Adding upstream version 2.9.3.upstream/2.9.3

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

8 files changed, 1282 insertions, 9 deletions

diff --git a/apt-pkg/CMakeLists.txt b/apt-pkg/CMakeLists.txt
index d13aed9..63052fa 100644
--- a/apt-pkg/CMakeLists.txt
+++ b/apt-pkg/CMakeLists.txt
@@ -3,7 +3,8 @@ include_directories(${PROJECT_BINARY_DIR}/include/apt-pkg)
 
 file(MAKE_DIRECTORY ${PROJECT_BINARY_DIR}/include/apt-pkg/)
 execute_process(COMMAND grep -v "^#" "${CMAKE_CURRENT_SOURCE_DIR}/tagfile-keys.list"
-		OUTPUT_FILE "${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.clean.list")
+		OUTPUT_FILE "${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.clean.list"
+		COMMAND_ERROR_IS_FATAL ANY)
 execute_process(COMMAND ${TRIEHASH_EXECUTABLE}
                         --ignore-case
                          --header ${PROJECT_BINARY_DIR}/include/apt-pkg/tagfile-keys.h
@@ -13,7 +14,8 @@ execute_process(COMMAND ${TRIEHASH_EXECUTABLE}
                          --function-name pkgTagHash
                          --include "<apt-pkg/tagfile.h>"
                          --include "<apt-pkg/header-is-private.h>"
-                         "${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.clean.list")
+                         "${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.clean.list"
+                COMMAND_ERROR_IS_FATAL ANY)
 set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS "tagfile-keys.list")
 set_source_files_properties("${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.cc" PROPERTIES COMPILE_DEFINITIONS APT_COMPILING_APT)
 
@@ -22,11 +24,13 @@ set_source_files_properties("${CMAKE_CURRENT_BINARY_DIR}/tagfile-keys.cc" PROPER
 execute_process(COMMAND awk -v ORS=. "/^\#define APT_PKG_M/ {print \$3}"
                 COMMAND sed "s/\\.\$//"
                 INPUT_FILE ${CMAKE_CURRENT_SOURCE_DIR}/contrib/macros.h
-                OUTPUT_VARIABLE MAJOR OUTPUT_STRIP_TRAILING_WHITESPACE)
+                OUTPUT_VARIABLE MAJOR OUTPUT_STRIP_TRAILING_WHITESPACE
+                COMMAND_ERROR_IS_FATAL ANY)
 execute_process(COMMAND grep "^#define APT_PKG_RELEASE"
                 COMMAND cut -d " " -f 3
                 INPUT_FILE ${CMAKE_CURRENT_SOURCE_DIR}/contrib/macros.h
-                OUTPUT_VARIABLE MINOR OUTPUT_STRIP_TRAILING_WHITESPACE)
+                OUTPUT_VARIABLE MINOR OUTPUT_STRIP_TRAILING_WHITESPACE
+                COMMAND_ERROR_IS_FATAL ANY)
 
 message(STATUS "Building libapt-pkg ${MAJOR} (release ${MINOR})")
 set(APT_PKG_MAJOR ${MAJOR} PARENT_SCOPE) # exporting for methods/CMakeLists.txt
diff --git a/apt-pkg/contrib/error.cc b/apt-pkg/contrib/error.cc
index 83e90a0..054435b 100644
--- a/apt-pkg/contrib/error.cc
+++ b/apt-pkg/contrib/error.cc
@@ -315,12 +315,14 @@ APT_HIDDEN std::ostream &operator<<(std::ostream &out, GlobalError::Item i)
       case GlobalError::FATAL:
       case GlobalError::ERROR:
       case GlobalError::WARNING:
-      case GlobalError::NOTICE:
-      case GlobalError::AUDIT:
 	 out << COLOR_RESET;
 	 if (out_ver >= 30)
 	    out << COLOR_BOLD;
 	 break;
+      case GlobalError::NOTICE:
+      case GlobalError::AUDIT:
+	 out << COLOR_RESET;
+	 break;
       default:
 	 break;
       }
diff --git a/apt-pkg/deb/deblistparser.cc b/apt-pkg/deb/deblistparser.cc
index 071189b..9177d54 100644
--- a/apt-pkg/deb/deblistparser.cc
+++ b/apt-pkg/deb/deblistparser.cc
@@ -887,7 +887,7 @@ bool debListParser::ParseProvides(pkgCache::VerIterator &Ver)
    bool const barbarianArch = not APT::Configuration::checkArchitecture(Arch);
    const char *Start;
    const char *Stop;
-   if (Section.Find(pkgTagSection::Key::Provides,Start,Stop) == true)
+   if (Section.Find(pkgTagSection::Key::Provides,Start,Stop) && Start != Stop)
    {
       StringView Package;
       StringView Version;
diff --git a/apt-pkg/depcache.cc b/apt-pkg/depcache.cc
index 76a5c09..72cbf8d 100644
--- a/apt-pkg/depcache.cc
+++ b/apt-pkg/depcache.cc
@@ -1463,7 +1463,7 @@ static bool MarkInstall_RemoveConflictsIfNotUpgradeable(pkgDepCache &Cache, bool
    return not failedToRemoveSomething;
 }
 									/*}}}*/
-static bool MarkInstall_CollectReverseDepends(pkgDepCache &Cache, bool const DebugAutoInstall, pkgCache::VerIterator const &PV, unsigned long Depth, APT::PackageVector &toUpgrade) /*{{{*/
+static bool MarkInstall_CollectReverseDepends(pkgDepCache &Cache, bool const DebugAutoInstall, pkgCache::VerIterator const &PV, unsigned long Depth, APT::PackageVector &toUpgrade, APT::PackageVector const &delayedRemove) /*{{{*/
 {
    auto CurrentVer = PV.ParentPkg().CurrentVer();
    if (CurrentVer.end())
@@ -1474,6 +1474,9 @@ static bool MarkInstall_CollectReverseDepends(pkgDepCache &Cache, bool const Deb
       // Skip non-installed versions and packages already marked for upgrade
       if (ParentPkg.CurrentVer() != D.ParentVer() || Cache[ParentPkg].Install())
 	 continue;
+      // Skip rev-depends we already tagged for removal
+      if (Cache[ParentPkg].Delete() || std::find(delayedRemove.begin(), delayedRemove.end(), ParentPkg) != delayedRemove.end())
+	 continue;
       // We only handle important positive dependencies, RemoveConflictsIfNotUpgradeable handles negative
       if (not Cache.IsImportantDep(D) || D.IsNegative())
 	 continue;
@@ -1722,7 +1725,7 @@ bool pkgDepCache::MarkInstall(PkgIterator const &Pkg, bool AutoInst,
 	    return false;
 	 hasFailed = true;
       }
-      if (not MarkInstall_CollectReverseDepends(*this, DebugAutoInstall, PV, Depth, toUpgrade))
+      if (not MarkInstall_CollectReverseDepends(*this, DebugAutoInstall, PV, Depth, toUpgrade, delayedRemove))
       {
 	 if (failEarly)
 	    return false;
diff --git a/apt-pkg/edsp.cc b/apt-pkg/edsp.cc
index a02e400..5894008 100644
--- a/apt-pkg/edsp.cc
+++ b/apt-pkg/edsp.cc
@@ -19,6 +19,7 @@
 #include <apt-pkg/pkgsystem.h>
 #include <apt-pkg/prettyprinters.h>
 #include <apt-pkg/progress.h>
+#include <apt-pkg/solver3.h>
 #include <apt-pkg/string_view.h>
 #include <apt-pkg/strutl.h>
 #include <apt-pkg/tagfile.h>
@@ -765,6 +766,18 @@ static bool CreateDumpFile(char const * const id, char const * const type, FileF
 // EDSP::ResolveExternal - resolve problems by asking external for help	{{{*/
 bool EDSP::ResolveExternal(const char* const solver, pkgDepCache &Cache,
 			 unsigned int const flags, OpProgress *Progress) {
+   if (strcmp(solver, "3.0") == 0)
+   {
+      APT::Solver s(Cache.GetCache(), Cache.GetPolicy());
+      FileFd output;
+      if (not s.FromDepCache(Cache))
+	 return false;
+      if (not s.Solve())
+	 return false;
+      if (not s.ToDepCache(Cache))
+	 return false;
+      return true;
+   }
 	if (strcmp(solver, "internal") == 0)
 	{
 		FileFd output;
diff --git a/apt-pkg/solver3.cc b/apt-pkg/solver3.cc
new file mode 100644
index 0000000..d43bd5b
--- /dev/null
+++ b/apt-pkg/solver3.cc
@@ -0,0 +1,960 @@
+/*
+ * solver3.cc - The APT 3.0 solver
+ *
+ * Copyright (c) 2023 Julian Andres Klode
+ * Copyright (c) 2023 Canonical Ltd
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ */
+
+#define APT_COMPILING_APT
+
+#include <config.h>
+
+#include <apt-pkg/algorithms.h>
+#include <apt-pkg/aptconfiguration.h>
+#include <apt-pkg/cachefilter.h>
+#include <apt-pkg/cacheset.h>
+#include <apt-pkg/error.h>
+#include <apt-pkg/macros.h>
+#include <apt-pkg/pkgsystem.h>
+#include <apt-pkg/solver3.h>
+#include <apt-pkg/version.h>
+
+#include <algorithm>
+#include <cassert>
+#include <sstream>
+
+// FIXME: Helpers stolen from DepCache, please give them back.
