Autoconf and defaults

Autoconf is based on a set of macros. these macros are put by developers into the configure.ac file, and the "autoconf" program generates a proper configure script from that. The configure script commonly accepts options such as --enable-foo and --with-foo. It is important that they behave in a sensible way.

The --with-foo argument is processed with the AC_ARG_WITH macro. It is documented as follows:

AC_ARG_WITH (package, help-string, [action-if-given], [action-if-not-given])

If the user gave configure the option --with-package or --without-package, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name package indicates another software package that this program should work with. It should consist only of alphanumeric characters, dashes, plus signs, and dots.

The option's argument is available to the shell commands action-if-given in the shell variable withval, which is actually just the value of the shell variable named with_package, with any non-alphanumeric characters in package changed into ‘_’. You may use that variable instead, if you wish.

 Let's see what's wrong with this piece of configure.ac, found in lxdm-0.4.1:

AC_ARG_WITH(pam,AC_HELP_STRING([--without-pam],[build without pam]),
[],[AC_CHECK_LIB([pam], [pam_open_session])])

This does nothing if the --without-pam option is not given, and checks for the PAM library if the option is given. Looks right? No!

This also does nothing if the --with-pam option is given, thus, resulting in a build without PAM support!

Note that the documentation for AC_ARG_WITH even provides examples how to use the macro properly. Use them as a reference, and have the following checklist:

• Use the $withval or $with_package variable.
• Test it for values such as "yes", "no", "auto" and explicit path.
• Check the presence of the package unless it is explicitly disabled or the support is experimental.
• Fail if the external package is requested but not available.

Yes, this becomes verbose, but it is necessary for correctness.

The first point also applies to the AC_ARG_ENABLE macro:

AC_ARG_ENABLE(debug,         AS_HELP_STRING([--enable-debug],
                                  [Enable debugging (default: disabled)]),
                                [DEBUGGING=$enableval], [DEBUGGING=no])

I wrote a DTS encoder

Let me announce a piece of software that I have published a week ago: dcaenc, an open-source DTS encoder. The package contains sources for a shared library, a command-line tool and an ALSA plugin.

DTS is one of the compressed formats that allow transfer of multichannel (e.g., 5.1) audio over SPDIF connections. The other common format is AC3. The SPDIF standard does not define a method for passing more than two channels of uncompressed PCM audio, so compression has to be used. Both AC3 and DTS are also used in DVD sound tracks.

Open-source decoders for both AC3 and DTS already exist: liba52 and libdca (side note: please don't use libdca, it is a security risk, there are some files that crash it or are decoded improperly). FFmpeg can also decode these formats. However, useful open-source encoders existed only for AC3: one in FFmpeg, and the other one (aften) based on it. The DTS "encoder" in FFmpeg was ported by someone else from my old proof-of-concept code that served as a tool to understand the DTS subband transform. It could only encode stereo PCM files into a valid DTS bitstream of the same bitrate, which is useless for any practical purpose. Now dcaenc provides a useful encoder that accepts multichannel sound and encodes it to the bitrate specified by the command line parameter.

As already mentioned, there are the following use cases for my encoder:

• On-the-fly encoding of multichannel PCM audio produced by arbitrary ALSA applications (e.g. games) for transmission via SPDIF
• Creation of DVD soundtracks and DTS CDs.

Some people ask me why I didn't integrate my encoder into FFmpeg instead of releasing it as a standalone package. Indeed, there are faster implementations of the basic DSP building blocks in FFmpeg, and the criticism that I reinvented a lot of wheels is valid. Integration with FFmpeg is indeed a desired long-term goal.

There are still several reasons why I decided not to integrate right from the beginning. First, I don't think that my work is in the necessary shape for integration yet. E.g., in FFmpeg, floating-point codecs are preferred, while my library currently uses fixed-point (I thought it would be beneficial for porting to the ARM architecture). Second and the most important reason: when the encoder is standalone, users can get it immediately and use it, without the hassle of replacing the distribution-provided FFmpeg package and potentially breaking distribution-provided software such as VLC that depends on it. Third, if I know that I wrote all the code myself, I can sell LGPL exceptions.

While dcaenc already produces "transparent" output at 1411 or 1536 kilobits per second, there is still room for quality improvement at lower bitrates. This is because the library does not yet use all compression possibilities offered by the DTS standard. I am going to implement at least linear prediction (incorrectly called ADPCM in the specification) and channel coupling in the future versions. Stay tuned!

