[Bug 1795407] Re: python-apt frontend locking

[Julian Andres Klode]

without SystemLock being held works fine:

root at xx:~#  strace -o $PWD/log -e openat,open,close,fcntl python3 $PWD/foo.py                                                                 
(Reading database ... 25835 files and directories currently installed.)                                                                       
Removing aptitude-common (0.7.4-2ubuntu2) ...                                                                                                 
COMMAND           PID  TYPE SIZE MODE  M START END PATH                                                                                       
iscsid            432 POSIX   4B WRITE 0     0   0 /run/iscsid.pid                                                                            
atd               421 POSIX   4B WRITE 0     0   0 /run/atd.pid                                                                               
cron              418 FLOCK   4B WRITE 0     0   0 /run/crond.pid                                                                             
python3          1207 POSIX   0B WRITE 0     0   0 /var/lib/dpkg/lock-frontend                                                                
python3          1207 POSIX   0B WRITE 0     0   0 /var/cache/apt/archives/lock                                                               
dpkg             1215 POSIX   0B WRITE 0     0   0 /var/lib/dpkg/lock                                                                         
Processing triggers for man-db (2.7.5-1) ...                                                   

with SystemLock held works fine:
# strace -o $PWD/log.locked -e openat,open,close,fcntl python3 $PWD/foo.py                                                          
(Reading database ... 25835 files and directories currently installed.)
Removing aptitude-common (0.7.4-2ubuntu2) ...                  
COMMAND           PID  TYPE SIZE MODE  M START END PATH          
iscsid            432 POSIX   4B WRITE 0     0   0 /run/iscsid.pid                                                                           
atd               421 POSIX   4B WRITE 0     0   0 /run/atd.pid                                                                              
cron              418 FLOCK   4B WRITE 0     0   0 /run/crond.pid    
dpkg             2285 POSIX   0B WRITE 0     0   0 /var/lib/dpkg/lock
python3          2277 POSIX   0B WRITE 0     0   0 /var/lib/dpkg/lock-frontend                                                               
python3          2277 POSIX   0B WRITE 0     0   0 /var/cache/apt/archives/lock                                                              
Processing triggers for man-db (2.7.5-1) ...

In the strace log, we can see that the frontend lock is closed after the
dpkg runs (after we have relocked the lock).

** Tags removed: verification-needed verification-needed-xenial
** Tags added: verification-done verification-done-xenial

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https://bugs.launchpad.net/bugs/1795407

Title:
  python-apt frontend locking

Status in python-apt package in Ubuntu:
  Fix Released
Status in python-apt source package in Xenial:
  Fix Committed
Status in python-apt source package in Bionic:
  Fix Released

Bug description:
  [Impact]
  Add support to python-apt for frontend locking. This is a bit more complicated, and also requires some other restructuring:

  (1) The archives lock was only taken for a short time, when it should
  have been kept for the duration of an installation, as otherwise debs
  could disappear from archives/ while the install is running.

  (2) There was no lock handling at all. Tools essentially acquire the
  lock, and then release it before commit().

  The fix is based on the apt fix, which makes apt_pkg.SystemLock()
  manage both the frontend and the normal lock, and allows code to call
  apt_pkg.pkgsystem_unlock_inner() and apt_pkg.pkgsystem_lock_inner()
  around dpkg invocations to release the inner lock.

  Cache.commit() takes care of locking itself now. It releases the inner
  lock as needed for dpkg invocations. It also now requires
  apt_pkg.SystemLock to be hold - if it is not, it will acquire it
  itself.

  [Test case]
  1. Mark a package in the cache for install/removal and commit() - FE lock should be taken while dpkg is running.
  2. Mark a package in the cache for install/removal and commit() while holding the lock. Releasing the lock afterwards should still work correctly, and the FE lock should never be released
  3. Observe that the archive lock is not released until the last dpkg run

  The locking behavior can be observed via strace.

  [Regression potential]
  The dpkg lock changed the most: Some lock counting might go wrong and programs might fail as a result. Any problems will be isolated, though - dpkg still acquires its locks, and if a lock miscount happens, apt would not tell dpkg to skip acquiring the frontend lock, so even if we were losing the lock anywhere, dpkg would still try to acquire it and not run unlocked.

  The archive lock done by commit()/fetch_archives() in apt.Cache() is
  now hold as long as the fetcher object exists though (as it uses the
  fetcher's get_lock method), which might cause unexpected behavior in
  apps not expecting that.

  The lists/ lock is unaffected by the changes, so there should not be
  any regressions when it comes to updating index files.

  [Other info (1)]
  CI changes: There were some improvements to type hints for mypy needed to make type checks pass, and changes to the CI's Dockerfile to pull a new apt from the apt stable PPA (as a new enough apt is only in unapproved atm). Neither of those files are used anywhere outside of the package, so they should not cause any problems.

  [Other info (2)]
  This is part of a wider series of SRUs for frontend locking

  - dpkg (bug 1796081)
  - apt (bug 1781169)
  - python-apt (bug 1795407)
  - packagekit (bug 1795614)
  - unattended-upgrades (bug 1789637)
  - aptdaemon (no bug filed yet)

  Further details about frontend locking can be found in
  https://lists.debian.org/debian-dpkg/2017/01/msg00044.html

  - This SRU depends on the apt SRU above, and breaks unattended-
  upgrades in bionic without its SRU due to a bug in the u-u code in
  handling the new API.

  - xenial depends on unattended-upgrades SRU

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