+struct CompareProviders3 /*{{{*/
+{
+   pkgCache &Cache;
+   pkgDepCache::Policy &Policy;
+   pkgCache::PkgIterator const Pkg;
+   bool upgrade{_config->FindB("APT::Solver::Upgrade", false)};
+
+   bool operator()(pkgCache::Version *AV, pkgCache::Version *BV)
+   {
+      return (*this)(pkgCache::VerIterator(Cache, AV), pkgCache::VerIterator(Cache, BV));
+   }
+   bool operator()(pkgCache::VerIterator const &AV, pkgCache::VerIterator const &BV)
+   {
+      pkgCache::PkgIterator const A = AV.ParentPkg();
+      pkgCache::PkgIterator const B = BV.ParentPkg();
+      // Compare versions for the same package. FIXME: Move this to the real implementation
+      if (A == B)
+      {
+	 if (AV == BV)
+	    return false;
+	 if (not upgrade && A->CurrentVer != 0 && A.CurrentVer() == AV)
+	    return true;
+	 if (Policy.GetPriority(AV) < Policy.GetPriority(BV))
+	    return false;
+
+	 return _system->VS->CmpVersion(AV.VerStr(), BV.VerStr()) > 0;
+      }
+      // Prefer MA:same packages if other architectures for it are installed
+      if ((AV->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same ||
+	  (BV->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same)
+      {
+	 bool instA = false;
+	 if ((AV->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same)
+	 {
+	    pkgCache::GrpIterator Grp = A.Group();
+	    for (pkgCache::PkgIterator P = Grp.PackageList(); P.end() == false; P = Grp.NextPkg(P))
+	       if (P->CurrentVer != 0)
+	       {
+		  instA = true;
+		  break;
+	       }
+	 }
+	 bool instB = false;
+	 if ((BV->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same)
+	 {
+	    pkgCache::GrpIterator Grp = B.Group();
+	    for (pkgCache::PkgIterator P = Grp.PackageList(); P.end() == false; P = Grp.NextPkg(P))
+	    {
+	       if (P->CurrentVer != 0)
+	       {
+		  instB = true;
+		  break;
+	       }
+	    }
+	 }
+	 if (instA != instB)
+	    return instA;
+      }
+      if ((A->CurrentVer == 0 || B->CurrentVer == 0) && A->CurrentVer != B->CurrentVer)
+	 return A->CurrentVer != 0;
+      // Prefer packages in the same group as the target; e.g. foo:i386, foo:amd64
+      if (A->Group != B->Group)
+      {
+	 if (A->Group == Pkg->Group && B->Group != Pkg->Group)
+	    return true;
+	 else if (B->Group == Pkg->Group && A->Group != Pkg->Group)
+	    return false;
+      }
+      // we like essentials
+      if ((A->Flags & pkgCache::Flag::Essential) != (B->Flags & pkgCache::Flag::Essential))
+      {
+	 if ((A->Flags & pkgCache::Flag::Essential) == pkgCache::Flag::Essential)
+	    return true;
+	 else if ((B->Flags & pkgCache::Flag::Essential) == pkgCache::Flag::Essential)
+	    return false;
+      }
+      if ((A->Flags & pkgCache::Flag::Important) != (B->Flags & pkgCache::Flag::Important))
+      {
+	 if ((A->Flags & pkgCache::Flag::Important) == pkgCache::Flag::Important)
+	    return true;
+	 else if ((B->Flags & pkgCache::Flag::Important) == pkgCache::Flag::Important)
+	    return false;
+      }
+      // prefer native architecture
+      if (strcmp(A.Arch(), B.Arch()) != 0)
+      {
+	 if (strcmp(A.Arch(), A.Cache()->NativeArch()) == 0)
+	    return true;
+	 else if (strcmp(B.Arch(), B.Cache()->NativeArch()) == 0)
+	    return false;
+	 std::vector<std::string> archs = APT::Configuration::getArchitectures();
+	 for (std::vector<std::string>::const_iterator a = archs.begin(); a != archs.end(); ++a)
+	    if (*a == A.Arch())
+	       return true;
+	    else if (*a == B.Arch())
+	       return false;
+      }
+      // higher priority seems like a good idea
+      if (AV->Priority != BV->Priority)
+	 return AV->Priority < BV->Priority;
+      if (auto NameCmp = strcmp(A.Name(), B.Name()))
+	 return NameCmp < 0;
+      // unable to decide…
+      return A->ID > B->ID;
+   }
+};
+
+/** \brief Returns \b true for packages matching a regular
+ *  expression in APT::NeverAutoRemove.
+ */
+class DefaultRootSetFunc2 : public pkgDepCache::DefaultRootSetFunc
+{
+   std::unique_ptr<APT::CacheFilter::Matcher> Kernels;
+
+   public:
+   DefaultRootSetFunc2(pkgCache *cache) : Kernels(APT::KernelAutoRemoveHelper::GetProtectedKernelsFilter(cache)){};
+   virtual ~DefaultRootSetFunc2(){};
+
+   bool InRootSet(const pkgCache::PkgIterator &pkg) APT_OVERRIDE { return pkg.end() == false && ((*Kernels)(pkg) || DefaultRootSetFunc::InRootSet(pkg)); };
+}; // FIXME: DEDUP with pkgDepCache.
