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authorJulian Andres Klode <jak@debian.org>2017-01-29 13:05:18 +0100
committerJulian Andres Klode <julian.klode@canonical.com>2018-08-07 15:07:52 +0200
commitc2c8b4787b0882234ba2772ec7513afbf97b563a (patch)
treef6c0634c45d174c00192356c49a3fd7fe583e396 /apt-private/private-update.cc
parent91671efc7f63b3e2ff2a573d86ac3225e9b5f9cd (diff)
Add support for dpkg frontend lock
The dpkg frontend lock is a lock dpkg tries to acquire except if the frontend already acquires it. This fixes a race condition in the install command where the dpkg lock is not held for a short period of time between different dpkg invocations. For this reason we also define an environment variable DPKG_FRONTEND_LOCKED for dpkg invocations so dpkg knows not to try to acquire the frontend lock because it's held by a parent process. We can set DPKG_FRONTEND_LOCKED only if the frontend lock really is held; that is, if our lock count is greater than 0 - otherwise an apt client not using the LockInner family of functions would run dpkg without the frontend lock set, but with DPKG_FRONTEND_LOCKED set. Such a process has a weaker guarantee: Because dpkg would not lock the frontend lock either, the process is prone to the existing races, and, more importantly, so is a new style process. Closes: #869546 [fixups: fix error messages, add public IsLocked() method, and make {Un,}LockInner return an error on !debSystem]
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