Understand dpkg and don’t get stuck with a maintainer script failure

Continuing my series of articles on dpkg’s errors, this time I’ll cover a pretty common one which has several variations:

Setting up acpid (1:2.0.12-1) ...
rm: cannot remove `/etc/rc1.d/K20acpid': No such file or directory
dpkg: error processing acpid (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 acpid

Even if dpkg is failing and outputting the error message, the real problem is not in dpkg but in the installed package (acpid in the example above). As we already learned, a package contains not only files but also “maintainer scripts” that are executed at various points of the installation process (see some useful graphics to understand how they are called, thanks to Margarita Manterola).

Maintainer scripts in a package upgrade

In the introductory example it was acpid’s “post-installation script” that failed, and dpkg is only forwarding that failure back to the caller. The maintainer scripts are stored in /var/lib/dpkg/info/. You can thus inspect them and even modify them if you hit a bug and want to work around it (do this only if you understand what you do!).

One common modification is to add “set -x” at the start of the script and to retry the failing operation. That way you can see what’s executed exactly. Here’s what the output could look like after the addition of “set -x” to /var/lib/dpkg/info/acpid.postinst:

$ sudo dpkg --configure acpid
Setting up acpid (1:2.0.12-1) ...
+ dpkg --compare-versions 1:2.0.11-1 lt-nl 1.0.10-3
+ dpkg --compare-versions 1:2.0.11-1 lt-nl 1.0.6-16
+ dpkg --compare-versions 1:2.0.11-1 lt 1.0.6-6
+ rm /etc/rc1.d/K20acpid
rm: cannot remove `/etc/rc1.d/K20acpid': No such file or directory
dpkg: error processing acpid (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 acpid

This output helps you locate the command that is actually failing. Here’s it’s relatively easy since we have an error message from “rm”. And the fix is trivial too, we replace “rm” with “rm -f” so that it doesn’t fail when the file doesn’t exist (this is a fake bug I made up for this article—I just added a failing rm call—but it’s inspired by real bugs I experienced).

Maintainer scripts are supposed to be idempotent: we should be able to execute them several times in a row without bad consequences. It happens from time to time that the maintainer gets this wrong… on the first try it works, so he uploads his package and we discover the problem only later once someone ended up executing the same code twice for some reason.

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Understanding dpkg’s file overwrite error

This is probably one of the most common errors. You’re very likely to encounter it, in particular if you tend to mix packages from various origins/distributions, or if you’re using unstable. It looks like this:

Unpacking gbonds-data (from .../gbonds-data_2.0.3-2_all.deb) ...
dpkg: error processing /var/cache/apt/archives/gbonds-data_2.0.3-2_all.deb (--unpack):
 trying to overwrite '/usr/share/omf/gbonds/gbonds-C.omf', which is also in package gbonds 2.0.2-9
dpkg-deb: subprocess paste killed by signal (Broken pipe)

A given file can only be provided by a single package. So if you try to install a package that provides a file that is already part of another installed package, it will fail with a message similar to the above one.

Sometimes this failure will be meaningful because dpkg prevented you to install two unrelated packages that happen to have a real file conflict. In other cases, like in the example above, this failure is just the result of a mistake.

Folder with gears

The version 2.0.3-1 of gbonds split the architecture independent files in a separate package called gbonds-data but the maintainer forgot to add the required control field in gbonds-data (Replaces: gbonds (<< 2.0.3-1)). That field allows dpkg to take over files from the listed packages.

If you want to ignore the file conflict and let dpkg take over the file (even without the Replaces), you can pass the --force-overwrite command-line option.

But you’re not using dpkg directly, you’re probably using an APT frontend (like apt-get or aptitude). Don’t worry, there’s a simple way to define custom dpkg options to use:

# apt-get -o Dpkg::Options::="--force-overwrite" install gbonds-data

The syntax is a bit weird, but the “::” after “Options” is important, it’s the syntax that defines a list item value instead of a single value. And you can effectively pass multiple options to dpkg by putting multiple -o Dpkg::Options::="…".

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Deciphering one of dpkg’s weirdest errors: unable to open ‘/path/to/foo.dpkg-new’

We already studied one weird error of dpkg, let’s do another one:

Unpacking replacement libexo-common ...
dpkg: error processing /var/cache/apt/archives/libexo-common_0.6.1-1_all.deb (--unpack):
 unable to open '/usr/share/doc/exo/html/C/images/exo-preferred-applications-internet.png.dpkg-new': No such file or directory

Rather difficult to understand on the first look right? Let’s see in detail what’s usually happening when you get this error.

The first hint comes from the file extension “.dpkg-new”. This extension is used by dpkg to unpack the updated files near the old files. When everything has been unpacked, they are renamed over the old files.

The failure happens precisely when dpkg tries to rename the file (in fact when it tries to fsync() it before the rename)… but why does it fail?

Usually because there are unexpected symlinks (or bind mounts) involved that resulted in two different files being installed in the same directory. For example consider package that provides /dir1/a-file and /dir2/a-file. Now imagine that on the target system, /dir1 is a real directory but /dir2 is a symbolic link that points to /dir1.

When dpkg processes /dir1/a-file.dpkg-new everything is fine, but when it tries to process /dir2/a-file.dpkg-new it will fail because that file is the same than /dir1/a-file.dpkg-new which has already been renamed.

Diagnosing further the problem requires to understand why there’s a symlink instead of a real directory. It might be two packages that were badly coordinated, or a problem in the package itself because it lacks some code that drops the symlink in the preinst (so that dpkg installs the real directory instead of keeping the symlink).

There might be variations in the way two files end up sharing the same directory, but this simple example should have clarified the nature of the underlying problem.

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Deciphering one of dpkg’s weirdest errors: short read on buffer copy

As a Debian/Ubuntu user, you’re likely to be exposed at some point to an error reported by dpkg. In a series of articles, I’ll explain some of the errors that you might encounter.

Some error messages can be confusing at times. Most of the error strings do not appear very often and developers thus tend to use very terse description of the underlying problem. In other cases the architecture of the software makes it difficult to pin-point the real problem because the part that displays the error is several layers above the one that generated the initial error.

This is for example the case with this error of dpkg:

Unpacking replacement xulrunner-1.9.2 ...
dpkg-deb (subprocess): data: internal gzip read error: '<fd:0>: too many length or distance symbols'
dpkg-deb: error: subprocess <decompress> returned error exit status 2
dpkg: error processing /var/cache/apt/archives/xulrunner-1.9.2_1.9.2.17+build3+nobinonly-0ubuntu1_amd64.deb (--unpack):
 short read on buffer copy for backend dpkg-deb during `./usr/lib/xulrunner-1.9.2.17/components/libdbusservice.so'

First, the decompression layer discovers something unexpected in the data read in the .deb file and dpkg-deb outputs the error message coming from zlib (“too many length or distance symbols”). This causes the premature end of dpkg-deb --fsys-tarfile that dpkg had executed to extract the .data.tar archive from the deb file. In turn, dpkg informs us that dpkg-deb did not send all the data that were announced (and hence the “short read” in the error message) and that were meant to be part of the file ‘/usr/lib/xulrunner-1.9.2.17/components/libdbusservice.so’.

That’s all nice but it doesn’t help you much in general. What you must understand from the above is that the .deb file is corrupted (sometimes just truncated). In theory it should not happen since APT verifies the checksums of files when they are downloaded. But computers are not infallible and even if the downloaded data was good, it can have been corrupted when stored on disk (for example cheap SSD disks are known to not last very well).

Try removing the file (usually with apt-get clean since it’s stored in APT’s cache) and let APT download it again. Chances are that it will work on the second try. Otherwise consider doing a memory and HDD check as something is probably broken in your computer.

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