+/*}}}*/
+
+APT::Solver::Solver(pkgCache &cache, pkgDepCache::Policy &policy)
+    : cache(cache),
+      policy(policy),
+      pkgStates{cache.Head().PackageCount},
+      verStates{cache.Head().VersionCount}
+{
+   static_assert(sizeof(APT::Solver::State<pkgCache::PkgIterator>) == 3 * sizeof(int));
+   static_assert(sizeof(APT::Solver::State<pkgCache::VerIterator>) == 3 * sizeof(int));
+   static_assert(sizeof(APT::Solver::Reason) == sizeof(map_pointer<pkgCache::Package>));
+   static_assert(sizeof(APT::Solver::Reason) == sizeof(map_pointer<pkgCache::Version>));
+}
+
+// This function determines if a work item is less important than another.
+bool APT::Solver::Work::operator<(APT::Solver::Work const &b) const
+{
+   if (optional && b.optional && reason.empty() && b.reason.empty() && upgrade != b.upgrade)
+   {
+      // Assuming we have libfoo-dev=5.1 Depends libfoo5.1-dev upgrade to libfoo-dev=5.3 Depends libfoo5.3-dev,
+      // We schedule libfoo-dev=5.3|libfoo-dev=5.1, libfoo5.1-dev. The latter would be resolved first, resulting
+      // in libfoo-dev being kept back.
+      //
+      // However, if we schedule not libfoo5.1-dev but bar Recommends libfoo5.1-dev, we should not be breaking that
+      // Recommends, hence we need to ensure that if we order an upgrade before an optional package that this optional
+      // package was a top level package, i.e. b.reason is empty (or our reason in the reverse case).
+      //
+      // So if we are the upgrade, and b also Depends on one of our versions, we need to satisfy b after we
+      // have scheduled the upgrade.
+      if (upgrade)
+	 return std::any_of(b.solutions.begin(), b.solutions.end(), [this](auto bsol) -> bool
+			    { return std::find(solutions.begin(), solutions.end(), bsol) != solutions.end(); });
+      else
+	 return std::any_of(solutions.begin(), solutions.end(), [b](auto sol) -> bool
+			    { return std::find(b.solutions.begin(), b.solutions.end(), sol) != b.solutions.end(); });
+   }
+   // An optional item is less important than a required one.
+   if (optional != b.optional)
+      return optional;
+   // More solutions to explore are more expensive.
+   if (size != b.size)
+      return size > b.size;
+   // We enqueue common dependencies at the package level to avoid choosing versions, so let's solve package items first,
+   // this improves the implication graph as it now tells you that common dependencies were installed by the package.
+   if (reason.Pkg() != b.reason.Pkg())
+      return reason.Pkg() == 0;
+
+   return false;
+}
+
+void APT::Solver::Work::Dump(pkgCache &cache)
+{
+   if (dirty)
+      std::cerr << "Dirty ";
+   if (optional)
+      std::cerr << "Optional ";
+   std::cerr << "Item (" << size << "@" << depth << (upgrade ? "u" : "") << ") ";
+   if (auto Pkg = reason.Pkg(); Pkg != 0)
+      std::cerr << pkgCache::PkgIterator(cache, cache.PkgP + Pkg).FullName();
+   if (auto Ver = reason.Ver(); Ver != 0)
+      std::cerr << pkgCache::VerIterator(cache, cache.VerP + Ver).ParentPkg().FullName() << "=" << pkgCache::VerIterator(cache, cache.VerP + Ver).VerStr();
+   std::cerr << " -> ";
+   for (auto sol : solutions)
+   {
+      auto Ver = pkgCache::VerIterator(cache, sol);
+      std::cerr << " | " << Ver.ParentPkg().FullName() << "=" << Ver.VerStr();
+   }
+}
+
+// Prints an implication graph part of the form A -> B -> C, possibly with "not"
+std::string APT::Solver::WhyStr(Reason reason)
+{
+   std::vector<std::string> out;
+
+   while (not reason.empty())
+   {
+      if (auto Pkg = pkgCache::PkgIterator(cache, cache.PkgP + reason.Pkg()); not Pkg.end())
+      {
+	 if ((*this)[Pkg].decision == Decision::MUSTNOT)
+	    out.push_back(std::string("not ") + Pkg.FullName());
+	 else
+	    out.push_back(Pkg.FullName());
+	 reason = (*this)[Pkg].reason;
+      }
+      if (auto Ver = pkgCache::VerIterator(cache, cache.VerP + reason.Ver()); not Ver.end())
+      {
+	 if ((*this)[Ver].decision == Decision::MUSTNOT)
+	    out.push_back(std::string("not ") + Ver.ParentPkg().FullName() + "=" + Ver.VerStr());
+	 else
+	    out.push_back(Ver.ParentPkg().FullName() + "=" + Ver.VerStr());
+	 reason = (*this)[Ver].reason;
+      }
+   }
+
+   std::string outstr;
+   for (auto I = out.rbegin(); I != out.rend(); ++I)
+   {
+      outstr += (outstr.size() == 0 ? "" : " -> ") + *I;
+   }
+   return outstr;
+}
+
+bool APT::Solver::Install(pkgCache::PkgIterator Pkg, Reason reason)
+{
+   if ((*this)[Pkg].decision == Decision::MUST)
+      return true;
+
+   // Check conflicting selections
+   if ((*this)[Pkg].decision == Decision::MUSTNOT)
+      return _error->Error("Conflict: %s -> %s but %s", WhyStr(reason).c_str(), Pkg.FullName().c_str(), WhyStr(Reason(Pkg)).c_str());
+
+   bool anyInstallable = false;
+   for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
+      if ((*this)[ver].decision != Decision::MUSTNOT)
+	 anyInstallable = true;
+
+   if (not anyInstallable)
+   {
+      _error->Error("Conflict: %s -> %s but no versions are installable",
+		    WhyStr(reason).c_str(), Pkg.FullName().c_str());
+      for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
+	 _error->Error("Uninstallable version: %s", WhyStr(Reason(ver)).c_str());
+      return false;
+   }
+
+   // Note decision
+   if (unlikely(debug >= 1))
+      std::cerr << "[" << depth() << "] Install:" << Pkg.FullName() << " (" << WhyStr(reason) << ")\n";
+   (*this)[Pkg] = {reason, depth(), Decision::MUST,};
+
+   // Insert the work item.
