dh_virtualenv and long package names (FileNotFound error)

Interesting tidbit about Linux:

A maximum line length of 127 characters is allowed for the first line in a #! executable shell script.

from man execve

Should be enough, right?

Wrong.

Well, not if you are using dh_virtualenv with long package names, anyway:

Installing pip...
  Error [Errno 2] No such file or directory while executing command /tmp/lo...me/bin/easy_install /usr/share/python-virtualenv/pip-1.1.tar.gz
...Installing pip...done.
Traceback (most recent call last):
  File "/usr/bin/virtualenv", line 3, in <module>
virtualenv.main()
  File "/usr/lib/python2.7/dist-packages/virtualenv.py", line 938, in main
never_download=options.never_download)
  File "/usr/lib/python2.7/dist-packages/virtualenv.py", line 1054, in create_environment
install_pip(py_executable, search_dirs=search_dirs, never_download=never_download)
  File "/usr/lib/python2.7/dist-packages/virtualenv.py", line 643, in install_pip
filter_stdout=_filter_setup)
  File "/usr/lib/python2.7/dist-packages/virtualenv.py", line 976, in call_subprocess
cwd=cwd, env=env)
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1249, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory
Traceback (most recent call last):
  File "/usr/bin/dh_virtualenv", line 106, in <module>
sys.exit(main() or 0)
  File "/usr/bin/dh_virtualenv", line 83, in main
deploy.create_virtualenv()
  File "/usr/lib/python2.7/dist-packages/dh_virtualenv/deployment.py", line 112, in create_virtualenv
subprocess.check_call(virtualenv)
  File "/usr/lib/python2.7/subprocess.py", line 511, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['virtualenv', '--system-site-packages', '--setuptools', 'debian/long-package-name/usr/share/python/long-package-name']' returned non-zero exit status 1
make: *** [binary-arch] Error 1
dpkg-buildpackage: error: fakeroot debian/rules binary gave error exit status 2

dh_virtualenv is used to create Debian packages that include Python virtualenvs. It is one of the better ways of packaging Python software, especially if there are Python dependencies that are not available in Debian or Ubuntu. When building a .deb package, it creates a virtualenv in a location such as:

/<build-directory>/debian/<packagename>/usr/share/python/<packagename>

This virtualenv has several tools under its bin/ directory, and they all have the absolute path of the virtualenv's Python interpreter hard-coded in their #! shebang line:

#!/<build-directory>/debian/<packagename>/usr/share/python/<packagename>/bin/python

Given that <build-directory> often contains the package name as well, it's easy to overflow the 128 byte limit of the #! shebang line. In my case, with a ~30 character package name, the path length grew to 160 characters!

Consequently, the kernel couldn't find the Python executable anymore, and running any of the tools from the bin/ directory gave an ENOENT (file not found) error. This is what happened when virtualenv tried to install pip during the initial setup. The root cause of this error is not immediately obvious, to say the least.

To check whether this affects you, check the line length of any script with wc:

head -n 1 /path/to/virtualenv/bin/easy_install | wc -c

If that's larger than 128, it's probably the cause of the problem.

The fix is to change the package name and/or the build location to something shorter. The alternative would be to patch the Linux kernel, which – depending on your preferences – sounds either fun or really unpleasant. Suit yourself!