From c2c8b4787b0882234ba2772ec7513afbf97b563a Mon Sep 17 00:00:00 2001 From: Julian Andres Klode Date: Sun, 29 Jan 2017 13:05:18 +0100 Subject: 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] --- apt-pkg/pkgsystem.h | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'apt-pkg/pkgsystem.h') diff --git a/apt-pkg/pkgsystem.h b/apt-pkg/pkgsystem.h index 4b13072e2..b42070532 100644 --- a/apt-pkg/pkgsystem.h +++ b/apt-pkg/pkgsystem.h @@ -119,6 +119,18 @@ class pkgSystem pkgSystem(char const * const Label, pkgVersioningSystem * const VS); virtual ~pkgSystem(); + + + /* companions to Lock()/UnLock + * + * These functions can be called prior to calling dpkg to release an inner + * lock without releasing the overall outer lock, so that dpkg can run + * correctly but no other APT instance can acquire the system lock. + */ + bool LockInner(); + bool UnLockInner(bool NoErrors = false); + /// checks if the system is currently locked + bool IsLocked(); private: pkgSystemPrivate * const d; }; -- cgit v1.2.3