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/debsystem.h b/apt-pkg/deb/debsystem.h
index 7b59fc073..4503676fa 100644
--- a/apt-pkg/deb/debsystem.h
+++ b/apt-pkg/deb/debsystem.h
@@ -50,6 +50,10 @@ class debSystem : public pkgSystem
    APT_HIDDEN static pid_t ExecDpkg(std::vector<std::string> const &sArgs, int * const inputFd, int * const outputFd, bool const DiscardOutput);
    APT_HIDDEN static bool SupportsMultiArch();
    APT_HIDDEN static std::vector<std::string> SupportedArchitectures();
+
+   APT_HIDDEN bool LockInner();
+   APT_HIDDEN bool UnLockInner(bool NoErrors=false);
+   APT_HIDDEN bool IsLocked();
 };
 
 extern debSystem debSys;