AP_Scripting: add mavport module

Abstracts the MAVLink SERIAL_CONTROL message to be usable as a Lua port,
in particular the DEV_SHELL device supported by QGroundControl's MAVLink
Console.

libraries/AP_Scripting/modules/MAVLink/mavlink_msg_SERIAL_CONTROL.lua

@@ -0,0 +1,11 @@
+local SERIAL_CONTROL = {}
+SERIAL_CONTROL.id = 126
+SERIAL_CONTROL.fields = {
+             { "baudrate", "<I4" },
+             { "timeout", "<I2" },
+             { "device", "<B" },
+             { "flags", "<B" },
+             { "count", "<B" },
+             { "data", "<B", 70 },
+             }
+return SERIAL_CONTROL

libraries/AP_Scripting/modules/mavport.lua

@@ -0,0 +1,166 @@
+-- class to cast mavlink SERIAL_CONTROL message functionality as a serial port.
+-- currently always uses the "dev shell" device (which is ignored by Ardupilot).
+-- designed for use with the Lua REPL applet.
+-- note that flush() must be called to empty internal transmit buffers.
+
+local mavport = {}
+
+local mavlink_msgs = require("MAVLink/mavlink_msgs")
+
+local SERIAL_CONTROL_DATA_LEN = 70
+local SERIAL_CONTROL = require("MAVLink/mavlink_msg_SERIAL_CONTROL")
+local msg_map = {
+    [SERIAL_CONTROL.id] = "SERIAL_CONTROL",
+}
+
+function mavport:begin(_)
+    local data = {}
+    for i = 1, SERIAL_CONTROL_DATA_LEN do
+        data[i] = 0
+    end
+    self._tx_template = {
+        baudrate = 0,
+        timeout = 0,
+        device = 10, -- dev shell
+        flags = 1, -- is reply
+        count = 0,
+        data = data,
+    }
+
+    self._tx_buf = data
+    self._tx_count = 0
+    self._tx_msg = nil
+
+    self._chan = 0 -- send to channel 0 by default
+
+    self._rx_buf = nil
+    self._rx_pos = 1
+
+    mavlink.init(1, 4) -- only one message we care about, don't need huge queue
+    mavlink.register_rx_msgid(SERIAL_CONTROL.id) -- register it
+end
+
+function mavport:write(value)
+    return self:writestring(string.char(value))
+end
+
+function mavport:writestring(str)
+    if self._tx_msg then -- message already queued?
+        self:flush()
+        if self._tx_msg then return 0 end -- reject if flush failed
+    end
+
+    local count = self._tx_count
+    local buf = self._tx_buf
+    for ci = 1, #str do
+        if count == SERIAL_CONTROL_DATA_LEN then break end
+        count = count + 1
+        buf[count] = str:byte(ci)
+    end
+    local sent = count - self._tx_count
+    self._tx_count = count
+
+    if count == SERIAL_CONTROL_DATA_LEN then
+        self:flush()
+    end
+
+    return sent
+end
+
+function mavport:flush() -- send queued data if possible
+    if not self._tx_msg and self._tx_count > 0 then
+        -- encode the message and store it for transmission
+        local msg = self._tx_template
+        msg.count = self._tx_count
+        _, self._tx_msg = mavlink_msgs.encode("SERIAL_CONTROL", msg)
+        self._tx_count = 0
+    end
+
+    if self._tx_msg then -- message to send?
+        if mavlink.send_chan(self._chan, SERIAL_CONTROL.id, self._tx_msg) then
+            self._tx_msg = nil -- successfully sent
+        end
+    end
+end
+
+function mavport:read()
+    if not self._rx_buf then
+        self:_receive()
+        if not self._rx_buf then return -1 end
+    end
+
+    local b = self._rx_buf
+    local pos = self._rx_pos
+    local c = b[pos]
+
+    self._rx_pos = pos + 1
+    if pos == #b then
+        self._rx_buf = nil
+    end
+
+    return c 
+end
+
+function mavport:readstring(count)
+    local avail = self:available() -- also fills rx buf
+    if avail == 0 then return "" end
+    if count > avail then count = avail end
+
+    local b = self._rx_buf
+    local pos = self._rx_pos
+    local s = string.char(table.unpack(b, pos, pos+count-1))
+
+    pos = pos + count
+    if pos > #b then
+        self._rx_buf = nil
+    end
+    self._rx_pos = pos
+
+    return s
+end
+
+function mavport:_receive()
+    local msg, chan
+    while true do
+        msg, chan = mavlink.receive_chan()
+        if not msg then return end -- no new messages
+
+        -- decode message and handle if it's for us
+        msg = mavlink_msgs.decode(msg, msg_map)
+        if msg.device == 10 then -- for the dev shell?
+            self._chan = chan -- reply on the same channel
+            break
+        end
+    end
+
+    local data = msg.data
+    local count = msg.count
+    -- remove trailing nulls, they shouldn't happen but they do
+    while data[count] == 0 do
+        count = count - 1
+    end
+
+    -- store received bytes
+    if count > 0 then
+        if count < SERIAL_CONTROL_DATA_LEN then -- remove trailing junk
+            data = table.move(data, 1, count, 1, {})
+        end
+        self._rx_buf = data
+        self._rx_pos = 1
+    end
+end
+
+function mavport:available()
+    if not self._rx_buf then
+        self:_receive()
+        if not self._rx_buf then return 0 end
+    end
+
+    return #self._rx_buf - self._rx_pos + 1
+end
+
+-- for completeness
+function mavport.set_flow_control(_, _)
+end
+
+return mavport