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Is it possible to force some preinst script to run before another?

What I would like to do, is create a package in my private repository, which when installed would FIRST ask some questions, and only then other packages (on which it depends) would be unpacked.

For example, let's say I want to create my-mysql package, which would depend on mysql-server-5.5 from standard repository. I would like when user types apt-get install my-mysql for my-mysql.preinst to run before mysql-server-5.5.preinst (so it can do debconf-set-selections to autogenerate root password and to stop mysql-server package from asking for it interactively).

I could do it without putting any depends in my-mysql package, and manually running two commands:

apt-get install my-mysql
apt-get install mysql-server-5.5

but I need it to work with only one command (apt-get install my-mysql). I've tried playing with Pre-depends on third package which would do config, and Depends on mysql-server-5.5, but it seems mysql-server-5.5.preinst always run first. Is there a way to change that and force my package preinst to run before it?

As alternative way, I also cannot run apt-get install mysql-server-5.5 from postinst because of dpkg locking issues, and would like to do it in much cleaner way than locking kludges like this one.

UPDATE: as mentioned in the comments, it seems that mysql-server package also has .config scripts (with associated .templates) which execute even before .preinst scripts, so I need to order them too.

  • I checked mysql-server-5.5, and it asks for the password in postinst, not preinst. I'd suggest you check whether your preinst is correctly setting the password debconf selection. – muru Feb 17 '16 at 1:35
  • @muru I'm pretty sure. I've set DEBIAN_SCRIPT_DEBUG and apt-get install my-mysql says as a first thing Preconfiguring packages ... + DEBIAN_SCRIPT_TRACE=1 ${DEBIAN_SCRIPT_TRACE:+ echo "#42#DEBUG# RUNNING $0 $*" 1>&2 } + echo '#42#DEBUG# RUNNING /tmp/mysql-server-5.5.config.WdEe3A configure ' '1>&2'. Only later does my-mysql.preinst does its output. – Matija Nalis Feb 17 '16 at 3:35
  • Pretty sure of what? That your preinst is setting the password correctly? The output you show doesn't indicate anything to that effect. – muru Feb 17 '16 at 3:45
  • @muru shortened Output shows that the first thing apt-get runs is mysql-server-5.5.config. Only after that is completed, does it run my-mysql.preinst script (first line of which outputs some text, so I know exactly when it is run). Since my-mysql.preinst is run only AFTER mysql-server already asked the password question, it does not matter what my-mysql.preinst does or does not do correctly, as it is already too late. – Matija Nalis Feb 17 '16 at 3:59
  • That's because that prompt is from a config script, not a preinst script. You might try running your own config script, but the timing of config isn't guaranteed, so there's no telling if it will work. – muru Feb 17 '16 at 4:34
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I've found a way to do it.

So, in order for single apt-get install my-mysql to work as described in question, I had to do several things:

  • make my-mysql package so it pre-depends on my-mysql-config and depends on mysql-server
  • create additional package my-mysql-config, which pre-depends on debconf and contains my-mysql-config.templates and my-mysql-config.config script (which among other things sets password so mysql-server-5.5 won't ask for it) containing code like:

    . /usr/share/debconf/confmodule
    db_set  mysql-server/root_password "$PW"
    db_fset mysql-server/root_password seen true
    
    db_set  mysql-server/root_password_again "$PW"
    db_fset mysql-server/root_password_again seen true
    
  • also my-mysql-config.preinst contains code that does db_get and creates config files before mysql-server gets started

  • you can use my-mysql package .postinst script to run stuff you want after mysql-server is installed and running.

The Debconf Programmer's Tutorial was great help there, as was using export DEBCONF_DEBUG=developer and set -x in scripts for debugging.

Another two time-consuming issues for debug was making sure that config/preinst/postinst scripts do not output ANYTHING to STDOUT (as it breaks debconf in strange and interesting ways) and that you need db_stop if you do stuff like invoke-rc.d mysql restart or similar (or your apt-get will hang hard, without even responding to Ctrl + C)

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