+   Work workItem{Reason(Pkg), depth()};
+   for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
+      if (IsAllowedVersion(ver))
+	 workItem.solutions.push_back(ver);
+   std::sort(workItem.solutions.begin(), workItem.solutions.end(), CompareProviders3{cache, policy, Pkg});
+   assert(workItem.solutions.size() > 0);
+
+   if (workItem.solutions.size() > 1 || workItem.optional)
+      AddWork(std::move(workItem));
+   else if (not Install(pkgCache::VerIterator(cache, workItem.solutions[0]), workItem.reason))
+      return false;
+
+   if (not EnqueueCommonDependencies(Pkg))
+      return false;
+
+   return true;
+}
+
+bool APT::Solver::Install(pkgCache::VerIterator Ver, Reason reason)
+{
+   if ((*this)[Ver].decision == Decision::MUST)
+      return true;
+
+   if (unlikely(debug >= 1))
+      assert(IsAllowedVersion(Ver));
+
+   // Check conflicting selections
+   if ((*this)[Ver].decision == Decision::MUSTNOT)
+      return _error->Error("Conflict: %s -> %s but %s",
+			   WhyStr(reason).c_str(),
+			   (Ver.ParentPkg().FullName() + "=" + Ver.VerStr()).c_str(),
+			   WhyStr(Reason(Ver)).c_str());
+   if ((*this)[Ver.ParentPkg()].decision == Decision::MUSTNOT)
+      return _error->Error("Conflict: %s -> %s but %s",
+			   WhyStr(reason).c_str(),
+			   (Ver.ParentPkg().FullName() + "=" + Ver.VerStr()).c_str(),
+			   WhyStr(Reason(Ver.ParentPkg())).c_str());
+   for (auto otherVer = Ver.ParentPkg().VersionList(); not otherVer.end(); otherVer++)
+      if (otherVer->ID != Ver->ID && (*this)[otherVer].decision == Decision::MUST)
+	 return _error->Error("Conflict: %s -> %s but %s",
+			      WhyStr(reason).c_str(),
+			      (Ver.ParentPkg().FullName() + "=" + Ver.VerStr()).c_str(),
+			      WhyStr(Reason(otherVer)).c_str());
+
+   // Note decision
+   if (unlikely(debug >= 1))
+      std::cerr << "[" << depth() << "] Install:" << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " (" << WhyStr(reason) << ")\n";
+   (*this)[Ver] = {reason, depth(), Decision::MUST,};
+   if ((*this)[Ver.ParentPkg()].decision != Decision::MUST)
+      (*this)[Ver.ParentPkg()] = {Reason(Ver), depth(), Decision::MUST,};
+
+   for (auto OV = Ver.ParentPkg().VersionList(); not OV.end(); ++OV)
+   {
+      if (OV != Ver && not Reject(OV, Reason(Ver)))
+	 return false;
+   }
+
+   for (auto dep = Ver.DependsList(); not dep.end();)
+   {
+      // Compute a single dependency element (glob or)
+      pkgCache::DepIterator start;
+      pkgCache::DepIterator end;
+      dep.GlobOr(start, end); // advances dep
+
+      if (not policy.IsImportantDep(start))
+	 continue;
+      if (not EnqueueOrGroup(start, end, Reason(Ver)))
+	 return false;
+   }
+
+   return true;
+}
+
+bool APT::Solver::Reject(pkgCache::PkgIterator Pkg, Reason reason)
+{
+   if ((*this)[Pkg].decision == Decision::MUSTNOT)
+      return true;
+
+   // Check conflicting selections
+   for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
+      if ((*this)[ver].decision == Decision::MUST)
+	 return _error->Error("Conflict: %s -> not %s but %s", WhyStr(reason).c_str(), Pkg.FullName().c_str(), WhyStr(Reason(ver)).c_str());
+   if ((*this)[Pkg].decision == Decision::MUST)
+      return _error->Error("Conflict: %s -> not %s but %s", WhyStr(reason).c_str(), Pkg.FullName().c_str(), WhyStr(Reason(Pkg)).c_str());
+
+   // Reject the package and its versions.
+   if (unlikely(debug >= 1))
+      std::cerr << "[" << depth() << "] Reject:" << Pkg.FullName() << " (" << WhyStr(reason) << ")\n";
+   (*this)[Pkg] = {reason, depth(), Decision::MUSTNOT,};
+   for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
+      if (not Reject(ver, Reason(Pkg)))
+	 return false;
+
+   needsRescore = true;
+
+   return true;
+}
+
+// \brief Do not install this version
+bool APT::Solver::Reject(pkgCache::VerIterator Ver, Reason reason)
+{
+   if ((*this)[Ver].decision == Decision::MUSTNOT)
+      return true;
+
+   // Check conflicting choices.
+   if ((*this)[Ver].decision == Decision::MUST)
+      return _error->Error("Conflict: %s -> not %s but %s",
+			   WhyStr(reason).c_str(),
+			   (Ver.ParentPkg().FullName() + "=" + Ver.VerStr()).c_str(),
+			   WhyStr(Reason(Ver)).c_str());
+
+   // Mark the package as rejected and propagate up as needed.
+   if (unlikely(debug >= 1))
+      std::cerr << "[" << depth() << "] Reject:" << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " (" << WhyStr(reason) << ")\n";
+   (*this)[Ver] = {reason, depth(), Decision::MUSTNOT,};
+   if (auto pkg = Ver.ParentPkg(); (*this)[pkg].decision != Decision::MUSTNOT)
+   {
+      bool anyInstallable = false;
+      for (auto otherVer = pkg.VersionList(); not otherVer.end(); otherVer++)
+	 if (otherVer->ID != Ver->ID && (*this)[otherVer].decision != Decision::MUSTNOT)
+	    anyInstallable = true;
+
+      if (anyInstallable)
+	 ;
+      else if ((*this)[pkg].decision == Decision::MUST) // Must install, but none available
+      {
+	 _error->Error("Conflict: %s but no versions are installable",
+		       WhyStr(Reason(pkg)).c_str());
+	 for (auto otherVer = pkg.VersionList(); not otherVer.end(); otherVer++)
+	    if ((*this)[otherVer].decision == Decision::MUSTNOT)
+	       _error->Error("Uninstallable version: %s", WhyStr(Reason(otherVer)).c_str());
+	 return _error->Error("Uninstallable version: %s -> not %s",
+			      WhyStr(reason).c_str(),
+			      (Ver.ParentPkg().FullName() + "=" + Ver.VerStr()).c_str());
+      }
+      else if ((*this)[Ver.ParentPkg()].decision != Decision::MUSTNOT) // Last installable invalidated
+	 (*this)[Ver.ParentPkg()] = {Reason(Ver), depth(), Decision::MUSTNOT};
+   }
+
+   if (not RejectReverseDependencies(Ver))
+      return false;
+
+   needsRescore = true;
+
+   return true;
+}
+
+bool APT::Solver::EnqueueCommonDependencies(pkgCache::PkgIterator Pkg)
+{
+   if (not _config->FindB("APT::Solver::Enqueue-Common-Dependencies", true))
+      return false;
+   for (auto dep = Pkg.VersionList().DependsList(); not dep.end();)
+   {
+      pkgCache::DepIterator start;
+      pkgCache::DepIterator end;
+      dep.GlobOr(start, end); // advances dep
+
+      bool allHaveDep = true;
+      for (auto ver = Pkg.VersionList()++; not ver.end(); ver++)
+      {
+	 bool haveDep = false;
+	 for (auto otherDep = ver.DependsList(); not haveDep && not otherDep.end(); otherDep++)
+	    haveDep = otherDep->DependencyData == start->DependencyData;
+	 if (!haveDep)
+	    allHaveDep = haveDep;
+      }
+      if (not allHaveDep)
+	 continue;
+      if (not policy.IsImportantDep(start))
+	 continue;
+      if (not EnqueueOrGroup(start, end, Reason(Pkg)))
+	 return false;
+   }
+
+   return true;
+}
+
+bool APT::Solver::EnqueueOrGroup(pkgCache::DepIterator start, pkgCache::DepIterator end, Reason reason)
+{
+   auto TgtPkg = start.TargetPkg();
+   auto Ver = start.ParentVer();
+   auto fixPolicy = _config->FindB("APT::Get::Fix-Policy-Broken");
+
+   if (unlikely(debug >= 3))
+      std::cerr << "Found dependency critical " << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " -> " << start.TargetPkg().FullName() << "\n";
+
+   Work workItem{reason, depth(), not start.IsCritical() /* optional */};
+
+   do
+   {
+      auto begin = workItem.solutions.size();
+      auto all = start.AllTargets();
+
+      for (auto tgt = all; *tgt; ++tgt)
+      {
+	 pkgCache::VerIterator tgti(cache, *tgt);
+
+	 if (start.IsNegative())
+	 {
+	    if (unlikely(debug >= 3))
+	       std::cerr << "Reject: " << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " -> " << tgti.ParentPkg().FullName() << "=" << tgti.VerStr() << "\n";
+	    // FIXME: We should be collecting these and marking the heap only once.
