hybrid-detect does not correctly detect if an alternative for the respective driver cannot be found

[linuxball]

$ lsb_release -rd
Description: Ubuntu 12.04.1 LTS
Release: 12.04

$ apt-cache policy nvidia-common
nvidia-common:
  Installed: 1:0.2.44.2
  Candidate: 1:0.2.44.2
  Version table:
 *** 1:0.2.44.2 0
        500 http://de.archive.ubuntu.com/ubuntu/ precise-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1:0.2.44 0
        500 http://de.archive.ubuntu.com/ubuntu/ precise/main i386 Packages

Hardware:
  LENOVO ThinkPad T520 (Type: 4240-6AG) with Nvidia Optimus technology
  BIOS Setup -> Config -> Display -> OS Detection for NVIDIA Optimus: [Disabled]

Expected behavior:

  If "BIOS Setup -> Config -> Display -> Graphics Device" was changed from "Integrated Graphics" [Intel Sandybridge Mobile (GT2+)] to "Discrete Graphics" (NVIDIA NVd9) then hybrid-detect should look for the master link of an alternative for the "i386-linux-gnu_gl_conf"/"x86_64-linux-gnu_gl_conf" nvidia driver and set the respective link using "update-alternatives". If an alternative cannot be found (e.g. because no nvidia driver is installed and the nouveau driver is used instead) then hybrid-detect should not call "update-alternatives --set <arch-prefix>_gl_conf" without path argument and write an error message to stderr:

  Gfx was changed in the BIOS
  Error: no alternative found

Actual behavior for i386 package architecture:

  In the above reported case hybrid-detect calls "update-alternatives --set <arch-prefix>_gl_conf" with the path argument missing. The respective log output in /var/log/upstart/hybrid-gfx.log is:

  Gfx was changed in the BIOS
  Select
  update-alternatives: --set needs <name> <path>

  Usage: update-alternatives [<option> ...] <command>

  [ rest of usage output removed ]

Fix:

  Because hybrid-detect does not check for an empty string in line 207 (only for a NULL pointer) after calling get_alternative_link() it does not detect if an alternative was not found. The attached patch, which will add the empty string detection will fix this problem.

[linuxball (linuxball)]

This (corrected) patch fixes the reported problem.

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "Fix for the reported problem" of this bug report has been identified as being a patch. The ubuntu-reviewers team has been subscribed to the bug report so that they can review the patch. In the event that this is in fact not a patch you can resolve this situation by removing the tag 'patch' from the bug report and editing the attachment so that it is not flagged as a patch. Additionally, if you are member of the ubuntu-reviewers team please also unsubscribe the team from this bug report.

[This is an automated message performed by a Launchpad user owned by Brian Murray. Please contact him regarding any issues with the action taken in this bug report.]