Automatically find a free display

shell/Emulator.py

@@ -1,20 +1,46 @@
+import logging
 import os
+import socket
 
 from Process import Process
 
 class XephyrProcess(Process):
 	def __init__(self):
-		# FIXME How to pick a free display number?
-		self._display = 100
+		self._display = self.get_display_number()
 		cmd = 'Xephyr :%d -ac -screen 640x480' % (self._display) 
 		Process.__init__(self, cmd)
 
+	def get_display_number(self):
+		"""Find a free display number trying to connect to 6000+ sockets"""
+		retries = 20
+		display_number = 1
+		display_is_free = False
+		
+		while not display_is_free and retries > 0:
+			s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+			try:
+				s.connect(('127.0.0.1', 6000 + display_number))
+				logging.info('Display %d is already in use. Trying next.' % (display_number))
+
+				display_number += 1
+				retries -= 1
+			except:
+				display_is_free = True
+
+		if display_is_free:
+			return display_number
+		
+		return -1
+		
 	def get_name(self):
 		return 'Xephyr'
 
 	def start(self):
-		Process.start(self)
-		os.environ['DISPLAY'] = ":%d" % (self._display)
+		if self._display < 0:
+			logging.error('Cannot find a free display.')
+		else:
+			Process.start(self)
+			os.environ['DISPLAY'] = ":%d" % (self._display)
 
 class Emulator:
 	"""The OLPC emulator"""