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]

(cherry picked from commit c2c8b4787b0882234ba2772ec7513afbf97b563a)
LP: #1781169
(cherry picked from commit 6c0c94ed32b8e679b14b0f89b51c1c336dc0ab9c)

1 files changed, 25 insertions, 0 deletions

diff --git a/apt-pkg/pkgsystem.cc b/apt-pkg/pkgsystem.cc
index 530150221..99d104310 100644
--- a/apt-pkg/pkgsystem.cc
+++ b/apt-pkg/pkgsystem.cc
@@ -13,6 +13,7 @@
 #include<config.h>
 
 #include <apt-pkg/debsystem.h>
+#include <apt-pkg/error.h>
 #include <apt-pkg/pkgsystem.h>
 #include <apt-pkg/macros.h>
 
@@ -64,4 +65,28 @@ std::vector<std::string> pkgSystem::ArchitecturesSupported() const	/*{{{*/
 }
 									/*}}}*/
 
+bool pkgSystem::LockInner()				/*{{{*/
+{
+   debSystem * const deb = dynamic_cast<debSystem *>(this);
+   if (deb != NULL)
+      return deb->LockInner();
+   return _error->Error("LockInner is not implemented");
+}
+									/*}}}*/
+bool pkgSystem::UnLockInner(bool NoErrors)				/*{{{*/
+{
+   debSystem * const deb = dynamic_cast<debSystem *>(this);
+   if (deb != NULL)
+      return deb->UnLockInner(NoErrors);
+   return _error->Error("UnLockInner is not implemented");
+}
+									/*}}}*/
+bool pkgSystem::IsLocked() 						/*{{{*/
+{
+   debSystem * const deb = dynamic_cast<debSystem *>(this);
+   if (deb != NULL)
+      return deb->IsLocked();
+   return true;
+}
+									/*}}}*/
 pkgSystem::~pkgSystem() {}