Improve the init system: support skipping startup files and running a custom main command

Changelog.md

@@ -1,6 +1,9 @@
 ## 0.9.6
 
  * Fixed a bug in `my_init`: child processes that have been adopted during execution of init scripts are now properly reaped.
+ * Much improved `my_init`:
+   * It is now possible to run and watch a custom command, possibly in addition to running runit. See "Running a one-shot command in the container" in the README.
+   * It is now possible to skip running startup files such as /etc/rc.local.
 
 ## 0.9.5 (release date: 2014-02-06)
 

README.md

@@ -40,6 +40,7 @@ You can configure the stock `ubuntu` image yourself from your Dockerfile, so why
    * [Getting started](#getting_started)
    * [Adding additional daemons](#adding_additional_daemons)
    * [Running scripts during container startup](#running_startup_scripts)
+   * [Running a one-shot command in the container](#oneshot)
    * [Login to the container via SSH](#login)
  * [Building the image yourself](#building)
  * [Conclusion](#conclusion)
@@ -154,6 +155,50 @@ The following example shows how you can add a startup script. This script simply
     RUN mkdir -p /etc/my_init.d
     ADD logtime.sh /etc/my_init.d/logtime.sh
 
+<a name="oneshot"></a>
+### Running a one-shot command in the container
+
+Normally, when you want to run a single command in a container, and exit immediately after the command, you invoke Docker like this:
+
+    docker run YOUR_IMAGE COMMAND ARGUMENTS...
+
+However the downside of this approach is that the init system is not started. That is, while invoking `COMMAND`, important daemons such as cron and syslog are not running. Also, orphaned child processes are not properly reaped, because `COMMAND` is PID 1.
+
+Baseimage-docker provides a facility to run a single one-shot command, while solving all of the aforementioned problems. Run a single command in the following manner:
+
+    docker run YOUR_IMAGE /sbin/my_init -- COMMAND ARGUMENTS ...
+
+This will perform the following:
+
+ * Runs all system startup files, such as /etc/my_init.d/* and /etc/rc.local.
+ * Starts all runit services.
+ * Runs the specified command.
+ * When the specified command exits, stops all runit services.
+
+For example:
+
+    $ docker run phusion/baseimage:<VERSION> /sbin/my_init -- ls
+    *** Running /etc/my_init.d/00_regen_ssh_host_keys.sh...
+    No SSH host key available. Generating one...
+    Creating SSH2 RSA key; this may take some time ...
+    Creating SSH2 DSA key; this may take some time ...
+    Creating SSH2 ECDSA key; this may take some time ...
+    *** Running /etc/rc.local...
+    *** Booting runit daemon...
+    *** Runit started as PID 80
+    *** Running ls...
+    bin  boot  dev  etc  home  image  lib  lib64  media  mnt  opt  proc  root  run  sbin  selinux  srv  sys  tmp  usr  var
+    *** ls exited with exit code 0.
+    *** Shutting down runit daemon (PID 80)...
+    *** Killing all processes...
+
+You may find that the default invocation is too noisy. Or perhaps you don't want to run the startup files. You can customize all this by passing arguments to `my_init`. Invoke `docker run YOUR_IMAGE /sbin/my_init --help` for more information.
+
+The following example runs `ls` without running the startup files and with less messages, while running all runit services:
+
+    $ docker run phusion/baseimage:<VERSION> /sbin/my_init --skip-startup-files --quiet -- ls
+    bin  boot  dev  etc  home  image  lib  lib64  media  mnt  opt  proc  root  run  sbin  selinux  srv  sys  tmp  usr  var
+
 <a name="login"></a>
 ### Login to the container via SSH
 

image/my_init

@@ -1,8 +1,42 @@
 #!/usr/bin/python2
-import os, sys, stat, signal, errno
+import os, sys, stat, signal, errno, argparse, time
 
-pid = None
+KILL_PROCESS_TIMEOUT = 5
-status = None
+KILL_ALL_PROCESSES_TIMEOUT = 5
+
+LOG_LEVEL_ERROR = 1
+LOG_LEVEL_WARN  = 1
+LOG_LEVEL_INFO  = 2
+LOG_LEVEL_DEBUG = 3
+
+log_level = None
+
+class AlarmException(Exception):
+	pass
+
+def error(message):
+	if log_level >= LOG_LEVEL_ERROR:
+		sys.stderr.write("*** %s\n" % message)
+
+def warn(message):
+	if log_level >= LOG_LEVEL_WARN:
+		print("*** %s" % message)
+
+def info(message):
+	if log_level >= LOG_LEVEL_INFO:
+		print("*** %s" % message)
+
+def debug(message):
+	if log_level >= LOG_LEVEL_DEBUG:
+		print("*** %s" % message)
+
+def ignore_signals_and_raise_keyboard_interrupt(signame):
+	signal.signal(signal.SIGTERM, signal.SIG_IGN)
+	signal.signal(signal.SIGINT, signal.SIG_IGN)
+	raise KeyboardInterrupt(signame)
+
+def raise_alarm_exception():
+	raise AlarmException('Alarm')
 
 def listdir(path):
 	try:
@@ -20,100 +54,185 @@ def is_exe(path):
 	except OSError:
 		return False
 