+	    if (not Reject(pkgCache::VerIterator(cache, *tgt), Reason(Ver)))
+	       return false;
+	 }
+	 else
+	 {
+	    if (unlikely(debug >= 3))
+	       std::cerr << "Adding work to  item " << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " -> " << tgti.ParentPkg().FullName() << "=" << tgti.VerStr() << "\n";
+	    if (IsAllowedVersion(*tgt))
+	       workItem.solutions.push_back(*tgt);
+	 }
+      }
+      delete[] all;
+
+      // If we are fixing the policy, we need to sort each alternative in an or group separately
+      // FIXME: This is not really true, though, we should fix the CompareProviders to ignore the
+      // installed state
+      if (fixPolicy)
+	 std::sort(workItem.solutions.begin() + begin, workItem.solutions.end(), CompareProviders3{cache, policy, TgtPkg});
+
+      if (start == end)
+	 break;
+      ++start;
+   } while (1);
+
+   if (not fixPolicy)
+      std::sort(workItem.solutions.begin(), workItem.solutions.end(), CompareProviders3{cache, policy, TgtPkg});
+
+   // Figure out if the reason is installed
+   bool reasonInstalled = false;
+   if (auto p = workItem.reason.Pkg())
+      reasonInstalled = pkgCache::PkgIterator(cache, cache.PkgP + p)->CurrentVer != 0;
+   else if (auto v = workItem.reason.Ver())
+      reasonInstalled = pkgCache::VerIterator(cache, cache.VerP + v).ParentPkg()->CurrentVer != 0;
+
+   // Try to perserve satisfied Recommends. FIXME: We should check if the Recommends was there in the installed version?
+   if (workItem.optional && reasonInstalled && not fixPolicy &&
+       not std::any_of(workItem.solutions.begin(), workItem.solutions.end(), [this](auto ver)
+		       { return pkgCache::VerIterator(cache, ver).ParentPkg()->CurrentVer != 0; }))
+   {
+      if (unlikely(debug >= 3))
+      {
+	 std::cerr << "Ignoring currently unsatisfied Recommends ";
+	 workItem.Dump(cache);
+	 std::cerr << "\n";
+      }
+      return true;
+   }
+   if (not workItem.solutions.empty())
+   {
+      // std::sort(workItem.solutions.begin(), workItem.solutions.end(), CompareProviders3{cache, TgtPkg});
+      if (unlikely(debug >= 3 && workItem.optional))
+      {
+	 std::cerr << "Enqueuing currently satisfied Recommends ";
+	 workItem.Dump(cache);
+	 std::cerr << "\n";
+      }
+      if (workItem.optional || workItem.solutions.size() > 1)
+	 AddWork(std::move(workItem));
+      else if (not Install(pkgCache::VerIterator(cache, workItem.solutions[0]), reason))
+	 return false;
+   }
+   else if (start.IsCritical() && not start.IsNegative())
+   {
+      return _error->Error("Unsatisfiable dependency group %s=%s -> %s", Ver.ParentPkg().FullName().c_str(), Ver.VerStr(), TgtPkg.FullName().c_str());
+   }
+   return true;
+}
+
+// \brief Find the or group containing the given dependency.
+static void FindOrGroup(pkgCache::DepIterator const &D, pkgCache::DepIterator &start, pkgCache::DepIterator &end)
+{
+   for (auto dep = D.ParentVer().DependsList(); not dep.end();)
+   {
+      dep.GlobOr(start, end); // advances dep
+
+      for (auto member = start;;)
+      {
+	 if (member == D)
+	    return;
+	 if (member == end)
+	    break;
+	 member++;
+      }
+   }
+
+   _error->Fatal("Found a dependency that does not exist in its parent version");
+   abort();
+}
+
+// This is the opposite of EnqueueOrDependencies, it rejects the reverse dependencies of the
+// given version iterator.
+bool APT::Solver::RejectReverseDependencies(pkgCache::VerIterator Ver)
+{
+   // This checks whether an or group is still satisfiable.
+   auto stillPossible = [this](pkgCache::DepIterator start, pkgCache::DepIterator end)
+   {
+      while (1)
+      {
+	 std::unique_ptr<pkgCache::Version *[]> Ts{start.AllTargets()};
+	 for (size_t i = 0; Ts[i] != nullptr; ++i)
+	    if ((*this)[Ts[i]].decision != Decision::MUSTNOT)
+	       return true;
+
+	 if (start == end)
+	    return false;
+
+	 start++;
+      }
+   };
+
+   for (auto RD = Ver.ParentPkg().RevDependsList(); not RD.end(); ++RD)
+   {
+      auto RDV = RD.ParentVer();
+      if (RD.IsNegative() || not RD.IsCritical() || not RD.IsSatisfied(Ver))
+	 continue;
+
+      if ((*this)[RDV].decision == Decision::MUSTNOT)
+	 continue;
+
+      pkgCache::DepIterator start;
+      pkgCache::DepIterator end;
+      FindOrGroup(RD, start, end);
+
+      if (stillPossible(start, end))
+	 continue;
+
+      if (unlikely(debug >= 3))
+	 std::cerr << "Propagate NOT " << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " to " << RDV.ParentPkg().FullName() << "=" << RDV.VerStr() << " for dependency group starting with" << start.TargetPkg().FullName() << std::endl;
+
+      if (not Reject(RDV, Reason(Ver)))
+	 return false;
+   }
+   return true;
+}
+
+bool APT::Solver::IsAllowedVersion(pkgCache::Version *V)
+{
+   pkgCache::VerIterator ver(cache, V);
+   if (not StrictPinning || ver.ParentPkg().CurrentVer() == ver || policy.GetCandidateVer(ver.ParentPkg()) == ver)
+      return true;
+
+   if (unlikely(debug >= 3))
+      std::cerr << "Ignoring: " << ver.ParentPkg().FullName() << "=" << ver.VerStr() << "(neither candidate nor installed)\n";
+   return false;
+}
+
+void APT::Solver::Push(Work work)
+{
+   if (unlikely(debug >= 2))
+   {
+      std::cerr << "Trying choice for ";
+      work.Dump(cache);
+      std::cerr << "\n";
+   }
+   choices.push_back(std::move(work));
+   // Pop() will call MergeWithStack() when reverting to level 0, or RevertToStack after dumping to the debug log.
+   _error->PushToStack();
+}
+
+bool APT::Solver::Pop()
+{
+   auto depth = APT::Solver::depth();
+   if (depth == 0)
+      return false;
+
+   if (unlikely(debug >= 2))
+      for (std::string msg; _error->PopMessage(msg);)
+	 std::cerr << "Branch failed: " << msg << std::endl;
+
+   _error->RevertToStack();
+
+   depth--;
+
+   // Clean up the higher level states.
+   // FIXME: Do not override the hints here.
+   for (auto &state : pkgStates)
+      if (state.depth > depth)
+	 state = {};
+   for (auto &state : verStates)
+      if (state.depth > depth)
+	 state = {};
+
+   // This destroys the invariants that `work` must be a heap. But this is ok:
+   // we are restoring the invariant below, because rejecting a package always
+   // calls std::make_heap.
+   work.erase(std::remove_if(work.begin(), work.end(), [depth](Work &w) -> bool
+			     { return w.depth > depth || w.dirty; }),
+	      work.end());
+   std::make_heap(work.begin(), work.end());
+
+   // Go over the solved items, see if any of them need to be moved back or deleted.
