re-implement daemonize

[richvdh]

This has long been something I've wanted to do. Basically the Daemonize code
is both too flexible and not flexible enough, in that it offers a bunch of
features that we don't use (changing UID, closing FDs in the child, logging to
syslog) and doesn't offer a bunch that we could do with (redirecting stdout/err
to a file instead of /dev/null; having the parent not exit until the child is
running).

As a first step, I've lifted the Daemonize code and removed the bits we don't
use. This should be a non-functional change. Fixing everything else will come
later.

The reveiewer is invited to compare with https://github.com/thesharp/daemonize/blob/master/daemonize.py.

[clokep]

The reveiewer is invited to compare with thesharp/daemonize@master/daemonize.py.

See a diff

--- daemonize.py	2020-08-03 10:19:48.000000000 -0400
+++ synapse/util/daemonize.py	2020-08-03 10:19:39.000000000 -0400
@@ -1,251 +1,120 @@
-# #!/usr/bin/python
+# -*- coding: utf-8 -*-
+# Copyright (c) 2012, 2013, 2014 Ilya Otyutskiy <[email protected]>
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
 
+import atexit
 import fcntl
+import logging
 import os
-import pwd
-import grp
-import sys
 import signal
-import resource
-import logging
-import atexit
-from logging import handlers
-import traceback
-
+import sys
 
-__version__ = "2.5.0"
 
+def daemonize_process(pid_file: str, logger: logging.Logger, chdir: str = "/") -> None:
+    """daemonize the current process
 
-class Daemonize(object):
+    This calls fork(), and has the main process exit. When it returns we will be
+    running in the child process.
     """
-    Daemonize object.
 
-    Object constructor expects three arguments.
+    # If pidfile already exists, we should read pid from there; to overwrite it, if
+    # locking will fail, because locking attempt somehow purges the file contents.
+    if os.path.isfile(pid_file):
+        with open(pid_file, "r") as pid_fh:
+            old_pid = pid_fh.read()
+
+    # Create a lockfile so that only one instance of this daemon is running at any time.
+    try:
+        lock_fh = open(pid_file, "w")
+    except IOError:
+        print("Unable to create the pidfile.")
+        sys.exit(1)
+
+    try:
+        # Try to get an exclusive lock on the file. This will fail if another process
+        # has the file locked.
+        fcntl.flock(lock_fh, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except IOError:
+        print("Unable to lock on the pidfile.")
+        # We need to overwrite the pidfile if we got here.
+        #
+        # XXX better to avoid overwriting it, surely. this looks racey as the pid file
+        # could be created between us trying to read it and us trying to lock it.
+        with open(pid_file, "w") as pid_fh:
+            pid_fh.write(old_pid)
+        sys.exit(1)
 
-    :param app: contains the application name which will be sent to syslog.
-    :param pid: path to the pidfile.
-    :param action: your custom function which will be executed after daemonization.
-    :param keep_fds: optional list of fds which should not be closed.
-    :param auto_close_fds: optional parameter to not close opened fds.
-    :param privileged_action: action that will be executed before drop privileges if user or
-                              group parameter is provided.
-                              If you want to transfer anything from privileged_action to action, such as
-                              opened privileged file descriptor, you should return it from
-                              privileged_action function and catch it inside action function.
-    :param user: drop privileges to this user if provided.
-    :param group: drop privileges to this group if provided.
-    :param verbose: send debug messages to logger if provided.
-    :param logger: use this logger object instead of creating new one, if provided.
-    :param foreground: stay in foreground; do not fork (for debugging)
-    :param chdir: change working directory if provided or /
-    """
-    def __init__(self, app, pid, action,
-                 keep_fds=None, auto_close_fds=True, privileged_action=None,
-                 user=None, group=None, verbose=False, logger=None,
-                 foreground=False, chdir="/"):
-        self.app = app
-        self.pid = os.path.abspath(pid)
-        self.action = action
-        self.keep_fds = keep_fds or []
-        self.privileged_action = privileged_action or (lambda: ())
-        self.user = user
-        self.group = group
-        self.logger = logger
-        self.verbose = verbose
-        self.auto_close_fds = auto_close_fds
-        self.foreground = foreground
-        self.chdir = chdir
-
-    def sigterm(self, signum, frame):
-        """
-        These actions will be done after SIGTERM.
-        """
-        self.logger.warning("Caught signal %s. Stopping daemon." % signum)
+    # Fork, creating a new process for the child.
+    process_id = os.fork()
+
+    if process_id != 0:
+        # parent process
         sys.exit(0)
 
