is dpkg install atomic?

[Han]

Hi, 

This might be a dumb question:  is dpkg installing (or updating) a package using a .deb file atomic in the event of power outage? 

By atomic, I meant either the new version of the application is fully installed, or not at all.  Is this always guaranteed? 

I read the `man dpkg` doc, but couldn't find a clear answer. I might have missed it though.

Thanks,

Han

[Roberto C. Sánchez]

It's not a dumb question. The installation of a package might seem,
from the user perspective, like it could be atomic, but it is really
made up of a variety of steps.

Broadly, the package is downloaded, then unpacked, then any included
pre-installation script is executed, the unpacked contents are placed in
their location on the system, and then any included post-installation
script is executed. A power outage at any point in the process could
leave the package in a state where is it is not fully installed.
Though, generally speaking, a situation like that can almost always be
recovered from by a command like 'apt-get install --reinstall <pkg>'.
However, there might circumstances where more advanced techniques might
be required.

Regards,

-Roberto
--
Roberto C. Sánchez

[Greg Wooledge]

On Tue, Oct 26, 2021 at 10:31:54AM -0700, Han wrote:

> Hi,
>
> This might be a dumb question: is dpkg installing (or updating) a package
> using a .deb file atomic in the event of power outage?
>
> By atomic, I meant either the new version of the application is fully
> installed, or not at all. Is this always guaranteed?

No. A package can be in a "half-installed" state.

unicorn:~$ dpkg -l | head -n3
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)

See where it says "Half-inst" in the middle? If an "H" is shown in
that column, the package is half-installed.

Also, the post-install scripts that most packages use are shell scripts,
so you already know a priori that the script could be interrupted
partway through.

[Brian]

On Tue 26 Oct 2021 at 13:57:13 -0400, Roberto C. Sánchez wrote:

> On Tue, Oct 26, 2021 at 10:31:54AM -0700, Han wrote:
> > Hi, 
> > This might be a dumb question:  is dpkg installing (or updating) a package
> > using a .deb file atomic in the event of power outage? 
> > By atomic, I meant either the new version of the application is fully
> > installed, or not at all.  Is this always guaranteed? 
> > I read the `man dpkg` doc, but couldn't find a clear answer. I might have
> > missed it though.
> > Thanks,
> > Han
>
> It's not a dumb question. The installation of a package might seem,
> from the user perspective, like it could be atomic, but it is really
> made up of a variety of steps.

Indeed.

> Broadly, the package is downloaded, then unpacked, then any included
> pre-installation script is executed, the unpacked contents are placed in
> their location on the system, and then any included post-installation
> script is executed. A power outage at any point in the process could
> leave the package in a state where is it is not fully installed.
> Though, generally speaking, a situation like that can almost always be
> recovered from by a command like 'apt-get install --reinstall <pkg>'.
> However, there might circumstances where more advanced techniques might
> be required.

Assuming that Ctrl-C while dpkg or apt is running is equivalent to a
power outage, I have found that running the specific command afterwards
is generally successful.

--
Brian.

[David Wright]

On Tue 26 Oct 2021 at 19:11:50 (+0100), Brian wrote:

> Assuming that Ctrl-C while dpkg or apt is running is equivalent to a
> power outage, I have found that running the specific command afterwards
> is generally successful.

And it might be worth adding that if apt is downloading a large
package when interrupted, it will continue the download from
wherever it got to, without wasting bandwidth.

Cheers,
David.