Fix bug where the problemresolve can put a pkg into a heisenstate

The problemresolver will set the candidate version for pkg P back
to the current version if it encounters an impossible to satisfy
critical dependency on P. However it did not set the State of
the package back as well which lead to a situation where P is
neither in Keep,Install,Upgrade,Delete state.

Note that this can not be tested via the traditional sh based
framework. I added a python-apt based test for this.

LP: #1550741

[jak@debian.org: Make the test not fail if apt_pkg cannot be
 imported]

1 files changed, 5 insertions, 0 deletions

diff --git a/apt-pkg/depcache.cc b/apt-pkg/depcache.cc
index 8281949f9..dd18305d2 100644
--- a/apt-pkg/depcache.cc
+++ b/apt-pkg/depcache.cc
@@ -1421,7 +1421,12 @@ bool pkgDepCache::IsInstallOkDependenciesSatisfiableByCandidates(PkgIterator con
 	 // the dependency is critical, but can't be installed, so discard the candidate
 	 // as the problemresolver will trip over it otherwise trying to install it (#735967)
 	 if (Pkg->CurrentVer != 0 && (PkgState[Pkg->ID].iFlags & Protected) != Protected)
+         {
 	    SetCandidateVersion(Pkg.CurrentVer());
+            StateCache &State = PkgState[Pkg->ID];
+            State.Mode = ModeKeep;
+            State.Update(Pkg, *this);
+         }
 	 return false;
       }
    }