[Bug 1778547] Re: Broken system lock counting

[Julian Andres Klode]

** Description changed:

  [Impact]
  System lock unrefing is broken, off-by-one error. Locking twice opens a second fd for the lock file, and the old fd gets lost and remains open for the remaining duration of the program
  
  [Test case]
- TODO
+ 
+ Executing the following script should show one lock file in the first
+ line, and none in the last line.
+ 
+ def maybe_readlink(path):
+   try:
+     return os.readlink("/proc/self/fd/" + p)
+   except:
+     return "<deleted>"
+ 
+ import os, apt_pkg
+ apt_pkg.init()
+ apt_pkg.pkgsystem_lock()
+ apt_pkg.pkgsystem_lock()
+ apt_pkg.pkgsystem_unlock()
+ apt_pkg.pkgsystem_unlock()
+ print([(p, maybe_readlink("/proc/self/fd/" + p)) for p in os.listdir("/proc/self/fd")])
+ 
+ 
+ [Test output]
+ good, cosmic:
+ [('0', '/dev/pts/2'), ('1', '/dev/pts/2'), ('2', '/dev/pts/2'), ('3', '/var/lib/dpkg/lock'), ('4', '<deleted>')]
+ [('0', '/dev/pts/2'), ('1', '/dev/pts/2'), ('2', '/dev/pts/2'), ('3', '<deleted>')]
+ 
+ bad, bionic:
+ [('0', '/dev/pts/0'), ('1', '/dev/pts/0'), ('2', '/dev/pts/0'), ('3', 'pipe:[40889331]'), ('4', '/var/lib/dpkg/lock'), ('5', '/var/lib/dpkg/lock'), ('6', '<deleted>'), ('17', 'socket:[53931]')]
+ [('0', '/dev/pts/0'), ('1', '/dev/pts/0'), ('2', '/dev/pts/0'), ('3', 'pipe:[40889331]'), ('4', '/var/lib/dpkg/lock'), ('5', '<deleted>'), ('17', 'socket:[53931]')]
+ 
  
  [Regression potential]
- TODO
+ A locking change can potentially break locking, but this one seems like an obvious off-by-one error.

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https://bugs.launchpad.net/bugs/1778547

Title:
  Broken system lock counting

Status in apt package in Ubuntu:
  Fix Committed
Status in apt source package in Xenial:
  Confirmed
Status in apt source package in Artful:
  Confirmed
Status in apt source package in Bionic:
  Triaged

Bug description:
  [Impact]
  System lock unrefing is broken, off-by-one error. Locking twice opens a second fd for the lock file, and the old fd gets lost and remains open for the remaining duration of the program

  [Test case]

  Executing the following script should show one lock file in the first
  line, and none in the last line.

  def maybe_readlink(path):
    try:
      return os.readlink("/proc/self/fd/" + p)
    except:
      return "<deleted>"

  import os, apt_pkg
  apt_pkg.init()
  apt_pkg.pkgsystem_lock()
  apt_pkg.pkgsystem_lock()
  apt_pkg.pkgsystem_unlock()
  apt_pkg.pkgsystem_unlock()
  print([(p, maybe_readlink("/proc/self/fd/" + p)) for p in os.listdir("/proc/self/fd")])

  
  [Test output]
  good, cosmic:
  [('0', '/dev/pts/2'), ('1', '/dev/pts/2'), ('2', '/dev/pts/2'), ('3', '/var/lib/dpkg/lock'), ('4', '<deleted>')]
  [('0', '/dev/pts/2'), ('1', '/dev/pts/2'), ('2', '/dev/pts/2'), ('3', '<deleted>')]

  bad, bionic:
  [('0', '/dev/pts/0'), ('1', '/dev/pts/0'), ('2', '/dev/pts/0'), ('3', 'pipe:[40889331]'), ('4', '/var/lib/dpkg/lock'), ('5', '/var/lib/dpkg/lock'), ('6', '<deleted>'), ('17', 'socket:[53931]')]
  [('0', '/dev/pts/0'), ('1', '/dev/pts/0'), ('2', '/dev/pts/0'), ('3', 'pipe:[40889331]'), ('4', '/var/lib/dpkg/lock'), ('5', '<deleted>'), ('17', 'socket:[53931]')]

  
  [Regression potential]
  A locking change can potentially break locking, but this one seems like an obvious off-by-one error.

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