+   solved.erase(std::remove_if(solved.begin(), solved.end(), [this, depth](Work &w) -> bool
+			       {
+				 if (w.depth > depth) // Deeper decision level is no longer valid.
+				    return true;
+				 // This item is still solved, keep it on the solved list.
+				 if (not std::any_of(w.solutions.begin(), w.solutions.end(), [this](auto ver)
+						     { return (*this)[ver].decision == Decision::MUST; }))
+				    return false;
+				 // We are not longer solved, move it back to work.
+				 AddWork(std::move(w));
+				 return true; }),
+		solved.end());
+
+   Work w = std::move(choices.back());
+   choices.pop_back();
+   if (unlikely(debug >= 2))
+   {
+      std::cerr << "Backtracking to choice ";
+      w.Dump(cache);
+      std::cerr << "\n";
+   }
+   if (unlikely(debug >= 4))
+   {
+      std::cerr << "choices: ";
+      for (auto &i : choices)
+      {
+	 std::cerr << pkgCache::VerIterator(cache, i.choice).ParentPkg().FullName(true) << "=" << pkgCache::VerIterator(cache, i.choice).VerStr();
+      }
+      std::cerr << std::endl;
+   }
+
+   assert(w.choice != nullptr);
+   // FIXME: There should be a reason!
+   if (not Reject(pkgCache::VerIterator(cache, w.choice), {}))
+      return false;
+
+   w.choice = nullptr;
+   AddWork(std::move(w));
+   return true;
+}
+
+void APT::Solver::AddWork(Work &&w)
+{
+   w.size = std::count_if(w.solutions.begin(), w.solutions.end(), [this](auto V)
+			  { return (*this)[V].decision != Decision::MUSTNOT; });
+   work.push_back(std::move(w));
+   std::push_heap(work.begin(), work.end());
+}
+
+void APT::Solver::RescoreWorkIfNeeded()
+{
+   if (not needsRescore)
+      return;
+
+   needsRescore = false;
+   std::vector<Work> resized;
+   for (auto &w : work)
+   {
+      if (w.dirty)
+	 continue;
+      size_t newSize = std::count_if(w.solutions.begin(), w.solutions.end(), [this](auto V)
+				     { return (*this)[V].decision != Decision::MUSTNOT; });
+
+      // Notably we only insert the work into the queue if it got smaller. Work that got larger
+      // we just move around when we get to it too early in Solve(). This reduces memory usage
+      // at the expense of counting each item we see in Solve().
+      if (newSize < w.size)
+      {
+	 Work newWork(w);
+	 newWork.size = newSize;
+	 resized.push_back(std::move(newWork));
+	 w.dirty = true;
+      }
+   }
+   if (unlikely(debug >= 2))
+      std::cerr << "Rescored: " << resized.size() << "items\n";
+   for (auto &w : resized)
+   {
+      work.push_back(std::move(w));
+      std::push_heap(work.begin(), work.end());
+   }
+}
+
+bool APT::Solver::Solve()
+{
+   while (not work.empty())
+   {
+      // Rescore the work if we need to
+      RescoreWorkIfNeeded();
+      // *NOW* we can pop the item.
+      std::pop_heap(work.begin(), work.end());
+
+      // This item has been replaced with a new one. Remove it.
+      if (work.back().dirty)
+      {
+	 work.pop_back();
+	 continue;
+      }
+
+      // If our size increased, queue again.
+      size_t newSize = std::count_if(work.back().solutions.begin(), work.back().solutions.end(), [this](auto V)
+				     { return (*this)[V].decision != Decision::MUSTNOT; });
+
+      if (newSize > work.back().size)
+      {
+	 work.back().size = newSize;
+	 std::push_heap(work.begin(), work.end());
+	 continue;
+      }
+      assert(newSize == work.back().size);
+
+      auto item = std::move(work.back());
+      work.pop_back();
+      solved.push_back(item);
+
+      if (std::any_of(item.solutions.begin(), item.solutions.end(), [this](auto ver)
+		      { return (*this)[ver].decision == Decision::MUST; }))
+      {
+	 if (unlikely(debug >= 2))
+	 {
+	    std::cerr << "ELIDED ";
+	    item.Dump(cache);
+	    std::cerr << "\n";
+	 }
+	 continue;
+      }
+
+      if (unlikely(debug >= 1))
+      {
+	 item.Dump(cache);
+	 std::cerr << "\n";
+      }
+
+      assert(item.solutions.size() > 1 || item.optional);
+
+      bool foundSolution = false;
+      for (auto &sol : item.solutions)
+      {
+	 pkgCache::VerIterator ver(cache, sol);
+	 if ((*this)[ver].decision == Decision::MUSTNOT)
+	 {
+	    if (unlikely(debug >= 3))
+	       std::cerr << "(existing conflict: " << ver.ParentPkg().FullName() << "=" << ver.VerStr() << ")\n";
+	    continue;
+	 }
+	 if (item.size > 1 || item.optional)
+	 {
+	    item.choice = ver;
+	    Push(item);
+	 }
+	 if (unlikely(debug >= 3))
+	    std::cerr << "(try it: " << ver.ParentPkg().FullName() << "=" << ver.VerStr() << ")\n";
+	 if (not Install(pkgCache::VerIterator(cache, ver), item.reason) && not Pop())
+	    return false;
+	 foundSolution = true;
+	 break;
+      }
+      if (not foundSolution && not item.optional)
+      {
+	 std::ostringstream dep;
+	 assert(item.solutions.size() > 0);
+	 for (auto &sol : item.solutions)
+	    dep << (dep.tellp() == 0 ? "" : " | ") << pkgCache::VerIterator(cache, sol).ParentPkg().FullName() << "=" << pkgCache::VerIterator(cache, sol).VerStr();
+	 _error->Error("Unsatisfiable dependency: %s -> %s", WhyStr(item.reason).c_str(), dep.str().c_str());
+	 for (auto &sol : item.solutions)
+	    if ((*this)[sol].decision == Decision::MUSTNOT)
+	       _error->Error("Not considered: %s=%s: %s", pkgCache::VerIterator(cache, sol).ParentPkg().FullName().c_str(),
+			     pkgCache::VerIterator(cache, sol).VerStr(),
+			     WhyStr(Reason(pkgCache::VerIterator(cache, sol))).c_str());
+	 if (not Pop())
+	    return false;
+      }
+   }
+
+   return true;
+}
+
+// \brief Apply the selections from the dep cache to the solver
+bool APT::Solver::FromDepCache(pkgDepCache &depcache)
+{
+   bool KeepAuto = not _config->FindB("APT::Get::AutomaticRemove");
+   bool AllowRemove = _config->FindB("APT::Solver::Remove", true);
+   bool AllowInstall = _config->FindB("APT::Solver::Install", true);
+   DefaultRootSetFunc2 rootSet(&cache);
+
+   for (auto P = cache.PkgBegin(); not P.end(); P++)
+   {
+      if (P->VersionList == nullptr)
+	 continue;
+
+      auto state = depcache[P];
+      auto maybeInstall = state.Install() || (state.Keep() && P->CurrentVer);
+      auto reject = state.Delete() || (depcache[P].Keep() && not P->CurrentVer && depcache[P].Protect());
+      if (P->SelectedState == pkgCache::State::Hold && not state.Protect())
+      {
+	 if (unlikely(debug >= 1))
+	    std::cerr << "Hold " << P.FullName() << "\n";
+	 if (P->CurrentVer ? not Install(P.CurrentVer(), {}) : not Reject(P, {}))
+	    return false;
+      }
+      else if (reject)
+      {
+	 if (unlikely(debug >= 1))