-    def exit(self):
-        """
-        Cleanup pid file at exit.
-        """
-        self.logger.warning("Stopping daemon.")
-        os.remove(self.pid)
+    # This is the child process. Continue.
+
+    # Stop listening for signals that the parent process receives.
+    # This is done by getting a new process id.
+    # setpgrp() is an alternative to setsid().
+    # setsid puts the process in a new parent group and detaches its controlling
+    # terminal.
+
+    os.setsid()
+
+    # point stdin, stdout, stderr at /dev/null
+    devnull = "/dev/null"
+    if hasattr(os, "devnull"):
+        # Python has set os.devnull on this system, use it instead as it might be
+        # different than /dev/null.
+        devnull = os.devnull
+
+    devnull_fd = os.open(devnull, os.O_RDWR)
+    os.dup2(devnull_fd, 0)
+    os.dup2(devnull_fd, 1)
+    os.dup2(devnull_fd, 2)
+    os.close(devnull_fd)
+
+    # Set umask to default to safe file permissions when running as a root daemon. 027
+    # is an octal number which we are typing as 0o27 for Python3 compatibility.
+    os.umask(0o27)
+
+    # Change to a known directory. If this isn't done, starting a daemon in a
+    # subdirectory that needs to be deleted results in "directory busy" errors.
+    os.chdir(chdir)
+
+    try:
+        lock_fh.write("%s" % (os.getpid()))
+        lock_fh.flush()
+    except IOError:
+        logger.error("Unable to write pid to the pidfile.")
+        print("Unable to write pid to the pidfile.")
+        sys.exit(1)
+
+    # write a log line on SIGTERM.
+    def sigterm(signum, frame):
+        logger.warning("Caught signal %s. Stopping daemon." % signum)
         sys.exit(0)
 
