheck disable=SC2086 # intentional word splitting
+stty $STTY_ARGS
 
-exec 3<"${DEVICE}"
-echo "$@" >"${DEVICE}"
+wait_dot () {
+    while read -r line; do
+        if [ "${line}" = "." ]; then
+            break
+        fi
+        echo "${line}"
+    done
+}
 
-while read -r line <&3; do
-    line="$(echo -n "${line}" | tr -d '\r')"
-    if [ "${line}" == "." ]; then
-        break
-    fi
-    echo "${line}"
-done
+# Flush any invalid commands out of input buffer.
+# This could happen after a failed upload.
+echo ' ' > "${DEVICE}"
+wait_dot
+echo "$@" >"${DEVICE}"
+wait_dot