+	    std::cerr << "Delete " << P.FullName() << "\n";
+	 if (!Reject(P, {}))
+	    return false;
+      }
+      else if (maybeInstall && P->Flags & (pkgCache::Flag::Essential | pkgCache::Flag::Important))
+      {
+	 if (unlikely(debug >= 1))
+	    std::cerr << "ESSENTIAL " << P.FullName() << "\n";
+	 if (depcache[P].Keep() ? not Install(P, {}) : not Install(depcache.GetCandidateVersion(P), {}))
+	    return false;
+      }
+      else if (maybeInstall && not(depcache[P].Flags & pkgCache::Flag::Auto))
+      {
+	 if (unlikely(debug >= 1))
+	    std::cerr << "MANUAL " << P.FullName() << "\n";
+	 if (depcache[P].Keep() ? not Install(P, {}) : not Install(depcache.GetCandidateVersion(P), {}))
+	    return false;
+      }
+      else if (maybeInstall && (KeepAuto || rootSet.InRootSet(P)) && (depcache[P].Flags & pkgCache::Flag::Auto))
+      {
+	 auto Upgrade = depcache.GetCandidateVersion(P) != P.CurrentVer();
+	 if (unlikely(debug >= 1))
+	    std::cerr << "AUTOMATIC " << P.FullName() << (Upgrade ? " - upgrade" : "") << "\n";
+
+	 if (not AllowRemove)
+	 {
+	    if (depcache[P].Keep() ? not Install(P, {}) : not Install(depcache.GetCandidateVersion(P), {}))
+	       return false;
+	 }
+	 else
+	 {
+	    Work w{Reason(), depth(), true, Upgrade};
+	    for (auto V = P.VersionList(); not V.end(); ++V)
+	       if (IsAllowedVersion(V))
+		  w.solutions.push_back(V);
+	    std::sort(w.solutions.begin(), w.solutions.end(), CompareProviders3{cache, policy, P});
+	    AddWork(std::move(w));
+	 }
+      }
+      else if (P->CurrentVer == 0 && not AllowInstall)
+      {
+	 if (unlikely(debug >= 1))
+	    std::cerr << "NOT ALLOWING INSTALL OF " << P.FullName() << "\n";
+	 if (not Reject(P, {}))
+	    return false;
+      }
+   }
+
+   return true;
+}
+
+bool APT::Solver::ToDepCache(pkgDepCache &depcache)
+{
+   pkgDepCache::ActionGroup group(depcache);
+   for (auto P = cache.PkgBegin(); not P.end(); P++)
+   {
+      if ((*this)[P].decision == Decision::MUST)
+      {
+	 for (auto V = P.VersionList(); not V.end(); V++)
+	 {
+	    if ((*this)[V].decision == Decision::MUST)
+	    {
+	       depcache.SetCandidateVersion(V);
+	       break;
+	    }
+	 }
+	 auto reason = (*this)[depcache.GetCandidateVersion(P)].reason;
+	 if (auto RP = reason.Pkg(); RP == P.MapPointer())
+	    reason = (*this)[P].reason;
+
+	 depcache.MarkInstall(P, false, 0, reason.empty());
+	 if (not P->CurrentVer)
+	    depcache.MarkAuto(P, not reason.empty());
+	 depcache[P].Marked = 1;
+	 depcache[P].Garbage = 0;
+      }
+      else if (P->CurrentVer)
+      {
+	 depcache.MarkDelete(P, false, 0, (*this)[P].reason.empty());
+	 depcache[P].Marked = 0;
+	 depcache[P].Garbage = 1;
+      }
+   }
+   return true;
+}
diff --git a/apt-pkg/solver3.h b/apt-pkg/solver3.h
new file mode 100644
index 0000000..cf2fb2e
--- /dev/null
+++ b/apt-pkg/solver3.h
@@ -0,0 +1,286 @@
+/*
+ * solver3.h - The APT 3.0 solver
+ *
+ * Copyright (c) 2023 Julian Andres Klode
+ * Copyright (c) 2023 Canonical Ltd
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ */
+
+#include <vector>
+
+#include <apt-pkg/configuration.h>
+#include <apt-pkg/depcache.h>
+#include <apt-pkg/pkgcache.h>
+#include <apt-pkg/policy.h>
+
+namespace APT
+{
+
+/*
+ * \brief APT 3.0 solver
+ *
+ * This is a simple solver focused on understandability and sensible results, it
+ * will not generally find all solutions to the problem but will try to find the best
+ * ones.
+ *
+ * It is a brute force solver with heuristics, conflicts learning, and 2**32 levels
+ * of backtracking.
+ */
+class Solver
+{
+   enum class Decision : uint16_t;
+   enum class Hint : uint16_t;
+   struct Reason;
+   template <typename T>
+   struct State;
+   struct Work;
+
+   // \brief Type to record depth at. This may very well be a 16-bit
+   // unsigned integer, then change Solver::State::Decision to be a
+   // uint16_t class enum as well to get a more compact space.
+   using depth_type = unsigned int;
+
+   // Documentation
+   template <typename T>
+   using heap = std::vector<T>;
+
+   static_assert(sizeof(depth_type) >= sizeof(map_id_t));
+
+   // Cache is needed to construct Iterators from Version objects we see
+   pkgCache &cache;
+   // Policy is needed for determining candidate version.
+   pkgDepCache::Policy &policy;
+   // States for packages
+   std::vector<State<pkgCache::Package>> pkgStates{};
+   // States for versions
+   std::vector<State<pkgCache::Version>> verStates{};
+
+   // \brief Helper function for safe access to package state.
+   inline State<pkgCache::Package> &operator[](pkgCache::Package *P)
+   {
+      return pkgStates[P->ID];
+   }
+
+   // \brief Helper function for safe access to version state.
+   inline State<pkgCache::Version> &operator[](pkgCache::Version *V)
+   {
+      return verStates[V->ID];
+   }
+
+   // \brief Heap of the remaining work.
+   //
+   // We are using an std::vector with std::make_heap(), std::push_heap(),
+   // and std::pop_heap() rather than a priority_queue because we need to
+   // be able to iterate over the queued work and see if a choice would
+   // invalidate any work.
+   heap<Work> work{};
+   // \brief Whether RescoreWork() actually needs to rescore the work.
+   bool needsRescore{false};
+
+   // \brief Current decision level.
+   //
+   // Each time a decision needs to be made we can push the item under
+   // consideration to our backlog of choices made and then later we can
+   // restore it easily.
+   std::vector<Work> choices{};
+   // \brief Backlog of solved work.
+   //
+   // Solved work may become invalidated when backtracking, so store it
+   // here to revisit it later.
+   std::vector<Work> solved{};
+
+   /// Various configuration options
+   // \brief Debug level
+   int debug{_config->FindI("Debug::APT::Solver")};
+   // \brief If set, we try to keep automatically installed packages installed.
+   bool KeepAuto{not _config->FindB("APT::Get::AutomaticRemove")};
+   // \brief If set, removals are allowed.
+   bool AllowRemove{_config->FindB("APT::Solver::Remove", true)};
+   // \brief If set, installs are allowed.
+   bool AllowInstall{_config->FindB("APT::Solver::Install", true)};
+   // \brief If set, we use strict pinning.
+   bool StrictPinning{_config->FindB("APT::Solver::Strict-Pinning", true)};
+
+   // \brief Enqueue dependencies shared by all versions of the package.
+   bool EnqueueCommonDependencies(pkgCache::PkgIterator Pkg);
+   // \brief Reject reverse dependencies. Must call std::make_heap() after.