-    def start(self):
-        """
-        Start daemonization process.
-        """
-        # If pidfile already exists, we should read pid from there; to overwrite it, if locking
-        # will fail, because locking attempt somehow purges the file contents.
-        if os.path.isfile(self.pid):
-            with open(self.pid, "r") as old_pidfile:
-                old_pid = old_pidfile.read()
-        # Create a lockfile so that only one instance of this daemon is running at any time.
-        try:
-            lockfile = open(self.pid, "w")
-        except IOError:
-            print("Unable to create the pidfile.")
-            sys.exit(1)
-        try:
-            # Try to get an exclusive lock on the file. This will fail if another process has the file
-            # locked.
-            fcntl.flock(lockfile, fcntl.LOCK_EX | fcntl.LOCK_NB)
-        except IOError:
-            print("Unable to lock on the pidfile.")
-            # We need to overwrite the pidfile if we got here.
-            with open(self.pid, "w") as pidfile:
-                pidfile.write(old_pid)
-            sys.exit(1)
-
-        # skip fork if foreground is specified
-        if not self.foreground:
-            # Fork, creating a new process for the child.
-            try:
-                process_id = os.fork()
-            except OSError as e:
-                self.logger.error("Unable to fork, errno: {0}".format(e.errno))
-                sys.exit(1)
-            if process_id != 0:
-                if self.keep_fds:
-                    # This is the parent process. Exit without cleanup,
-                    # see https://github.com/thesharp/daemonize/issues/46
-                    os._exit(0)
-                else:
-                    sys.exit(0)
-            # This is the child process. Continue.
-
-            # Stop listening for signals that the parent process receives.
-            # This is done by getting a new process id.
-            # setpgrp() is an alternative to setsid().
-            # setsid puts the process in a new parent group and detaches its controlling terminal.
-            process_id = os.setsid()
-            if process_id == -1:
-                # Uh oh, there was a problem.
-                sys.exit(1)
-
-            # Add lockfile to self.keep_fds.
-            self.keep_fds.append(lockfile.fileno())
-
-            # Close all file descriptors, except the ones mentioned in self.keep_fds.
-            devnull = "/dev/null"
-            if hasattr(os, "devnull"):
-                # Python has set os.devnull on this system, use it instead as it might be different
-                # than /dev/null.
-                devnull = os.devnull
-
-            if self.auto_close_fds:
-                for fd in range(3, resource.getrlimit(resource.RLIMIT_NOFILE)[0]):
-                    if fd not in self.keep_fds:
-                        try:
-                            os.close(fd)
-                        except OSError:
-                            pass
-
-            devnull_fd = os.open(devnull, os.O_RDWR)
-            os.dup2(devnull_fd, 0)
-            os.dup2(devnull_fd, 1)
-            os.dup2(devnull_fd, 2)
-            os.close(devnull_fd)
-
-        if self.logger is None:
-            # Initialize logging.
-            self.logger = logging.getLogger(self.app)
-            self.logger.setLevel(logging.DEBUG)
-            # Display log messages only on defined handlers.
-            self.logger.propagate = False
-
-            # Initialize syslog.
-            # It will correctly work on OS X, Linux and FreeBSD.
-            if sys.platform == "darwin":
-                syslog_address = "/var/run/syslog"
-            else:
-                syslog_address = "/dev/log"
-
-            # We will continue with syslog initialization only if actually have such capabilities
-            # on the machine we are running this.
-            if os.path.exists(syslog_address):
-                syslog = handlers.SysLogHandler(syslog_address)
-                if self.verbose:
-                    syslog.setLevel(logging.DEBUG)
-                else:
-                    syslog.setLevel(logging.INFO)
-                # Try to mimic to normal syslog messages.
-                formatter = logging.Formatter("%(asctime)s %(name)s: %(message)s",
-                                              "%b %e %H:%M:%S")
-                syslog.setFormatter(formatter)
-
-                self.logger.addHandler(syslog)
-
-        # Set umask to default to safe file permissions when running as a root daemon. 027 is an
-        # octal number which we are typing as 0o27 for Python3 compatibility.
-        os.umask(0o27)
-
-        # Change to a known directory. If this isn't done, starting a daemon in a subdirectory that
-        # needs to be deleted results in "directory busy" errors.
-        os.chdir(self.chdir)
-
-        # Execute privileged action
-        privileged_action_result = self.privileged_action()
-        if not privileged_action_result:
-            privileged_action_result = []
-
-        # Change owner of pid file, it's required because pid file will be removed at exit.
-        uid, gid = -1, -1
-
-        if self.group:
-            try:
-                gid = grp.getgrnam(self.group).gr_gid
-            except KeyError:
-                self.logger.error("Group {0} not found".format(self.group))
-                sys.exit(1)
-
-        if self.user:
-            try:
-                uid = pwd.getpwnam(self.user).pw_uid
-            except KeyError:
-                self.logger.error("User {0} not found.".format(self.user))
-                sys.exit(1)
-
-        if uid != -1 or gid != -1:
-            os.chown(self.pid, uid, gid)
-
-        # Change gid
-        if self.group:
-            try:
-                os.setgid(gid)
-            except OSError:
-                self.logger.error("Unable to change gid.")
-                sys.exit(1)
-
-        # Change uid
-        if self.user:
-            try:
-                uid = pwd.getpwnam(self.user).pw_uid
-            except KeyError:
-                self.logger.error("User {0} not found.".format(self.user))
-                sys.exit(1)
-            try:
-                os.setuid(uid)
-            except OSError:
-                self.logger.error("Unable to change uid.")
-                sys.exit(1)
-
-        try:
-            lockfile.write("%s" % (os.getpid()))
-            lockfile.flush()
-        except IOError:
-            self.logger.error("Unable to write pid to the pidfile.")
-            print("Unable to write pid to the pidfile.")
-            sys.exit(1)
-
-        # Set custom action on SIGTERM.
-        signal.signal(signal.SIGTERM, self.sigterm)
-        atexit.register(self.exit)
-
-        self.logger.warning("Starting daemon.")
-
-        try:
-            self.action(*privileged_action_result)
-        except Exception:
-            for line in traceback.format_exc().split("\n"):
-                self.logger.error(line)
+    signal.signal(signal.SIGTERM, sigterm)
+
+    # Cleanup pid file at exit.
+    def exit():
+        logger.warning("Stopping daemon.")
+        os.remove(pid_file)
+        sys.exit(0)
+
+    atexit.register(exit)
+
+    logger.warning("Starting daemon.")

Edit: This doesn't really seem too useful since there's a conversion from a class to a function. The non-whitespace version didn't seem to help either.

[clokep]

Seems OK. I left a couple of queries about removed error handling.

[clokep]

Curious why the error handling code here was removed from the original library?

[richvdh]

basically: I kinda felt that we should be allowing exceptions raised in this function to propagate, rather than suddenly calling sys.exit. It's not particularly clear though. I could be persuaded to go either way.

[clokep]

The result should be the same either way. Showing the stack trace should give more info, I suppose. 👍

[clokep]

Again, this differs from the original code by removing error handling.

[richvdh]

os.setsid returns None, so that was dead code.

[richvdh]

I've realised that the original code wrapped this with a try/except for a reason, which is that after stderr gets redirected to /dev/null, any uncaught exceptions will otherwise get thrown away.

I'm currently wondering how best to solve that.