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, 4 insertions, 0 deletions

diff --git a/apt-pkg/deb/dpkgpm.cc b/apt-pkg/deb/dpkgpm.cc
index c6900ec77..790019b98 100644
--- a/apt-pkg/deb/dpkgpm.cc
+++ b/apt-pkg/deb/dpkgpm.cc
@@ -2010,6 +2010,10 @@ bool pkgDPkgPM::Go(APT::Progress::PackageManager *progress)
 	 else
 	    setenv("DPKG_COLORS", "never", 0);
 
+	 if (dynamic_cast<debSystem*>(_system) != nullptr
+	    && dynamic_cast<debSystem*>(_system)->IsLocked() == true) {
+	    setenv("DPKG_FRONTEND_LOCKED", "true", 1);
+	 }
 	 execvp(Args[0], (char**) &Args[0]);
 	 cerr << "Could not exec dpkg!" << endl;
 	 _exit(100);