Comment 0 for bug 1851675

Seen on: Ubuntu 18.04.3 LTS
dpkg version: 1.19.0.5ubuntu2.3 (also in 1.19.0.5ubuntu2.2)

Regression compared to dpkg 1.19.0.5ubuntu2.1

To reproduce this:
1. You need a package that depends on a library for running a script in its postinst hook. For instance, the attached minimal package has `Depends: python-augeas`, and tries to do “from augeas import Augeas” in a script invoked from the postinst hook. (It does nothing else.)
2. Make sure the dependent library is not yet installed. In this example: libaugeas0, which is a dependency of python-augeas.
3. apt-get install the package. For the attached exmple, you can use `dpkg -i`, followed by `apt-get -f install`.

Result:

Setting up the-package (1.2.3ubuntu1-2-1) ...
Traceback (most recent call last):
  File "/usr/lib/the-package/setup-package.py", line 3, in <module>
    from augeas import Augeas
  File "/usr/lib/python2.7/dist-packages/augeas.py", line 78, in <module>
    class Augeas(object):
  File "/usr/lib/python2.7/dist-packages/augeas.py", line 82, in Augeas
    _libaugeas = _dlopen("augeas")
  File "/usr/lib/python2.7/dist-packages/augeas.py", line 75, in _dlopen
    raise ImportError("Unable to import lib%s!" % args[0])
ImportError: Unable to import libaugeas!
dpkg: error processing package the-package (--configure):
 installed the-package package post-installation script subprocess returned error exit status 1

Simply running apt-get -f install afterwards will succeed and run the postinst script.

The cause of this problem seems to be that any invocations of `ldconfig` during the installation of the dependent libraries, do nothing at all.
When adding `ldconfig -p >/tmp/ldconfig-output` to the postinst hook, one can see that neither libaugeas0 nor any other libraries that were just installed, are in the ldconfig cache at the moment the script is invoked.
Even when adding an explicit `ldconfig` in the postinst script before running the python script, it still fails.
Very curiously, calling `ldconfig -v >/dev/null` instead, somehow makes ldconfig work. This seems to indicate that there is some problem with I/O redirection, but for some reason the `-v` is also essential.