Make merge checks its own phase

Fixes: ticket:1085

23 files changed, 262 insertions, 145 deletions

diff --git a/paludis/repositories/accounts/accounts_id.cc b/paludis/repositories/accounts/accounts_id.cc
index 1b196e4..6755da6 100644
--- a/paludis/repositories/accounts/accounts_id.cc
+++ b/paludis/repositories/accounts/accounts_id.cc
@@ -504,24 +504,46 @@ AccountsID::perform_action(Action & action) const
 
     std::string used_config_protect;
 
+    MergeParams merge_params(make_named_values<MergeParams>(
+                n::build_start_time() = build_start_time,
+                n::check() = true,
+                n::environment_file() = FSPath("/dev/null"),
+                n::image_dir() = fs_location_key()->value(),
+                n::merged_entries() = std::make_shared<FSPathSet>(),
+                n::options() = MergerOptions() + mo_rewrite_symlinks + mo_allow_empty_dirs,
+                n::output_manager() = output_manager,
+                n::package_id() = shared_from_this(),
+                n::perform_uninstall() = install_action->options.perform_uninstall(),
+                n::replacing() = install_action->options.replacing(),
+                n::used_this_for_config_protect() = std::bind(
+                    &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
+                ));
+
+    switch (install_action->options.want_phase()("check_merge"))
+    {
+        case wp_yes:
+            {
+                merge_params.check() = true;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
+            }
+            break;
+
+        case wp_skip:
+            break;
+
+        case wp_abort:
+            throw ActionAbortedError("Told to abort install");
+
+        case last_wp:
+            throw InternalError(PALUDIS_HERE, "bad WantPhase");
+    }
+
     switch (install_action->options.want_phase()("merge"))
     {
         case wp_yes:
             {
-                (*install_action->options.destination()).destination_interface()->merge(
-                        make_named_values<MergeParams>(
-                            n::build_start_time() = build_start_time,
-                            n::environment_file() = FSPath("/dev/null"),
-                            n::image_dir() = fs_location_key()->value(),
-                            n::merged_entries() = std::make_shared<FSPathSet>(),
-                            n::options() = MergerOptions() + mo_rewrite_symlinks + mo_allow_empty_dirs,
-                            n::output_manager() = output_manager,
-                            n::package_id() = shared_from_this(),
-                            n::perform_uninstall() = install_action->options.perform_uninstall(),
-                            n::replacing() = install_action->options.replacing(),
-                            n::used_this_for_config_protect() = std::bind(
-                                &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
-                            ));
+                merge_params.check() = false;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
             }
             break;
 
diff --git a/paludis/repositories/accounts/accounts_repository.cc b/paludis/repositories/accounts/accounts_repository.cc
index 1ca62ac..a84d72a 100644
--- a/paludis/repositories/accounts/accounts_repository.cc
+++ b/paludis/repositories/accounts/accounts_repository.cc
@@ -426,6 +426,9 @@ AccountsRepository::merge(const MergeParams & m)
     if (! _imp->handler_if_installed)
         throw ActionFailedError("Can't merge to here");
 
+    if (m.check())
+        return;
+
     _imp->handler_if_installed->merge(m);
 }
 
diff --git a/paludis/repositories/e/do_install_action.cc b/paludis/repositories/e/do_install_action.cc
index f804ac0..d123490 100644
--- a/paludis/repositories/e/do_install_action.cc
+++ b/paludis/repositories/e/do_install_action.cc
@@ -275,7 +275,7 @@ paludis::erepository::do_install_action(
             continue;
         }
 
-        if (phase->option("merge"))
+        if (phase->option("merge") || phase->option("check_merge"))
         {
             if (! (*install_action.options.destination()).destination_interface())
                 throw ActionFailedError("Can't install '" + stringify(*id)
@@ -290,6 +290,7 @@ paludis::erepository::do_install_action(
             (*install_action.options.destination()).destination_interface()->merge(
                     make_named_values<MergeParams>(
                         n::build_start_time() = build_start_time,
+                        n::check() = phase->option("check_merge"),
                         n::environment_file() = package_builddir / "temp" / "loadsaveenv",
                         n::image_dir() = package_builddir / "image",
                         n::merged_entries() = merged_entries,
diff --git a/paludis/repositories/e/e_repository.cc b/paludis/repositories/e/e_repository.cc
index acd9524..eda10b2 100644
--- a/paludis/repositories/e/e_repository.cc
+++ b/paludis/repositories/e/e_repository.cc
@@ -1889,8 +1889,12 @@ ERepository::merge(const MergeParams & m)
                 n::tar_file() = _imp->params.binary_distdir() / (bin_dist_base + pbin_tar_extension)
             ));
 
-    if (! merger.check())
-        throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+    if (m.check())
+    {
+        if (! merger.check())
+            throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        return;
+    }
 
     merger.merge();
 