I am going to Desktop Summit 2011

I am going to Desktop Summit. Actually, I have already arrived in Berlin.

At the summit, I will mostly follow the Platform track (Rm2002) and then try to participate in BoFs.

The case of a non-raised exception

As you probably know, python uses exceptions for error handling. It is considered a good style to avoid adding error-handling code in the form of conditional statements. Instead, one should rely on the fact that an appropriate exception is raised once an error condition is detected, and caught when it can be dealt with.

So, let'a assume that you are given a task of downloading a file given the URL and the file name on disk, using Python. You may want to write the following code and hope that you don't have to add any error-handling because (as you think) all errors that can happen are either network errors or file write errors, and those two types of errors already raise exceptions for you.

#!/usr/bin/python

import urllib2
import sys
import socket

def download(url, fname):
    net = urllib2.urlopen(url)
    f = open(fname, "wb")
    
    while True:
        data = net.read(4096)
        if not data:
            break
        f.write(data)
    
    net.close()
    f.close()

if __name__ == "__main__":
    if len(sys.argv) != 3:
        print "Usage: download.py URL filename"
    
    url = sys.argv[1]
    fname = sys.argv[2]
    
    socket.setdefaulttimeout(30)
    
    download(url, fname)

Indeed, this code downloads existing files via HTTP just fine. Also, it provides sensible tracebacks for non-existing hosts, 404 errors, full-disk situations, and socket timeouts. So, it looks like the result of calling the download() fnction is either a successfully downloaded file, or an exception that the other part of the application will likely be able to deal with.

But actually, it only looks like this. Consider a situation when the HTTP server closes the connection gracefully at the TCP level, but prematurely. You can test this by starting your own Apache web server, putting a large file there, and calling "apache2ctl restart" while the client is downloading the file. Result: an incompletely downloaded file, and no exceptions.

I don't know if it should be considered a bug in urllib2 or in the example download() function above. In fact, urllib2 could have noticed the mismatch of the total number of bytes before the EOF and the value in the Content-Length HTTP header.

Here is a version of the download() function that detects incomplete downloads based on the Content-Length header:

def download(url, fname):
    net = urllib2.urlopen(url)
    contentlen = net.info().get("Content-Length", "")
    f = open(fname, "wb")
    datalen = 0
    
    while True:
        data = net.read(4096)
        if not data:
            break
        f.write(data)
 datalen += len(data)
    
    net.close()
    f.close()

    try:
        contentlen = int(contentlen)
    except ValueError:
        contentlen = None

    if contentlen is not None and contentlen != datalen:
        raise urllib2.URLError("Incomplete download")

Writing systemd service files

Today I tried to convert a Gentoo initscript for VDE to a systemd service file. This post documents the required steps. VDE has been chosen as an example because it is a simple daemon (commonly used for communication between several instances of QEMU and the host) that illustrates the matter well.

Background

For those who don't know, systemd is a next-generation replacement for /sbin/init written by Lennart Poettering. While systemd supports traditional initscripts, service files are its native form of configuration. Service files have a structure similar to that of desktop files or Windows ini files. Their syntax is well documented in the manual pages.

Initscripts typically contain boilerplate code that checks whether the daemon is already running or figures out which process to kill when the daemon has to be stopped. Tools like start-stop-daemon help, but systemd reduces the syntax overhead to the minimum. This simplification occurs because service files specify what should be done, not how it should be done. I.e., unlike initscripts, they follow the declarative style and are not programs.

Simple services

Here is the simplest possible service file that starts VDE with options that work for me on my computer. Save it as /etc/systemd/system/vde.service:

[Unit]
Description=Virtual Distributed Ethernet

[Service]
ExecStart=/usr/bin/vde_switch --tap tap0 --mode 0660 \
 --dirmode 0750 --group qemu

[Install]
WantedBy=multi-user.target

Note the difference from a traditional init script: for simplicity, vde_switch is started in such a way that it doesn't become a daemon. In fact, it is possible to start real daemons that fork, you just have to tell systemd about that:

[Unit]
Description=Virtual Distributed Ethernet

[Service]
Type=forking
# The PID file is optional, but recommended in the manpage
# "so that systemd can identify the main process of the daemon"
PIDFile=/var/run/vde.pid
ExecStart=/usr/bin/vde_switch --tap tap0 --mode 0660 \
 --dirmode 0750 --group qemu \
 --daemon --pidfile /var/run/vde.pid

[Install]
WantedBy=multi-user.target

The difference is in the way the dependencies are handled. If some other services depend on vde.service, then, in the first example, systemd will be able to run them as soon as it starts vde_switch. In the second example, systemd will wait until vde_switch forks. The difference matters, because vde_switch creates its control socket after starting, but before forking. So, in the first example, there is some chance that systemd will start something that tries to connect to the socket before vde_switch creates it.

