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]

1 files changed, 2 insertions, 2 deletions

diff --git a/apt-private/private-install.cc b/apt-private/private-install.cc
index f90e7097f..4e888a9bd 100644
--- a/apt-private/private-install.cc
+++ b/apt-private/private-install.cc
@@ -346,7 +346,7 @@ bool InstallPackages(CacheFile &Cache,bool ShwKept,bool Ask, bool Safety)
       }
 
       auto const progress = APT::Progress::PackageManagerProgressFactory();
-      _system->UnLock();
+      _system->UnLockInner();
       pkgPackageManager::OrderResult const Res = PM->DoInstall(progress);
       delete progress;
 
@@ -355,7 +355,7 @@ bool InstallPackages(CacheFile &Cache,bool ShwKept,bool Ask, bool Safety)
       if (Res == pkgPackageManager::Completed)
 	 break;
 
-      _system->Lock();
+      _system->LockInner();
 
       // Reload the fetcher object and loop again for media swapping
       Fetcher.Shutdown();