+   bool RejectReverseDependencies(pkgCache::VerIterator Ver);
+   // \brief Enqueue a single or group
+   bool EnqueueOrGroup(pkgCache::DepIterator start, pkgCache::DepIterator end, Reason reason);
+   // \brief Check if a version is allowed by policy.
+   bool IsAllowedVersion(pkgCache::Version *V);
+
+   // \brief Return the current depth (choices.size() with casting)
+   depth_type depth()
+   {
+      return static_cast<depth_type>(choices.size());
+   }
+
+   public:
+   // \brief Create a new decision level.
+   bool Pop();
+   // \brief Revert to the previous decision level.
+   void Push(Work work);
+   // \brief Add work to our work queue.
+   void AddWork(Work &&work);
+   // \brief Rescore the work after a reject or a pop
+   void RescoreWorkIfNeeded();
+
+   // \brief Basic solver initializer. This cannot fail.
+   Solver(pkgCache &Cache, pkgDepCache::Policy &Policy);
+
+   // \brief Mark the package for install. This is annoying as it incurs a decision
+   bool Install(pkgCache::PkgIterator Pkg, Reason reason);
+   // \brief Install a version.
+   bool Install(pkgCache::VerIterator Ver, Reason reason);
+   // \brief Do not install this package
+   bool Reject(pkgCache::PkgIterator Pkg, Reason reason);
+   // \brief Do not install this version.
+   bool Reject(pkgCache::VerIterator Ver, Reason reason);
+
+   // \brief Apply the selections from the dep cache to the solver
+   bool FromDepCache(pkgDepCache &depcache);
+   // \brief Apply the solver result to the depCache
+   bool ToDepCache(pkgDepCache &depcache);
+
+   // \brief Solve the dependencies
+   bool Solve();
+
+   // Print dependency chain
+   std::string WhyStr(Reason reason);
+};
+
+}; // namespace APT
+
+/**
+ * \brief Tagged union holding either a package, version, or nothing; representing the reason for installing something.
+ *
+ * We want to keep track of the reason why things are being installed such that
+ * we can have sensible debugging abilities.
+ *
+ * If the reason is empty, this means the package is automatically installed.
+ */
+struct APT::Solver::Reason
+{
+   uint32_t IsVersion : 1;
+   uint32_t MapPtr : 31;
+
+   Reason() : IsVersion(0), MapPtr(0) {}
+   explicit Reason(pkgCache::PkgIterator const &Pkg) : IsVersion(0), MapPtr(Pkg.MapPointer()) {}
+   explicit Reason(pkgCache::VerIterator const &Ver) : IsVersion(1), MapPtr(Ver.MapPointer()) {}
+
+   // \brief Return the package, if any, otherwise 0.
+   map_pointer<pkgCache::Package> Pkg() const
+   {
+      return IsVersion ? 0 : map_pointer<pkgCache::Package>{(uint32_t)MapPtr};
+   }
+   // \brief Return the version, if any, otherwise 0.
+   map_pointer<pkgCache::Version> Ver() const
+   {
+      return IsVersion ? map_pointer<pkgCache::Version>{(uint32_t)MapPtr} : 0;
+   }
+   // \brief Check if there is no reason.
+   bool empty() const
+   {
+      return IsVersion == 0 && MapPtr == 0;
+   }
+};
+
+/**
+ * \brief A single work item
+ *
+ * A work item is a positive dependency that still needs to be resolved. Work
+ * is ordered, by depth, length of solutions, and optionality.
+ *
+ * The work can always be recalculated from the state by iterating over dependencies
+ * of all packages in there, finding solutions to them, and then adding all dependencies
+ * not yet resolved to the work queue.
+ */
+struct APT::Solver::Work
+{
+   // \brief Reason for the work
+   Reason reason;
+   // \brief The depth at which the item has been added
+   depth_type depth;
+
+   // \brief Possible solutions to this task, ordered in order of preference.
+   std::vector<pkgCache::Version *> solutions{};
+
+   // This is a union because we only need to store the choice we made when adding
+   // to the choice vector, and we don't need the size of valid choices in there.
+   union
+   {
+      // The choice we took
+      pkgCache::Version *choice;
+      // Number of valid choices
+      size_t size;
+   };
+
+   // \brief Whether this is an optional work item, they will be processed last
+   bool optional;
+   // \brief Whether this is an ugprade
+   bool upgrade;
+   // \brief This item should be removed from the queue.
+   bool dirty;
+
+   bool operator<(APT::Solver::Work const &b) const;
+   // \brief Dump the work item to std::cerr
+   void Dump(pkgCache &cache);
+
+   inline Work(Reason reason, depth_type depth, bool optional = false, bool upgrade = false) : reason(reason), depth(depth), size(0), optional(optional), upgrade(upgrade), dirty(false) {}
+};
+
+// \brief This essentially describes the install state in RFC2119 terms.
+enum class APT::Solver::Decision : uint16_t
+{
+   // \brief We have not made a choice about the package yet
+   NONE,
+   // \brief We need to install this package
+   MUST,
+   // \brief We cannot install this package (need conflicts with it)
+   MUSTNOT,
+};
+
+// \brief Hints for the solver about the item.
+enum class APT::Solver::Hint : uint16_t
+{
+   // \brief We have not made a choice about the package yet
+   NONE,
+   // \brief This package was listed as a Recommends of a must package,
+   SHOULD,
+   // \brief This package was listed as a Suggests of a must-not package
+   MAY,
+};
+
+/**
+ * \brief The solver state
+ *
+ * For each version, the solver records a decision at a certain level. It
+ * maintains an array mapping from version ID to state.
+ */
+template <typename T>
+struct APT::Solver::State
+{
+   // \brief The reason for causing this state (invalid for NONE).
+   //
+   // Rejects may have been caused by a later state. Consider we select
+   // between x1 and x2 in depth = N. If we now find dependencies of x1
+   // leading to a conflict with a package in K < N, we will record all
+   // of them as REJECT in depth = K.
+   //
+   // You can follow the reason chain upwards as long as the depth
+   // doesn't increase to unwind.
+   //
+   // Reasons < 0 are package ID, reasons > 0 are version IDs.
+   Reason reason{};
+
+   // \brief The depth at which the decision has been taken
+   depth_type depth{0};
+
+   // \brief This essentially describes the install state in RFC2119 terms.
+   Decision decision{Decision::NONE};
+
+   // \brief Any hint.
+   Hint hint{Hint::NONE};
+};
diff --git a/apt-pkg/upgrade.cc b/apt-pkg/upgrade.cc
index fad4783..ac0b71c 100644
--- a/apt-pkg/upgrade.cc
+++ b/apt-pkg/upgrade.cc
@@ -312,6 +312,11 @@ bool pkgMinimizeUpgrade(pkgDepCache &Cache)
 // APT::Upgrade::Upgrade - Upgrade using a specific strategy		/*{{{*/
 bool APT::Upgrade::Upgrade(pkgDepCache &Cache, int mode, OpProgress * const Progress)
 {
+   _config->Set("APT::Solver::Upgrade", "true");
+   if (mode & FORBID_REMOVE_PACKAGES)
+      _config->Set("APT::Solver::Remove", "false");
+   if (mode & FORBID_INSTALL_NEW_PACKAGES)
+      _config->Set("APT::Solver::Install", "false");
    if (mode == ALLOW_EVERYTHING)
       return pkgDistUpgrade(Cache, Progress);
    else if ((mode & ~FORBID_REMOVE_PACKAGES) == 0)