Simple MSolver::add_pin to play along all_problems_structured

libmamba/src/core/solver.cpp

@@ -417,78 +417,22 @@ namespace mamba
 
     void MSolver::add_pin(const std::string& pin)
     {
-        // if we pin a package, we need to remove all packages that don't match the
-        // pin from being available for installation! This is done by adding
-        // SOLVER_LOCK to the packages, so that they are prevented from being
-        // installed A lock basically says: keep the state of the package. I.e.
-        // uninstalled packages stay uninstalled, installed packages stay installed.
-        // A lock is a hard requirement, we could also use SOLVER_FAVOR for soft
-        // requirements
-
-        // First we need to check if the pin is OK given the currently installed
-        // packages
-        Pool* pool = m_pool;
-        MatchSpec ms(pin);
-
-        // TODO
-        // if (m_prefix_data)
-        // {
-        //     for (auto& [name, record] : m_prefix_data->records())
-        //     {
-        //         LOG_ERROR << "NAME " << name;
-        //         if (name == ms.name)
-        //         {
-        //             LOG_ERROR << "Found pinned package in installed packages, need
-        //             to check pin now."; LOG_ERROR << record.version << " vs " <<
-        //             ms.version;
-        //         }
-        //     }
-        // }
-
-        Id match = pool_conda_matchspec(pool, ms.conda_build_form().c_str());
-
-        std::set<Id> matching_solvables;
-        const Channel& c = make_channel(ms.channel);
-
-        for (Id* wp = pool_whatprovides_ptr(pool, match); *wp; wp++)
-        {
-            if (!ms.channel.empty())
-            {
-                if (!channel_match(pool_id2solvable(pool, *wp), c))
-                {
-                    continue;
-                }
-            }
-            matching_solvables.insert(*wp);
-        }
-
-        std::set<Id> all_solvables;
-        Id name_id = pool_str2id(pool, ms.name.c_str(), 1);
-        for (Id* wp = pool_whatprovides_ptr(pool, name_id); *wp; wp++)
-        {
-            all_solvables.insert(*wp);
-        }
-
-        if (all_solvables.size() != 0 && matching_solvables.size() == 0)
-        {
-            throw std::runtime_error(fmt::format("No package can be installed for pin: {}", pin));
-        }
-        m_pinned_specs.push_back(ms);
-
-        solv::ObjQueue selected_pkgs;
-
-        for (auto& id : all_solvables)
-        {
-            if (matching_solvables.find(id) == matching_solvables.end())
-            {
-                // the solvable is _NOT_ matched by our pinning expression! So we have to
-                // lock it to make it un-installable
-                selected_pkgs.push_back(id);
-            }
-        }
-
-        Id d = pool_queuetowhatprovides(pool, selected_pkgs.raw());
-        m_jobs->push_back(SOLVER_LOCK | SOLVER_SOLVABLE_ONE_OF, d);
+        // A previous version of this function would use ``SOLVER_LOCK`` to lock all packages not
+        // matching the pin.
+        // In libsolv, locking means that a package keep the same state: if it is installed,
+        // it remains installed, if not it remains uninstalled.
+        // Locking on a spec applies the lock to all packages matching the spec.
+        // In mamba, we do not want to lock the package because we want to allow other variants
+        // (mathcing the same spec) to unlock more solutions.
+        // For instance we may pin ``libfmt=8.*`` but allow it to be swaped with a version built
+        // by a more recent compiler.
+        // To achieve that, the previous version was locking (out) all solvables not matching the
+        // spec.
+        //
+        // Given that in Mamba, only a single version of a package can be installed at a time,
+        // and that libsolv knows which packages are already installed, it seems that one can
+        // simply add the pin as an extra job in order for it to be treated as a constraint.
+        return add_jobs({ pin }, SOLVER_INSTALL | SOLVER_SOLVABLE_PROVIDES);
     }
 
     void MSolver::add_pins(const std::vector<std::string>& pins)