dpkg frontend locking

Hello Julian, or anyone else affected,

Accepted dpkg into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/dpkg/1.18.4ubuntu1.5 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested and change the tag from verification-needed-xenial to verification-done-xenial. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-xenial. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello Julian, or anyone else affected,

Accepted dpkg into bionic-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/dpkg/1.19.0.5ubuntu2.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested and change the tag from verification-needed-bionic to verification-done-bionic. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-bionic. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Before:

1. with /var/lib/dpkg/lock locked:
   dpkg: error: dpkg status database is locked by another process
2. with lock-frontend locked (not yet recognized):
   dpkg: error: --install needs at least one package archive file argument
3. same as 2

after upgrade to 1.18.4ubuntu1.5:

1.
root@xxx:~# dpkg -i /var/cache/apt/archives/dpkg_1.18.4ubuntu1.5_amd64.deb
dpkg: error: dpkg status database is locked by another process

2.
root@xxx:~# dpkg -i /var/cache/apt/archives/dpkg_1.18.4ubuntu1.5_amd64.deb
dpkg: error: dpkg frontend is locked by another process

3.
root@xxx:~# DPKG_FRONTEND_LOCKED=1 dpkg -i /var/cache/apt/archives/dpkg_1.18.4ubuntu1.5_amd64.deb
(Reading database ... 25800 files and directories currently installed.)
Preparing to unpack .../dpkg_1.18.4ubuntu1.5_amd64.deb ...
Unpacking dpkg (1.18.4ubuntu1.5) over (1.18.4ubuntu1.5) ...
Setting up dpkg (1.18.4ubuntu1.5) ...
Processing triggers for man-db (2.7.5-1) ...

=> Verified.

Same with bionic (1.19.0.5ubuntu2.1)

1. lock locked:
root@bbb:~# dpkg -i /var/cache/apt/archives/dpkg_1.19.0.5ubuntu2.1_amd64.deb
dpkg: error: dpkg status database is locked by another process

2. Frontend locked
root@bbb:~# dpkg -i /var/cache/apt/archives/dpkg_1.19.0.5ubuntu2.1_amd64.deb
dpkg: error: dpkg frontend is locked by another process

3. Frontend locked, but overriden by env:

root@bbb:~# DPKG_FRONTEND_LOCKED=1 dpkg -i /var/cache/apt/archives/dpkg_1.19.0.5ubuntu2.1_amd64.deb
(Reading database ... 28496 files and directories currently installed.)
Preparing to unpack .../dpkg_1.19.0.5ubuntu2.1_amd64.deb ...
Unpacking dpkg (1.19.0.5ubuntu2.1) over (1.19.0.5ubuntu2.1) ...
Setting up dpkg (1.19.0.5ubuntu2.1) ...
Processing triggers for man-db (2.8.3-2) ...

=> verified

I have confirmed that the autopkgtest regressions in xenial are not actual regressions from that upload, but in release. (bionic next)

Same in bionic. In both cases, not all errors are exactly regressions in release, but some (sbuild, pbuilder) fail because they want to do stuff with devel, but there is no devel yet:

Please check for an update for distro-info-data. See /usr/share/doc/distro-info-data/README.Debian for details.

The verification of the Stable Release Update for dpkg has completed successfully and the package has now been released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package dpkg - 1.18.4ubuntu1.5

---------------
dpkg (1.18.4ubuntu1.5) xenial; urgency=medium

  * Apply patch from upstream to add frontend locking (LP: #1796081):
    - Add support for frontend locking. This makes it possible for frontends
      using this new protocol, to safely lock the dpkg database w/o risk of
      race conditions with other dpkg instances or frontends supporting the
      same protocol.

 -- Julian Andres Klode <email address hidden> Thu, 04 Oct 2018 14:21:49 +0200

This bug was fixed in the package dpkg - 1.19.0.5ubuntu2.1

---------------
dpkg (1.19.0.5ubuntu2.1) bionic; urgency=medium

  * Apply patch from upstream to add frontend locking (LP: #1796081):
    - Add support for frontend locking. This makes it possible for frontends
      using this new protocol, to safely lock the dpkg database w/o risk of
      race conditions with other dpkg instances or frontends supporting the
      same protocol.

 -- Julian Andres Klode <email address hidden> Thu, 04 Oct 2018 14:21:49 +0200

I do not see why we need this new frontend lock. If there are higher level frontends, they may have to do some locking before calling apt, but they would have to release the locks before calling apt, and then there is race condition. Why not just use the old dpkg lock and make the package managers ignore the lock, if there is DPKG_LOCKED in environment?

The reason is fairly simple: If you kill apt/frontend, you still want the running dpkg process to hold a lock. Sadly sharing the lock or handing it around is not possible, hence approximation with a second one.

I think it can be made to work. I do not know how apt/dpkg is implemented, but I made an example by Bash:

script.sh file:

#!/bin/bash
#
exec {var}>/tmp/lock
flock -n $var || { echo already locked; exit 1; }
LOCK= /path/to/subscript.sh # edit this path

subscript.sh file:

#!/bin/bash
#
[[ -v LOCK ]] && { kill $PPID; sleep 3; } # kill the caller
exec {var}>/tmp/lock
flock -n $var && echo was not locked || echo was locked

The subscript.sh above kills the caller (if LOCK variable is set in its environment). The subscript shows the lock is set even thereafter until the subscript finishes.

Anyway I am missing something: Even if I lock /var/lib/dpkg/lock or /var/lib/dpkg/lock-frontend this way (instead of locking /tmp/lock), that does not prevent installing a package by apt install or dpkg --install. But if I run Synaptic package manager by pkexec "/usr/sbin/synaptic", it locks the files and prevents installing by apt or dpkg from outside.

I have a related question at https://askubuntu.com/q/1197364/21005