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]

.travis.yml

@@ -8,7 +8,7 @@ before_install:
  - sudo apt-get update -qq
 install:
  - sudo ./prepare-release travis-ci
- - sudo apt-get -qq -y -t wily install gettext liblz4-dev
+ - sudo apt-get -qq -y -t wily install gettext liblz4-dev python3-apt
  - make
 script:
  - make test

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;
       }
    }

debian/tests/control

@@ -1,4 +1,4 @@
 Tests: run-tests
 Restrictions: allow-stderr
 Depends: @, @builddeps@, fakeroot, wget, stunnel4, lsof, db-util, gnupg, gnupg2,
-         libfile-fcntllock-perl
+         libfile-fcntllock-perl, python3-apt

test/integration/framework

@@ -175,6 +175,7 @@ runapt() {
 	esac
 	MALLOC_PERTURB_=21 MALLOC_CHECK_=2 APT_CONFIG="$(getaptconfig)" LD_LIBRARY_PATH="${LIBRARYPATH}:${LD_LIBRARY_PATH}" "$CMD" "$@"
 }
+runpython3() { runapt command python3 "$@"; }
 aptconfig() { runapt apt-config "$@"; }
 aptcache() { runapt apt-cache "$@"; }
 aptcdrom() { runapt apt-cdrom "$@"; }

test/integration/test-bug-lp1550741-heisestate

@@ -0,0 +1,48 @@
+#!/bin/sh
+set -e
+
+TESTDIR="$(readlink -f "$(dirname "$0")")"
+. "$TESTDIR/framework"
+
+setupenvironment
+configarchitecture 'amd64'
+
+insertpackage 'unstable' 'module-init-tools' 'amd64' '1.0' 'Depends: libkmod2 (= 21-1)'
+insertpackage 'unstable' 'libkmod2' 'amd64' '0.22-1'
+insertinstalledpackage 'module-init-tools' 'amd64' '0.1'
+
+setupaptarchive
+
+# this test only works if the python-apt is build against the same
+# ABI version as the apt we are testing here
+PYAPT_LIB_VER=$(runpython3 -c 'import apt_pkg;print(apt_pkg.LIB_VERSION)' 2>/dev/null || true)
+if [ ! -f $LIBRARYPATH/libapt-pkg.so.$PYAPT_LIB_VER ]; then
+    msgskip "python-apt build with the wrong library version: $PYAPT_LIB_VER"
+    exit 0
+fi
+
+# we can not test this using our normal sh tests
+cat > test.py <<EOF
+#!/usr/bin/python3
+import sys
+import apt
+def in_valid_state(pkg):
+   return (pkg.marked_keep or
+           pkg.marked_install or
+           pkg.marked_upgrade or
+           pkg.marked_delete or
+           pkg.marked_downgrade or
+           pkg.marked_reinstall)
+# main
+cache=apt.Cache()
+pkgname="module-init-tools"
+if not in_valid_state(cache[pkgname]):
+    print("the test is broken, %s should be in a valid state" % pkgname)
+    sys.exit(99)
+cache.upgrade(True)
+if not in_valid_state(cache[pkgname]):
+    print("package %s is in a heisen-state" % pkgname)
+    sys.exit(2)
+
+EOF
+testsuccess runpython3 test.py