[Bug 1931178] [NEW] apt-key fails when POSIXLY_CORRECT environmental variable is set

[Adam Bell]

Public bug reported:

apt-key fails when the POSIXLY_CORRECT environmental variable is set.
This is due to chmod acting differently with POSIXLY_CORRECT set: chmod tries to use `--` as a parameter with this variable.

This prevents some third-party utilities, like pmrun, from using apt-
key.

---

chmod with POSIXLY_CORRECT unset succeeds with the following commands:
```
touch testfile
chmod 0644 -- testfile
echo $?
```
The final command prints "0".
(This is what the apt-key script expects to happen.)

---

chmod with POSIXLY_CORRECT set fails with the following commands:
```
export POSIXLY_CORRECT=
touch testfile
chmod 0644 -- testfile
echo $?
```
The final two commands print these two lines, respectively:
"""
chmod: cannot access '--': No such file or directory
1
"""
(This is unexpected for the apt-key script.)

---

As such, when a third party utility that sets the POSIXLY_CORRECT environmental variable, like pmrun, tries to run apt-key, it will fail.
Example use similar to what the reporting customer tried: `cat some-repo-key.gpg | pmrun apt-key add -`

The chmod examples are run on an up-to-date Bionic container, /usr/bin/apt-key has `chmod #### -- path` commands at line 338 and line 499:
GNU coreutils package version 8.28-1ubuntu1
apt package version 1.6.13

** Affects: apt (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1931178

Title:
  apt-key fails when POSIXLY_CORRECT environmental variable is set

Status in apt package in Ubuntu:
  New

Bug description:
  apt-key fails when the POSIXLY_CORRECT environmental variable is set.
  This is due to chmod acting differently with POSIXLY_CORRECT set: chmod tries to use `--` as a parameter with this variable.

  This prevents some third-party utilities, like pmrun, from using apt-
  key.

  ---

  chmod with POSIXLY_CORRECT unset succeeds with the following commands:
  ```
  touch testfile
  chmod 0644 -- testfile
  echo $?
  ```
  The final command prints "0".
  (This is what the apt-key script expects to happen.)

  ---

  chmod with POSIXLY_CORRECT set fails with the following commands:
  ```
  export POSIXLY_CORRECT=
  touch testfile
  chmod 0644 -- testfile
  echo $?
  ```
  The final two commands print these two lines, respectively:
  """
  chmod: cannot access '--': No such file or directory
  1
  """
  (This is unexpected for the apt-key script.)

  ---

  As such, when a third party utility that sets the POSIXLY_CORRECT environmental variable, like pmrun, tries to run apt-key, it will fail.
  Example use similar to what the reporting customer tried: `cat some-repo-key.gpg | pmrun apt-key add -`

  The chmod examples are run on an up-to-date Bionic container, /usr/bin/apt-key has `chmod #### -- path` commands at line 338 and line 499:
  GNU coreutils package version 8.28-1ubuntu1
  apt package version 1.6.13

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