Description:
There is strange behaviour, when pacman's binary with SETUID attribute (i.e. chmod +s /usr/bin/pacman): pacman can extract files from package to / but cannot run any install scripts because bash runs without root acess (i.e. bash sets EUID to UID). As result we have not full install of package.
I think that's will be more correct prevent pacman's execution from the not priveleged user (i.e. in src/pacman/pacman.c:778 use not geteuid() but getuid()).
Or if pacman must work with enabled SETUID attribute, it's necessary set UID to EUID before call bash.

Additional info:
* package version(s)
* config and/or log files etc.


Steps to reproduce:
[user@localhost ~]$ id
uid=1000(user) gid=1000(user) groups=1000(user)
[user@localhost ~]$ ls -l /usr/bin/pacman
-rwsr-sr-x 1 root root 108856 Jun 18 04:03 /usr/bin/pacman
[user@localhost ~]$ pacman -S --noprogressbar gpm
resolving dependencies...
looking for inter-conflicts...

Packages (1): gpm-1.20.7-3

Total Download Size: 0.12 MiB
Total Installed Size: 0.39 MiB

:: Proceed with installation? [Y/n] y
warning: couldn't find or create package cache, using /tmp/ instead
:: Retrieving packages ...
downloading gpm-1.20.7-3-i686.pkg.tar.xz...
checking keyring...
checking package integrity...
loading package files...
checking for file conflicts...
checking available disk space...
installing gpm...
/usr/bin/bash: /tmp/alpm_UktsMO/.INSTALL: Permission denied
/usr/bin/bash: post_install: command not found
error: command failed to execute correctly
[user@localhost ~]$ pacman -Qi gpm
Name : gpm
Version : 1.20.7-3
Description : A mouse server for the console and xterm
<cut>
[user@localhost ~]$ ls -l /etc/profile.d/gpm.sh
-rwxr-xr-x 1 root root 102 May 12 08:09 /etc/profile.d/gpm.sh