Support --color option to indicate whether to use colors in the terminal

Mimic the behavior of the --color=<auto|always|never> found in tools
like `ls' and `grep'. Default to `auto' which checks whether stderr is a
tty to define whether colors are used.

Tested:
- Ran ./boostrap-vz --color=never and --color=always, confirmed
  colorization worked as expected.
- Ran ./boostrap-vz --color=auto ${manifest_file} 2>bootstrap.log,
  confirmed colors were not added to boostrap.log by default, repeated
  the test with --color=always and confirmed escape sequences were
  output.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

bootstrapvz/base/log.py

@@ -4,18 +4,20 @@ both to a file and to the console.
 import logging
 
 
-def get_console_handler(debug):
+def get_console_handler(debug, colorize):
 	"""Returns a log handler for the console
 	The handler color codes the different log levels
 
 	:params bool debug: Whether to set the log level to DEBUG (otherwise INFO)
+	:params bool colorize: Whether to colorize console output
 	:return: The console logging handler
 	"""
 	# Create a console log handler
 	import sys
 	console_handler = logging.StreamHandler(sys.stderr)
-	# We want to colorize the output to the console, so we add a formatter
-	console_handler.setFormatter(ConsoleFormatter())
+	if colorize:
+		# We want to colorize the output to the console, so we add a formatter
+		console_handler.setFormatter(ConsoleFormatter())
 	# Set the log level depending on the debug argument
 	if debug:
 		console_handler.setLevel(logging.DEBUG)

bootstrapvz/base/main.py

@@ -44,6 +44,8 @@ Options:
                      If <path> is `-' file logging will be disabled.
   --pause-on-error   Pause on error, before rollback
   --dry-run          Don't actually run the tasks
+  --color=auto|always|never
+                     Colorize the console output [default: auto]
   --debug            Print debugging information
   -h, --help         show this help
 	"""
@@ -68,7 +70,15 @@ def setup_loggers(opts):
 		file_handler = log.get_file_handler(path=logpath, debug=True)
 		root.addHandler(file_handler)
 
-	console_handler = log.get_console_handler(debug=opts['--debug'])
+	if opts['--color'] == 'never':
+		colorize = False
+	elif opts['--color'] == 'always':
+		colorize = True
+	else:
+		# If --color=auto (default), decide whether to colorize by whether stderr is a tty.
+		import os
+		colorize = os.isatty(2)
+	console_handler = log.get_console_handler(debug=opts['--debug'], colorize=colorize)
 	root.addHandler(console_handler)