Automatic restarts

Let's also add the proper dependency on the system logger and tell systemd to restart vde_switch if it crashes due to an uncaught signal (although it never happened to me):

[Unit]
Description=Virtual Distributed Ethernet
After=syslog.target

[Service]
Type=forking
PIDFile=/var/run/vde.pid
ExecStart=/usr/bin/vde_switch --tap tap0 --mode 0660 \
 --dirmode 0750 --group qemu \
 --daemon --pidfile /var/run/vde.pid
Restart=on-abort

[Install]
WantedBy=multi-user.target

Now try to start and crash the daemon:

home ~ # systemctl start vde.service
home ~ # systemctl status vde.service
vde.service - Virtual Distributed Ethernet
   Loaded: loaded (/etc/systemd/system/vde.service)
   Active: active (running) since Tue, 04 Jan 2011 22:08:10 +0500;
15s ago
  Process: 31434 (/usr/bin/vde_switch --tap tap0...,
code=exited, status=0/SUCCESS)
 Main PID: 31435 (vde_switch)
   CGroup: name=systemd:/system/vde.service
    └ 31435 /usr/bin/vde_switch --tap tap0...
home ~ # kill -SEGV 31435
home ~ # systemctl status vde.service
vde.service - Virtual Distributed Ethernet
   Loaded: loaded (/etc/systemd/system/vde.service)
   Active: failed since Tue, 04 Jan 2011 22:11:27 +0500;
4s ago
  Process: 31503 (/usr/bin/vde_switch --tap tap0...,
code=exited, status=0/SUCCESS)
 Main PID: 31504 (code=exited, status=1/FAILURE)
   CGroup: name=systemd:/system/vde.service

I.e., restarting didn't work. The system log tells us why:

Jan  4 22:11:27 home vde_switch[31504]: Error in pidfile
creation: File exists

So, VDE has a bug in its pidfile creation. There are two ways how one can deal with this: either tell systemd to remove the PID file before starting vde_switch, or drop the PID file altogether (because vde_switch has exactly one process, there can be no confusion which process is the main one). Both ways work. Here is how to implement the first alternative:

[Unit]
Description=Virtual Distributed Ethernet
After=syslog.target

[Service]
Type=forking
PIDFile=/var/run/vde.pid
# Note the -f: don't fail if there is no PID file
ExecStartPre=/bin/rm -f /var/run/vde.pid
ExecStart=/usr/bin/vde_switch --tap tap0 --mode 0660 \
 --dirmode 0750 --group qemu \
 --daemon --pidfile /var/run/vde.pid
Restart=on-abort

[Install]
WantedBy=multi-user.target

And here is the second alternative:

[Unit]
Description=Virtual Distributed Ethernet
After=syslog.target

[Service]
Type=forking
ExecStart=/usr/bin/vde_switch --tap tap0 --mode 0660 \
 --dirmode 0750 --group qemu \
 --daemon
Restart=on-abort

[Install]
WantedBy=multi-user.target

Configuration

Many initscripts come with configuration files that allow the user to customize how the daemon is started. For example, some users may want to pass the --hub argument to vde_switch, and others may want to enable the management socket. So, in Gentoo, the traditional configuration file for the initscript looks like this:

# load the tun module
VDE_MODPROBE_TUN="yes"
# virtual tap networking device to be used for vde
VDE_TAP="tap0"
# mode and group for the socket
VDE_SOCK_CHMOD="770"
VDE_SOCK_CHOWN=":qemu"

# This is the actual options string passed to VDE.
# Change this at your own risk.
VDE_OPTS="--tap ${VDE_TAP} -daemon"

Systemd service files typically use the EnvironmentFile key to provide users with a file where they can put their preferences regarding the service.

Traditional initscripts source their configuration files. Thus, any syntax construction supported by /bin/sh will work in the configuration file. In the example above, we see comments, assignment of values to variables, and reusing the values in later assignments. Systemd uses a different syntax from bash, so please resist the temptation to reuse the same configuration file for the traditional initscript and the service file. Resist even though some service files in the Gentoo systemd overlay do use the same configuration files as the corresponding traditional initscripts -- they are just buggy. Let me explain this in more detail.