-def reap_child(signum, frame):
+def waitpid_reap_other_children(pid):
-	global pid, status, waiting_for_runit
+	done = False
-	try:
+	status = None
-		result = os.wait3(os.WNOHANG)
+	while not done:
-		if result is not None and pid == result[0]:
+		this_pid, status = os.waitpid(-1, 0)
-			status = result[1]
+		done = this_pid == pid
-	except OSError:
+	return status
-		pass
 
-def stop_child_process(name, pid):
+def stop_child_process(name, pid, signo = signal.SIGTERM, time_limit = KILL_PROCESS_TIMEOUT):
-	print("*** Shutting down %s (PID %d)..." % (name, pid))
+	info("Shutting down %s (PID %d)..." % (name, pid))
 	try:
-		os.kill(pid, signal.SIGHUP)
+		os.kill(pid, signo)
 	except OSError:
 		pass
+	signal.alarm(time_limit)
+	try:
+		try:
+			waitpid_reap_other_children(pid)
+		except OSError:
+			pass
+	except AlarmException:
+		warn("%s (PID %d) did not shut down in time. Forcing it to exit.")
+		try:
+			os.kill(pid, signal.SIGKILL)
+		except OSError:
+			pass
+		try:
+			waitpid_reap_other_children(pid)
+		except OSError:
+			pass
+	finally:
+		signal.alarm(0)
 
 def run_command_killable(*argv):
-	global pid
 	filename = argv[0]
+	status = None
 	pid = os.spawnvp(os.P_NOWAIT, filename, argv)
-	signal.signal(signal.SIGINT, lambda signum, frame: stop_child_process(filename, pid))
-	signal.signal(signal.SIGTERM, lambda signum, frame: stop_child_process(filename, pid))
 	try:
+		status = waitpid_reap_other_children(pid)
+	except BaseException as s:
+		warn("An error occurred. Aborting.")
+		stop_child_process(filename, pid)
+		raise
+	if status != 0:
+		error("%s failed with exit code %d\n" % (filename, status))
+		sys.exit(1)
+
+def kill_all_processes(time_limit):
+	info("Killing all processes...")
+	try:
+		os.kill(-1, signal.SIGTERM)
+	except OSError:
+		pass
+	signal.alarm(time_limit)
+	try:
+		# Wait until no more child processes exist.
 		done = False
 		while not done:
 			try:
-				this_pid, status = os.waitpid(-1, 0)
+				os.waitpid(-1, 0)
-				done = this_pid == pid
 			except OSError as e:
-				if e.errno == errno.EINTR:
+				if e.errno == errno.ECHILD:
-					sys.exit(2)
+					done = True
 				else:
 					raise
+	except AlarmException:
+		warn("Not all processes have exited in time. Forcing them to exit.")
+		try:
+			os.kill(-1, signal.SIGKILL)
+		except OSError:
+			pass
 	finally:
-		signal.signal(signal.SIGINT, signal.SIG_DFL)
+		signal.alarm(0)
-		signal.signal(signal.SIGTERM, signal.SIG_DFL)
-	if status != 0:
-		sys.stderr.write("*** %s failed with exit code %d\n" % (filename, status))
-		sys.exit(1)
 
+def run_startup_files():
 	# Run /etc/my_init.d/*
 	for name in listdir("/etc/my_init.d"):
 		filename = "/etc/my_init.d/" + name
 		if is_exe(filename):
-		print("*** Running %s..." % filename)
+			info("Running %s..." % filename)
 			run_command_killable(filename)
 
 	# Run /etc/rc.local.
 	if is_exe("/etc/rc.local"):
-	print("*** Running /etc/rc.local...")
+		info("Running /etc/rc.local...")
 		run_command_killable("/etc/rc.local")
 
-# Start runit.
+def start_runit():
-signal.signal(signal.SIGCHLD, reap_child)
+	info("Booting runit daemon...")
-print("*** Booting runit...")
+	pid = os.spawnl(os.P_NOWAIT, "/usr/bin/runsvdir", "/usr/bin/runsvdir",
-pid = os.spawnl(os.P_NOWAIT, "/usr/bin/runsvdir", "/usr/bin/runsvdir", "-P", "/etc/service", "log: %s" % ('.' * 395))
+		"-P", "/etc/service", "log: %s" % ('.' * 395))
-print("*** Runit started as PID %d" % pid)
+	info("Runit started as PID %d" % pid)
-signal.signal(signal.SIGTERM, lambda signum, frame: stop_child_process("runit", pid))
+	return pid
 
