Resolve issue nutechsoftware/alarmdecoder#issue_40

Solution adds previous leftover to front of buf so leftovers can be
searched for newline.  It also will process leftover data even if
there is no more serial data arriving.

alarmdecoder/devices/serial_device.py

@@ -210,8 +210,14 @@ class SerialDevice(Device):
             timeout_event.reading = False
         timeout_event.reading = True
 
-        if purge_buffer:
-            self._buffer = b''
+        buf = b''
+        if not purge_buffer:
+            #Add previous leftovers to front of buf so leftovers can be searched for newlines
+            #If there were no leftovers then self._buffer will be empty
+            buf = self._buffer
+
+        self._buffer = b''
+
 
         got_line, data = False, ''
 
@@ -222,17 +228,27 @@ class SerialDevice(Device):
         leftovers = b''
         try:
             while timeout_event.reading and not got_line:
-                read_ready, _, _ = select.select([self._device.fileno()], [], [], 0.5)
-                if len(read_ready) == 0:
+                #If there is leftover data then only perform a poll to set the read_ready flag
+                #otherwise block here until short timeout waiting for more data
+                select_timeout = 0.5
+                if len(self._buffer) > 0:
+                    select_timeout = 0.1
+                read_ready, _, _ = select.select([self._device.fileno()], [], [], select_timeout)
+                if len(read_ready) == 0 and len(self._buffer) == 0:
                     continue
 
-                bytes_avail = 0
-                if hasattr(self._device, "in_waiting"):
-                    bytes_avail = self._device.in_waiting
-                else:
-                    bytes_avail = self._device.inWaiting()
+                #Only read data if there is some waiting
+                if len(read_ready) > 0:
+                    bytes_avail = 0
+                    if hasattr(self._device, "in_waiting"):
+                        bytes_avail = self._device.in_waiting
+                    else:
+                        bytes_avail = self._device.inWaiting()
+
+                    device_data = self._device.read(bytes_avail)
 
-                buf = self._device.read(bytes_avail)
+                    #Append new data to previous leftovers, if any
+                    buf = buf + device_data
 
                 for idx in range(len(buf)):
                     c = buf[idx]
@@ -242,15 +258,14 @@ class SerialDevice(Device):
                         ub = bytes([ub])
 
                     # NOTE: AD2SERIAL and AD2PI apparently sends down \xFF on boot.
-                    if ub != b'' and ub != b"\xff":
-                        self._buffer += ub
-
-                        if ub == b"\n":
-                            self._buffer = self._buffer.strip(b"\r\n")
-
+                    if ub != b'' and ub != b"\xff" and ub != b"\r":
+                        if ub != b"\n":
+                            self._buffer += ub
+                        else:
+                            #found end of line
                             if len(self._buffer) > 0:
                                 got_line = True
-                                leftovers = buf[idx:]
+                                leftovers = buf[idx+1:]
                                 break
 
         except (OSError, serial.SerialException) as err: