QueueItemRequire.cc

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: t; c-basic-offset: 4 -*- */
/* QueueItemRequire.cc
 *
 * Copyright (C) 2000-2002 Ximian, Inc.
 * Copyright (C) 2005 SUSE Linux Products GmbH
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
 * 02111-1307, USA.
 */

#include <sstream>

#include "zypp/CapSet.h"
#include "zypp/base/Logger.h"
#include "zypp/base/String.h"
#include "zypp/base/Gettext.h"

#include "zypp/base/Algorithm.h"
#include "zypp/ResPool.h"
#include "zypp/ResFilters.h"
#include "zypp/CapFilters.h"
#include "zypp/ResStatus.h"

#include "zypp/ZYppFactory.h"

#include "zypp/solver/detail/QueueItemRequire.h"
#include "zypp/solver/detail/QueueItemBranch.h"
#include "zypp/solver/detail/QueueItemUninstall.h"
#include "zypp/solver/detail/QueueItemInstall.h"
#include "zypp/solver/detail/QueueItem.h"
#include "zypp/solver/detail/ResolverContext.h"
#include "zypp/solver/detail/ResolverInfoDependsOn.h"
#include "zypp/solver/detail/ResolverInfoMisc.h"
#include "zypp/solver/detail/ResolverInfoNeededBy.h"
#include "zypp/solver/detail/Helper.h"

namespace zypp
{ 

  namespace solver
  { 

    namespace detail
    { 

using namespace std;

IMPL_PTR_TYPE(QueueItemRequire);

//---------------------------------------------------------------------------

std::ostream &
QueueItemRequire::dumpOn( std::ostream & os ) const
{
    os << "[" << (_soft?"Soft":"") << "Require: ";
    os << _capability;
    if (_requiring_item) {
        os << ", Required by " << _requiring_item;
    }
    if (_upgraded_item) {
        os << ", Upgrades " << _upgraded_item;
    }
    if (_lost_item) {
        os << ", Lost " << _lost_item;
    }
    if (_remove_only) os << ", Remove Only";
    return os << "]";
}

//---------------------------------------------------------------------------

QueueItemRequire::QueueItemRequire (const ResPool & pool, const Capability & cap, bool soft)
    : QueueItem (QUEUE_ITEM_TYPE_REQUIRE, pool)
    , _capability (cap)
    , _soft (soft)
    , _remove_only (false)
{
    _XDEBUG("QueueItemRequire::QueueItemRequire(" << cap << (soft?", soft":"") << ")");
}


QueueItemRequire::~QueueItemRequire()
{
}

//---------------------------------------------------------------------------

void
QueueItemRequire::addPoolItem (PoolItem_Ref item)
{
    assert (!_requiring_item);
    _requiring_item = item;
}


//---------------------------------------------------------------------------
struct UniqTable
{
  struct Order
  {
    bool operator()( PoolItem_Ref lhs, PoolItem_Ref rhs ) const
    {
      int res = lhs->name().compare( rhs->name() );
      if ( res )
        return res == -1; // lhs < rhs ?
      // here: lhs == rhs, so compare edition:
      return lhs->edition() < rhs->edition();
    }
  };
  typedef std::set<PoolItem,Order> UTable;


  bool has( PoolItem_Ref item_r ) const
  { return _table.find( item_r ) != _table.end(); }

  void remember( PoolItem_Ref item_r )
  { _table.insert(  item_r ); }


  UTable _table;
};
//---------------------------------------------------------------------------

struct RequireProcess
{
    PoolItem_Ref _requirer;
    const Capability _capability;
    ResolverContext_Ptr _context;
    ResPool _pool;

    PoolItemList providers;             // the provider which matched

    // only keep 'best' candidate for a package name
    typedef map<string,PoolItem_Ref> UniqMap;
    UniqMap uniq;

    RequireProcess (PoolItem_Ref r, const Capability & c, ResolverContext_Ptr ctx, const ResPool & p)
        : _requirer (r)
        , _capability (c)
        , _context (ctx)
        , _pool (p)
    { }

    bool operator()( const CapAndItem & cai )
    {
        PoolItem provider = cai.item;
        Capability match = cai.cap;

        ResStatus status = _context->getStatus( provider );
        PoolItem_Ref upgrades = Helper::findInstalledItem (_pool, provider);

        XXX << "RequireProcessInfo (" << provider << " provides " << match << ", is " << status << ")" << endl;
// ERR << "RequireProcessInfo(required: " << *capability << ")" << endl;
// ERR << "require_process_cb(itemIsPossible -> " <<  context->itemIsPossible (*provider) << ")" << endl;

        /* capability is set for item set childern only. If it is set
           allow only exactly required version */

        if (_capability != Capability()
            && _capability != match) {          // exact match required
            return true;
        }

        if (!provider->arch().compatibleWith( _context->architecture() )) {
            MIL << "provider " << provider << " has incompatible arch '" << provider->arch() << "'" << endl;
            return true;
        }

        if ( upgrades
             && upgrades.resolvable()->arch() != provider->arch()
             && provider->kind() != ResTraits<Atom>::kind )             // if patch provides arch upgrade, allow it (#266178)
                                                                        // (because of #168840 and #170098, the patch parser grabs the 'best' atom
                                                                        //  and does not have knowledge about already installed atom with the same name.
                                                                        //  The problem #266178 shows is a previously installed patch (noarch) and atom (ppc)
                                                                        //  conflict with a later patch which offers an arch upgrade (ppc -> ppc64)
                                                                        //  This has no effect on the patch, since the patch is noarch. But is has effect
                                                                        //  on the atom, since it is installed as ppc and the upgrade is ppc64.
                                                                        //  Here, we look at arch changes only if they don't affect an atom. So atoms are
                                                                        //  allowed for arch upgrades.
                                                                        //  However, this only applies to atoms, not to packages. The package will stay
                                                                        //  at its architecture. Not doing arch upgrades was one of the requirements for Code10.)
        {
            MIL << "provider " << provider << " has OTHER arch '" << provider->arch() << "' than the updated item "
                << upgrades << endl;
            PoolItemList ignore = _context->getIgnoreArchitectureItem();
            PoolItemList::iterator it;
            for (it = ignore.begin(); it != ignore.end(); ++it) {
                if (provider == *it) break;
            }
            if (it != ignore.end()) {
                MIL << " ---> will be ignored " << endl;
            } else {
                return true;
            }
        }

        if (! (status.isToBeUninstalled() || status.isImpossible())
            && ! _context->isParallelInstall( provider )
            && _context->itemIsPossible( provider )
            && ! provider.status().isLocked()
            && ! (provider.status().isKept()
                  &&provider.status().isByUser())
            ) {

            // if we found a to-be-installed provider, choose this and drop all others
            if (status.isToBeInstalled()                        // scheduled for install
                || (status.isUninstalled()
                    && provider.status().isToBeInstalled()))    // or will-be-scheduled 
            {
                providers.clear();
                providers.push_front( provider );
                return false;
            }


            if (!_context->tryAllPossibilities()) {
                // if we already have same name
                //   check for better architecture, then edition
                //   see yast2-pkg-bindings, Package.cc, ProvideProcess

                UniqMap::iterator upos = uniq.find( provider->name() );

                if (upos != uniq.end()) {
                    if ((upos->second->arch().compare( provider->arch() ) < 0)  // better arch
                        || ((upos->second->arch().compare( provider->arch() ) == 0)                 // or same arch
                            && (upos->second->edition().compare( provider->edition() ) < 0) ) ) // and better edition
                    {
                        // new provider is 'better'

                        // erase the old provider
                        for (PoolItemList::iterator it = providers.begin(); it != providers.end(); ++it) {
                            if (*it == upos->second) {
                                _context->setScippedPossibilities( true ); // Flag that there are other possibilities
                                // which we are currently ignore
                                _XDEBUG("Kicking " << *it << " for " << provider)
                                    providers.erase( it );
                                break;
                            }
                        } 
                        upos = uniq.end();      // trigger insertion of new provider below
                    }
                }
                if (upos == uniq.end()) {
                    providers.push_front( provider );
                    uniq[provider->name()] = provider;
                }
            } else {
                // try all alternatives
                providers.push_front( provider );
            }
        }

        return true;
    }
};


struct NoInstallableProviders
{
    PoolItem_Ref requirer;
    ResolverContext_Ptr context;

    bool operator()( const CapAndItem cai)
    {
        PoolItem provider = cai.item;
        Capability match = cai.cap;

        string msg_str;
        //const Capability match;

        ResStatus status = context->getStatus( provider );

        ResolverInfoMisc_Ptr misc_info;

        if (status.isToBeUninstalled()) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_UNINSTALL_PROVIDER, requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);
        } else if (context->isParallelInstall (provider)) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_PARALLEL_PROVIDER, requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);
        } else if (status.isImpossible()
                  || ! context->itemIsPossible (provider)) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NOT_INSTALLABLE_PROVIDER, requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);
        } else if (provider.status().isLocked()) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_LOCKED_PROVIDER, requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);
        } else if (provider.status().isKept() && provider.status().isByUser()) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_KEEP_PROVIDER, requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);         
        } else  if (provider->arch().compatibleWith( context->architecture() )) {
            misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_OTHER_ARCH_PROVIDER,
                                                                   requirer, RESOLVER_INFO_PRIORITY_VERBOSE, match);
            misc_info->setOtherPoolItem (provider);
        }

        if (misc_info != NULL) {
            context->addInfo (misc_info);
        }

        return true;
    }
};

typedef map<string, PoolItem_Ref> UpgradesMap;

struct LookForUpgrades
{
    PoolItem_Ref installed;
    ResolverContext_Ptr _context;    
    UpgradesMap upgrades;

    LookForUpgrades (PoolItem_Ref i, ResolverContext_Ptr ctx)
        : installed (i)
        , _context (ctx)        
    { }

    bool operator()( PoolItem_Ref provider )
    {
        UpgradesMap::iterator it = upgrades.find( provider->name() );

        if (it != upgrades.end()) {                             // provider with same name found
            if (!_context->upgradeMode()
                && it->second->arch() != installed->arch()
                && provider->arch() == installed->arch()) {
                // prefering the same architecture as the installed item
                // NOT in upgrade mode
                it->second = provider;
            } else {
                int cmp = it->second->arch().compare( provider->arch() );
                if ((_context->upgradeMode()                                    // only in upgrade mode
                     || it->second->arch() != installed->arch())                // or have not found the same arch as installed item
                    && cmp < 0) {                                               // new provider has better arch
                    it->second = provider;
                }
                else if (cmp == 0) {                                    // new provider has equal arch
                    if (it->second->edition().compare( provider->edition() ) < 0) {
                        it->second = provider;                          // new provider has better edition
                    }
                }
            }
        } else {
            upgrades[provider->name()] = provider;
        }
        return true;
    }
};


// check for an installed or to-be-installed item
// (used to match supplements/enhances of multiple providers)

struct HintItem
{
    PoolItem_Ref match;

    bool operator()( const CapAndItem & cai )
    {
        if (cai.item.status().staysInstalled()
            || cai.item.status().isToBeInstalled())
        {
            match = cai.item;
            return false;               // have one, we're done
        }
        return true;
    }
};


// check 'codependent' items
//   by looking at the name prefix: 'foo' and 'foo-bar' are codependent

static bool
codependent_items (const PoolItem_Ref item1, const PoolItem_Ref item2)
{
    string name1 = item1->name();
    string name2 = item2->name();
    string::size_type len1 = name1.size();
    string::size_type len2 = name2.size();

    if (len2 < len1) {
        string swap = name1;
        string::size_type swap_len = len1;
        name1 = name2;
        name2 = swap;
        len1 = len2;
        len2 = swap_len;
    }

    // foo and foo-bar are automatically co-dependent
    if (len1 < len2
        && name1.compare (0, len1, name2) == 0
        && name2[len1] == '-') {
        return true;
    }

    return false;
}


// check if we have a match for a (supplements/enhances) hint

static bool
hint_match( const CapSet & cset, ResPool pool )
{
    HintItem info;
    
    for (CapSet::const_iterator cit = cset.begin(); cit != cset.end(); ++cit) {
        Dep dep( Dep::PROVIDES );
        invokeOnEach( pool.byCapabilityIndexBegin( cit->index(), dep ),
                      pool.byCapabilityIndexEnd( cit->index(), dep ),
                      resfilter::ByCapMatch( *cit ),
                      functor::functorRef<bool,CapAndItem>(info) );
    }
    if (info.match) MIL << "hint_match(" << info.match << ")" << endl;
    return info.match; // as bool !
}

//----------------------------------------------------------------------------

bool
QueueItemRequire::process (ResolverContext_Ptr context, QueueItemList & new_items)
{
    _XDEBUG("QueueItemRequire::process(" << *this << ")");

    bool fulfilled = false;
            
    if (_requiring_item)
    {
        fulfilled = context->requirementIsInstalledOrUnneeded (_requiring_item->kind(),_capability);
    } else {
        fulfilled = context->requirementIsMet (_capability);
    }
    
    if (fulfilled) {
        _XDEBUG("requirement is already met in current context");
        return true;
    }

    // checking for ignoring dependencies
    IgnoreMap ignoreMap = context->getIgnoreRequires();
    for (IgnoreMap::iterator it = ignoreMap.begin();
         it != ignoreMap.end(); it++) {
        if ( (!(it->first) // ignore ALL requirements on this capability
              || it->first == _requiring_item)
             && it->second == _capability) {
            _XDEBUG("Found ignoring requires " << _capability << " for " << _requiring_item);
            return true;
        } else {
            _XDEBUG("Ignoring requires " << it->second << " for " <<  it->first << " does not fit");        
        }
    }

    RequireProcess info (_requiring_item,  Capability(), context,  pool());

    int num_providers = 0;

    if (! _remove_only) {

        Dep dep( Dep::PROVIDES );
        XXX << "Look for providers of " << _capability << endl;
        // world->foreachProvidingResItem (_capability, require_process_cb, &info);
        invokeOnEach( pool().byCapabilityIndexBegin( _capability.index(), dep ),
                      pool().byCapabilityIndexEnd( _capability.index(), dep ),
                      resfilter::ByCapMatch( _capability ),
                      functor::functorRef<bool,CapAndItem>(info) );

        _XDEBUG("Look for providers of " << _capability);

        num_providers = info.providers.size();

        _XDEBUG( "requirement is met by " << num_providers << " resolvable");


        // if there are multiple providers, try to reduce branching by cleaning up the providers list
        // first check all providers if they are to-be-installed or uninstalled
        //   if there are to-be-installed providers, erase all uninstalled providers
        //   if there are locale providers and none match, try a language fallback
        //   prefer providers which enhance an installed or to-be-installed resolvables

        if (num_providers > 1) {                                        // prefer to-be-installed providers
#if 1 // see also line 618
            // if there are exactly two providers which differ in architecture
            // prefer the better arch
            // this will reduce the number of branches for X-32bit.x86_64 vs. X.i586 dramatically
            //
            // - left commented out as advised by mls@suse.de, might be problematic on non-x86 archs
            // - Due the new behaviour of the solver this code will be executed.
            //   New: If there is no solution for the "best-architecture" way the solver tries
            //        the other branches too.

            if (num_providers == 2) {
                PoolItemList::iterator it = info.providers.begin();
                PoolItem first( *it++ );
                PoolItem second( *it );

                if (NVRA(first.resolvable()) == NVRA(second.resolvable()))
                {
                    // regarding items with the same NVRA only. Bug238284 
                    int cmp = first->arch().compare( second->arch() );
                    if (cmp < 0) {              // second is better
                        --it;
                    }

                    if (cmp != 0) {
                        info.providers.erase( it );             // erase one of both
                        num_providers = 1;
                        goto provider_done;
                    }
                }
            }
#endif
            MIL << "Have " << num_providers << " providers for " << _capability << endl;
            int to_be_installed = 0;
            int uninstalled = 0;
            std::map<std::string,PoolItem> language_freshens;
            ZYpp::Ptr z = zypp::getZYpp();
            ZYpp::LocaleSet requested_locales = z->getRequestedLocales();
            bool requested_locale_match = false;
            PoolItemSet hints;                  // those which supplement or enhance an installed or to-be-installed

            for (PoolItemList::iterator it = info.providers.begin(); it != info.providers.end(); ++it) {
                PoolItem item = *it;
                if (item.status().isToBeInstalled()) {
                    to_be_installed++;
                }
                if (item.status().staysUninstalled()) {
                    uninstalled++;
                }
                if (!requested_locale_match) {
                    CapSet freshens( item->dep( Dep::FRESHENS ) );

                    // try to find a match of the locale freshens with one of the requested locales
                    //   if we have a match, we're done.

                    for (CapSet::const_iterator cit = freshens.begin(); cit != freshens.end(); ++cit) {
                        if (cit->refers() == ResTraits<Language>::kind) {
                            string loc = cit->index();
                            MIL << "Look for language fallback " << loc << ":" << item << endl;
                            if (requested_locales.find( Locale( loc ) ) != requested_locales.end()) {
                                MIL << "Locale '" << loc << "' is requested, not looking further" << endl;
                                requested_locale_match = true;
                                break;
                            }
                            language_freshens[loc] = item;
                        }
                    }
                }


                // now check if a provider supplements or enhances an installed or to-be-installed resolvable

                if (hint_match( item->dep( Dep::SUPPLEMENTS ), pool() )
                    || hint_match( item->dep( Dep::ENHANCES ), pool() ))
                {
                    hints.insert( item );
                }

            } // for

            if (hints.empty()
                && to_be_installed == 0
                && !requested_locale_match)
            {
                // loop over requested locales, loop over their fallbacks, and try to find a matching provider

                for (ZYpp::LocaleSet::const_iterator rit = requested_locales.begin(); rit != requested_locales.end(); ++rit) {

                    // loop over possible fallbacks
                    Locale l = *rit;
                    for (;;) {
                        Locale fallback = l.fallback();
                        if (fallback == Locale::noCode
                            || fallback == l)
                        {
                            break;
                        }
                        MIL << "requested " << l << " fallback " << fallback << endl;
                        std::map<std::string,PoolItem>::const_iterator match = language_freshens.find( fallback.code() );
                        if (match != language_freshens.end()) {
                            MIL << match->second << " matches the fallback" << endl;
                            info.providers.clear();
                            info.providers.push_back( match->second );
                            break;
                        }
                        l = fallback;
                    }
                }
#if 0   // just debug
                std::string mil = "language_freshens ";
                for (std::map<std::string,PoolItem>::const_iterator it = language_freshens.begin(); it != language_freshens.end(); ++it) {
                    if (it != language_freshens.begin()) mil += ", ";
                    mil += it->first;
                }
                MIL << mil << endl;
#endif
            }
            else if (to_be_installed == 0
                     && !hints.empty())
            {
                MIL << "Have " << hints.size() << " hints" << endl;
                info.providers.clear();
                for (PoolItemSet::const_iterator it = hints.begin(); it != hints.end(); ++it)
                    info.providers.push_back( *it );
            }
            else { 

            // if we have one or more to-be-installed, erase all uninstalled (if there are any)

            MIL << to_be_installed << " to-be-installed, " << uninstalled << " uninstalled" << endl;

            if (to_be_installed > 0
                && uninstalled > 0)
            {
                PoolItemList::iterator next;
                for (PoolItemList::iterator it = info.providers.begin(); it != info.providers.end(); ++it) {
                    next = it; ++next;
                    if (it->status().staysUninstalled()) {
                        MIL << "Not considering " << *it << endl;
                        info.providers.erase (it);
                    }
                    it = next;
                }
            }
            }

            num_providers = info.providers.size();

        } // num_providers > 1
#if 1 // see also line 474
provider_done:;
#endif
    } // !_remove_only

    //
    // No providers found
    //

    if (num_providers == 0) {

        if (_soft) goto finished;                       // don't care for soft requires

        _DEBUG( "Unfulfilled requirement '" << _capability << "'. trying different solution");

        QueueItemUninstall_Ptr uninstall_item = NULL;
        QueueItemBranch_Ptr branch_item = NULL;
        bool explore_uninstall_branch = true;

        

        if (!_upgraded_item
            || _lost_item)
        {
            ResolverInfo_Ptr err_info;
            NoInstallableProviders info;
            info.requirer = _requiring_item;
            info.context = context;

            // Maybe we can add some extra info on why none of the providers are suitable.

            // pool()->foreachProvidingResItem (_capability, no_installable_providers_info_cb, (void *)&info);

            Dep dep( Dep::PROVIDES );

            invokeOnEach( pool().byCapabilityIndexBegin( _capability.index(), dep ), // begin()
                          pool().byCapabilityIndexEnd( _capability.index(), dep ),   // end()
                          resfilter::ByCapMatch( _capability ),
                          functor::functorRef<bool,CapAndItem>(info) );

        }
        //
        // If this is an upgrade, we might be able to avoid removing stuff by upgrading it instead.
        //

        if ((_upgraded_item
             || _lost_item // foo-devel requires foo; foo is lost due obsolete --> try to upgrade foo-devel
             || context->verifying()) // We are in the verify mode. So there could be already missing requirements
                                      // So try to solve it via update
            && _requiring_item)
        {

            LookForUpgrades info (_requiring_item, context);

//          pool()->foreachUpgrade (_requiring_item, new Channel(CHANNEL_TYPE_ANY), look_for_upgrades_cb, (void *)&upgrade_list);

            invokeOnEach( pool().byNameBegin( _requiring_item->name() ), pool().byNameEnd( _requiring_item->name() ),
                          functor::chain (resfilter::ByKind( _requiring_item->kind() ),
                                          resfilter::byEdition<CompareByGT<Edition> >( _requiring_item->edition() ) ),
                                          functor::functorRef<bool,PoolItem>(info) );

            if (!info.upgrades.empty()) {
                string label;

                branch_item = new QueueItemBranch (pool());

                ostringstream req_str;  req_str << _requiring_item;
                ostringstream up_str;
                if (_upgraded_item)
                    up_str << _upgraded_item;
                else
                    up_str << _requiring_item;
                ostringstream cap_str; cap_str << _capability;

                 // Translator: 1.%s = dependency; 2.%s and 3.%s = name of package,patch,...
                label = str::form (_("for requiring %s for %s when upgrading %s"),
                                   cap_str.str().c_str(), req_str.str().c_str(), up_str.str().c_str());
                branch_item->setLabel (label);
                _DEBUG("Branching: " + label)

                for (UpgradesMap::const_iterator iter = info.upgrades.begin(); iter != info.upgrades.end(); ++iter) {
                    PoolItem_Ref upgrade_item = iter->second;
                    QueueItemInstall_Ptr install_item;

                    if (context->itemIsPossible (upgrade_item)) {

                        install_item = new QueueItemInstall (pool(), upgrade_item, _soft);
                        install_item->setUpgrades (_requiring_item);
                        branch_item->addItem (install_item);

                        ResolverInfoNeededBy_Ptr upgrade_info = new ResolverInfoNeededBy (upgrade_item);
                        if (_upgraded_item)
                            upgrade_info->addRelatedPoolItem (_upgraded_item);
                        install_item->addInfo (upgrade_info);

                        // If an upgrade item has its requirements met, don't do the uninstall branch.
                        //   FIXME: should we also look at conflicts here?

                        if (explore_uninstall_branch) {
                            CapSet requires = upgrade_item->dep (Dep::REQUIRES);
                            CapSet::const_iterator iter = requires.begin();
                            for (; iter != requires.end(); iter++) {
                                const Capability req = *iter;
                                if (! context->requirementIsMet (req)) {
                                        break;
                                }
                            }
                            if (iter == requires.end()) {
                                explore_uninstall_branch = false;
                            }
                        }

                    } /* if (context->itemIsPossible ( ... */
                } /* for (iter = upgrade_list; ... */
            } /* if (info.upgrades) ... */

            if (!info.upgrades.empty()
                && branch_item->isEmpty ())
            {

                for (UpgradesMap::const_iterator iter = info.upgrades.begin(); iter != info.upgrades.end(); ++iter) {
                    ResolverInfoMisc_Ptr misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NO_UPGRADE, _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE);
                    if (iter == info.upgrades.begin()) {
                        misc_info->setOtherPoolItem( iter->second );
                    }
                    misc_info->addRelatedPoolItem( iter->second );
                    context->addInfo( misc_info );

                    explore_uninstall_branch = true;
                }

                //
                //  The exception: we always want to consider uninstalling
                //  when the requirement has resulted from a item losing
                //  one of it's provides.

            } else if (!info.upgrades.empty()
                       && explore_uninstall_branch
                       && _requiring_item
                       && _upgraded_item
                       && codependent_items (_requiring_item, _upgraded_item)
                       && !_lost_item)
            {
                explore_uninstall_branch = false;
            }

        } /* if (_upgrade_item && _requiring_item) ... */

        ResStatus status = context->getStatus(_requiring_item);
        
        if (context->verifying()) {
            // We always consider uninstalling when in verification mode.
            explore_uninstall_branch = true;
        }
        else if (status.isToBeInstalled()           // scheduled for installation
                 && !status.isToBeUninstalled()     // not scheduled for uninstallation
                 || _requiring_item.status().staysInstalled()) // not scheduled at all but installed
        {
            // The item has to be set for installing/updating explicity.
            // So the uninstall branch could be not useful if upgrade is not successful.
            // Adding the info for solution handling at the end of solving
            ResolverInfo_Ptr misc_info;
            if (!_upgraded_item) {
                if (_remove_only) {
                    misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NO_OTHER_PROVIDER,
                                                     _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE, _capability);
                } else {
                    misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NO_PROVIDER,
                                                     _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE, _capability);
                }
            } else {
                misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_CANT_SATISFY,
                                                  _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE,
                                                  _capability);
            }       
            context->addInfo (misc_info);
        }

        if (explore_uninstall_branch && _requiring_item) {
            ResolverInfo_Ptr log_info;
            uninstall_item = new QueueItemUninstall (pool(), _requiring_item, QueueItemUninstall::UNSATISFIED);
            uninstall_item->setCapability (_capability);

            if (_lost_item) {
                log_info = new ResolverInfoDependsOn (_requiring_item, _lost_item);
                uninstall_item->addInfo (log_info);
            }

