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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Comments

  1. Paul Grandperrin says:

    Hi, I’m having another weird error message with aptitude which is not easily understandable…

    “Internal error: found 2 (choice -> promotion) mappings for a single choice.”

    Aptitude is telling me this error dozens or even hundreds times while searching for solutions.

    Do you know what this means?

    And BTW, thanks for all your work.

  2. Benjamin says:

    It would be nice if the dpkg error message would be improved. It should tell me the possible causes for it and give me a hint how to deal with it. dpkg should print what you wrote, but in short.

  3. Thanks very much! This article helped me clear up an upgrade problem I was having due to the mysterious “short read on buffer copy for backend dpkg-deb” message.

Trackbacks

  1. […] a maintainer script failure September 13, 2011 By Raphaël Hertzog Leave a Comment Continuing my series of articles on dpkg’s errors, this time I’ll cover a pretty common one which has […]