Of course, the configuration file example above is not suitable for systemd because variable interpolation is not supported in systemd environment files. But let's suppose that we want to invent something that is suitable both as a bash script fragment and a systemd environment file, and still allows the user to configure vde_switch according to his wishes.

Let's focus on the VDE_OPTS variable only, as it is the only thing that matters. Indeed, module loading can be done directly by systemd (man modules-load.d), and the options related to the socket group and octal permissions can be expressed using vde_switch command line options, as illustrated in the examples above. Since the value of the VDE_OPTS variable can contain spaces, we have to quote it if we want bash to be able to understand what we mean:

VDE_OPTS="--tap tap0 --mode 0660 --dirmode 0750 --group qemu"

Without quotes, bash would interpret this as follows: with the variable VDE_OPTS that has value "--tap" in the environment, start the "tap0" process and pass 6 parameters to it. So, quotes are essential here.

Let's try to use this variable from the service file. We want it to be split by the spaces, so that each part becomes a separate vde_switch parameter. So, according to the systemd.service manpage, we have to use the $VDE_OPTS form, not ${VDE_OPTS}. So here is what we have:

[Unit]
Description=Virtual Distributed Ethernet
After=syslog.target

[Service]
Type=forking
EnvironmentFile=/etc/conf.d/vde2
ExecStart=/usr/bin/vde_switch --daemon $VDE_OPTS
Restart=on-abort

[Install]
WantedBy=multi-user.target

Result: no "tap0" interface and wrong permissions on the control socket. This happened because quotes play a special role in systemd environment files, different from their role in bash scripts. For systemd, they mean that the spaces inside them should not be treated as argument separators. So, all the arguments in $VDE_OPTS were passed to vde_switch as one long argument. No surprise that it didn't work.

In fact, the service file is correct. Its configuration just can't be made compatible with bash, because it is in a different language. The service works with the following configuration file (alas, incompatible with bash):

VDE_OPTS=--tap tap0 --mode 0660 --dirmode 0750 --group qemu

Conclusion

Let's hope that all of the above will get you started writing your own systemd service files. Since there are many initscripts in Gentoo still not converted, the project needs your help.

Thanks

The following people from #systemd IRC channel on freenode provided me with valuable support: MK_FG, zdzichuBG, miti1.

Minimizing server downtime

Suppose that you are responsible for a typical LAMP server running Debian. The goal is to have the needed security updates and minimize downtime. Unfortunately, due to installation of security updates, services need to be restarted, and sometimes (after kernel updates) even a reboot is required. Here are some tips that will help you shrink the time of service unavailability.

1. Upgrade your key services (apache, php and mysql) separately from the rest of software. Reason: if you just run "apt-get dist-upgrade" to upgrade everything, then your MySQL server will be stopped at the beginning of the whole upgrade and started only at the end, after unpacking all other packages. So, run "apt-get install mysql-server" before "apt-get dist-upgrade" to minimize the delay.
2. If you know that kexec works on your hardware, use it: apt-get install kexec-tools. This way, you will avoid the long time (sometimes more than a minute) while your server shows useless BIOS screens. Instead of rebooting to the BIOS, the old kernel will load the new kernel directly and pass control to it, thus avoiding the delay.

DNS servers in resolv.conf

Yesterday I found a problem with my Linode VPS. When running "aptitude update", it complained about not being able to resolve the name for security.debian.org (a server from where Debian distributes security updates).

Here is the /etc/resolv.conf file from that VPS, generated by dhclient:

domain members.linode.com
search members.linode.com
nameserver 109.74.194.20
nameserver 109.74.192.20
nameserver 109.74.193.20

It looks like the 109.74.194.20 server is overloaded and returns a SERVFAIL ("server failure - The name server was unable to process this query due to a problem with the name server") answer from time to time. Unfortunately, Debian has a bug in its version of libc that prevents the resolver from trying the other (working) servers if the first one returns an answer indicating a temporary failure.

The bug does not exist in glibc-2.12.1 on my Gentoo box at home. However, if the server does not respond at all, the Gentoo resolver spends 15 seconds before trying the next one, which is also not nice.

To work around the bug, you can run your own DNS server on 127.0.0.1 that forwards all queries to the official servers, and put 127.0.0.1 as the only nameserver in /etc/resolv.conf.