            if (_remove_only)
                uninstall_item->setRemoveOnly ();
        }

        if (uninstall_item && branch_item) {
            branch_item->addItem (uninstall_item);
            new_items.push_back (branch_item);
        } else if (uninstall_item) {
            new_items.push_front (uninstall_item);
        } else if (branch_item) {
            new_items.push_back (branch_item);
        } else {
            // We can't do anything to resolve the missing requirement, so we fail.
            ResolverInfo_Ptr misc_info;
            if (!_upgraded_item) {
                if (_remove_only) {
                    misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NO_OTHER_PROVIDER,
                                                     _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE, _capability);
                } else {
                    misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_NO_PROVIDER,
                                                     _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE, _capability);
                }
            } else {
                ResolverInfo_Ptr misc_info = new ResolverInfoMisc (RESOLVER_INFO_TYPE_CANT_SATISFY,
                                                                   _requiring_item, RESOLVER_INFO_PRIORITY_VERBOSE,
                                                                   _capability);
            }
            context->addError (misc_info);
        }

    }

    //
    // exactly 1 provider found
    //

    else if (num_providers == 1) {

        _XDEBUG( "Found exactly one resolvable, installing it.");

        PoolItem item = info.providers.front();

        // if this a soft require (a recommends), don't override a KEEP_STATE (-> bug #154650)
        // see also below
        if (_soft
            && item.status().isUninstalled()
            && !item.status().maySetSoftTransact( true, ResStatus::SOLVER ) )
        {
            _DEBUG("Can't soft-transact " << item);
            goto finished;
        }
        QueueItemInstall_Ptr install_item = new QueueItemInstall (pool(), item, _soft);
        install_item->addDependency (_capability);

        // The requiring item could be NULL if the requirement was added as an extra dependency.
        if (_requiring_item) {
            install_item->addNeededBy (_requiring_item);
        }
        new_items.push_front (install_item);

    }

    //
    // multiple providers found
    //

    else if (num_providers > 1) {

        _DEBUG( "Branching: Found more than one provider of " << _capability);

        QueueItemBranch_Ptr branch_item = new QueueItemBranch( pool() );

        for (PoolItemList::const_iterator iter = info.providers.begin(); iter != info.providers.end(); iter++) {

            PoolItem item = *iter;

            // if this a soft require (a recommends), don't override a KEEP_STATE (-> bug #154650)
            // see also above
            if (_soft
                && item.status().isUninstalled()
                && !item.status().maySetSoftTransact( true, ResStatus::SOLVER ) )
            {
                _DEBUG("Can't soft-transact " << item);
                continue;
            }

            QueueItemInstall_Ptr install_item = new QueueItemInstall( pool(), item, _soft );
            install_item->addDependency( _capability );
            branch_item->addItem( install_item );

            // The requiring item could be NULL if the requirement was added as an extra dependency.
            if (_requiring_item) {
                install_item->addNeededBy( _requiring_item );
            }
        }

        if (!branch_item->isEmpty())
            new_items.push_back (branch_item);

    } else {
        abort ();
    }

finished:

    return true;
}

//---------------------------------------------------------------------------

QueueItem_Ptr
QueueItemRequire::copy (void) const
{
    QueueItemRequire_Ptr new_require = new QueueItemRequire (pool(), _capability);

    new_require->QueueItem::copy(this);

    new_require->_requiring_item = _requiring_item;
    new_require->_upgraded_item  = _upgraded_item;
    new_require->_remove_only    = _remove_only;

    return new_require;
}


int
QueueItemRequire::cmp (QueueItem_constPtr item) const
{
    int cmp = this->compare (item);             // assures equal type
    if (cmp != 0)
        return cmp;

    QueueItemRequire_constPtr require = dynamic_pointer_cast<const QueueItemRequire>(item);

    if (_capability != require->capability())
    {
        cmp = -1;
    }
    return cmp;
}

    };// namespace detail
  };// namespace solver
};// namespace zypp

---