-# Wait for runit, and while waiting, reap any adopted orphans.
+def wait_for_runit_or_interrupt(pid):
-done = False
-while not done:
 	try:
-		this_pid, status = os.waitpid(pid, 0)
+		status = waitpid_reap_other_children(pid)
-		done = True
+		return (True, status)
-	except OSError as e:
+	except KeyboardInterrupt:
-		if e.errno == errno.EINTR:
+		return (False, None)
-			# Try again
-			pass
-		else:
-			# The SIGCHLD handler probably caught it.
-			done = True
 
-# Runit has exited. Reset signal handlers.
+def shutdown_runit_services():
-print("*** Runit exited with code %s. Waiting for all services to shut down..." % status)
+	debug("Begin shutting down runit services...")
-signal.signal(signal.SIGCHLD, signal.SIG_DFL)
+	os.system("/usr/bin/sv down /etc/service/*")
-signal.signal(signal.SIGTERM, signal.SIG_DFL)
-signal.siginterrupt(signal.SIGCHLD, False)
-signal.siginterrupt(signal.SIGTERM, False)
 
-# Wait at most 5 seconds for services to shut down.
+def wait_for_runit_services():
-import time
+	debug("Waiting for runit services to exit...")
-
-def shutdown(signum = None, frame = None):
-	global status
-	if status is not None:
-		sys.exit(status)
-
-signal.signal(signal.SIGALRM, shutdown)
-signal.alarm(5)
 	done = False
 	while not done:
 		done = os.system("/usr/bin/sv status /etc/service/* | grep -q '^run:'") != 0
 		if not done:
-		time.sleep(0.5)
+			time.sleep(0.1)
-shutdown()
 
+def main(args):
+	if not args.skip_startup_files:
+		run_startup_files()
+	
+	runit_exited = False
+	exit_code = None
+
+	if not args.skip_runit:
+		runit_pid = start_runit()
+	try:
+		if len(args.main_command) == 0:
+			runit_exited, exit_code = wait_for_runit_or_interrupt(runit_pid)
+			if runit_exited:
+				info("Runit exited with code %d" % exit_code)
+		else:
+			info("Running %s..." % " ".join(args.main_command))
+			pid = os.spawnvp(os.P_NOWAIT, args.main_command[0], args.main_command)
+			try:
+				exit_code = waitpid_reap_other_children(pid)
+				info("%s exited with exit code %d." % (args.main_command[0], exit_code))
+			except KeyboardInterrupt:
+				stop_child_process(args.main_command[0], pid)
+			except BaseException as s:
+				warn("An error occurred. Aborting.")
+				stop_child_process(args.main_command[0], pid)
+				raise
+		sys.exit(exit_code)
+	finally:
+		if not args.skip_runit:
+			shutdown_runit_services()
+			if not runit_exited:
+				stop_child_process("runit daemon", runit_pid)
+			wait_for_runit_services()
+
+# Parse options.
+parser = argparse.ArgumentParser(description = 'Initialize the system.')
+parser.add_argument('main_command', metavar = 'MAIN_COMMAND', type = str, nargs = '*',
+	help = 'The main command to run. (default: runit)')
+parser.add_argument('--skip-startup-files', dest = 'skip_startup_files',
+	action = 'store_const', const = True, default = False,
+	help = 'Skip running /etc/my_init.d/* and /etc/rc.local')
+parser.add_argument('--skip-runit', dest = 'skip_runit',
+	action = 'store_const', const = True, default = False,
+	help = 'Do not run runit services')
+parser.add_argument('--no-kill-all-on-exit', dest = 'kill_all_on_exit',
+	action = 'store_const', const = False, default = True,
+	help = 'Don\'t kill all processes on the system upon exiting')
+parser.add_argument('--quiet', dest = 'log_level',
+	action = 'store_const', const = LOG_LEVEL_WARN, default = LOG_LEVEL_INFO,
+	help = 'Only print warnings and errors')
+args = parser.parse_args()
+log_level = args.log_level
+
+if args.skip_runit and len(args.main_command) == 0:
+	error("When --skip-runit is given, you must also pass a main command.")
+	sys.exit(1)
+
+# Run main function.
+signal.signal(signal.SIGTERM, lambda signum, frame: ignore_signals_and_raise_keyboard_interrupt('SIGTERM'))
+signal.signal(signal.SIGINT, lambda signum, frame: ignore_signals_and_raise_keyboard_interrupt('SIGINT'))
+signal.signal(signal.SIGALRM, lambda signum, frame: raise_alarm_exception())
+try:
+	main(args)
+except KeyboardInterrupt:
+	warn("Init system aborted.")
+	exit(2)
+finally:
+	if args.kill_all_on_exit:
+		kill_all_processes(KILL_ALL_PROCESSES_TIMEOUT)