diff --git a/paludis/repositories/e/eapis/0.conf b/paludis/repositories/e/eapis/0.conf
index 55848ea..bd9f700 100644
--- a/paludis/repositories/e/eapis/0.conf
+++ b/paludis/repositories/e/eapis/0.conf
@@ -117,6 +117,7 @@ ebuild_install = \
     skipname=install skipifno=install sandbox clearenv : loadenv install saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/2.conf b/paludis/repositories/e/eapis/2.conf
index c841935..37a7385 100644
--- a/paludis/repositories/e/eapis/2.conf
+++ b/paludis/repositories/e/eapis/2.conf
@@ -24,6 +24,7 @@ ebuild_install = \
     skipname=install skipifno=install sandbox clearenv : loadenv install saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/4.conf b/paludis/repositories/e/eapis/4.conf
index a346fc2..006a2d9 100644
--- a/paludis/repositories/e/eapis/4.conf
+++ b/paludis/repositories/e/eapis/4.conf
@@ -20,6 +20,7 @@ ebuild_install = \
     skipname=install skipifno=*sources,unpack,prepare,configure,compile,test,install sandbox clearenv : loadenv install saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/exheres-0.conf b/paludis/repositories/e/eapis/exheres-0.conf
index 34f5233..318d731 100644
--- a/paludis/repositories/e/eapis/exheres-0.conf
+++ b/paludis/repositories/e/eapis/exheres-0.conf
@@ -149,6 +149,7 @@ ebuild_install = \
     skipname=install skipifno=*sources,unpack,prepare,configure,compile,test,install sydbox clearenv : loadenv install saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst skipifno=preinst sydbox clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst skipifno=postinst sydbox clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/paludis-1.conf b/paludis/repositories/e/eapis/paludis-1.conf
index fef8b79..7416c61 100644
--- a/paludis/repositories/e/eapis/paludis-1.conf
+++ b/paludis/repositories/e/eapis/paludis-1.conf
@@ -119,6 +119,7 @@ ebuild_install = \
     skipname=install skipifno=install sandbox clearenv : loadenv install saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst skipifno=preinst sandbox clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst skipifno=postinst sandbox clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+0.conf b/paludis/repositories/e/eapis/pbin-1+0.conf
index dcfac15..ff471d5 100644
--- a/paludis/repositories/e/eapis/pbin-1+0.conf
+++ b/paludis/repositories/e/eapis/pbin-1+0.conf
@@ -33,6 +33,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+1.conf b/paludis/repositories/e/eapis/pbin-1+1.conf
index da518a4..bfebb90 100644
--- a/paludis/repositories/e/eapis/pbin-1+1.conf
+++ b/paludis/repositories/e/eapis/pbin-1+1.conf
@@ -33,6 +33,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+2.conf b/paludis/repositories/e/eapis/pbin-1+2.conf
index bd8f979..0c57a14 100644
--- a/paludis/repositories/e/eapis/pbin-1+2.conf
+++ b/paludis/repositories/e/eapis/pbin-1+2.conf
@@ -33,6 +33,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+3.conf b/paludis/repositories/e/eapis/pbin-1+3.conf
index 9fed0bf..60bcb98 100644
--- a/paludis/repositories/e/eapis/pbin-1+3.conf
+++ b/paludis/repositories/e/eapis/pbin-1+3.conf
@@ -34,6 +34,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+4.conf b/paludis/repositories/e/eapis/pbin-1+4.conf
index 3965443..210964c 100644
--- a/paludis/repositories/e/eapis/pbin-1+4.conf
+++ b/paludis/repositories/e/eapis/pbin-1+4.conf
@@ -35,6 +35,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+exheres-0.conf b/paludis/repositories/e/eapis/pbin-1+exheres-0.conf
index 36500e4..3ccfb5b 100644
--- a/paludis/repositories/e/eapis/pbin-1+exheres-0.conf
+++ b/paludis/repositories/e/eapis/pbin-1+exheres-0.conf
@@ -32,6 +32,7 @@ ebuild_install = \
     skipname=installbin sydbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/eapis/pbin-1+paludis-1.conf b/paludis/repositories/e/eapis/pbin-1+paludis-1.conf
index fc315ac..2a00c62 100644
--- a/paludis/repositories/e/eapis/pbin-1+paludis-1.conf
+++ b/paludis/repositories/e/eapis/pbin-1+paludis-1.conf
@@ -31,6 +31,7 @@ ebuild_install = \
     skipname=installbin sandbox clearenv : loadenv installbin saveenv ; \
     skipname=strip strip : ; \
     skipname=preinst clearenv prepost : loadenv preinst saveenv ; \
+    skipname=check_merge check_merge : ; \
     skipname=merge merge : ; \
     skipname=postinst clearenv prepost : loadenv postinst saveenv ; \
     skipname=tidyup tidyup clearenv : loadenv tidyup
diff --git a/paludis/repositories/e/exndbam_repository.cc b/paludis/repositories/e/exndbam_repository.cc
index 7b49622..152e754 100644
--- a/paludis/repositories/e/exndbam_repository.cc
+++ b/paludis/repositories/e/exndbam_repository.cc
@@ -359,9 +359,11 @@ ExndbamRepository::merge(const MergeParams & m)
     {
         std::string uid(stringify(m.package_id()->name().category()) + "---" + stringify(m.package_id()->name().package()));
         uid_dir /= "data";
-        uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
+        if (! m.check())
+            uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
         uid_dir /= uid;
-        uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
+        if (! m.check())
+            uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
     }
 
     FSPath target_ver_dir(uid_dir);
@@ -370,39 +372,43 @@ ExndbamRepository::merge(const MergeParams & m)
     if (target_ver_dir.stat().exists())
         throw ActionFailedError("Temporary merge directory '" + stringify(target_ver_dir) + "' already exists, probably "
                 "due to a previous failed install. If it is safe to do so, please remove this directory and try again.");
-    target_ver_dir.mkdir(0755, { });
+    if (! m.check())
+        target_ver_dir.mkdir(0755, { });
 
-    WriteVDBEntryCommand write_vdb_entry_command(
-            make_named_values<WriteVDBEntryParams>(
-                n::environment() = _imp->params.environment(),
-                n::environment_file() = m.environment_file(),
-                n::maybe_output_manager() = m.output_manager(),
-                n::output_directory() = target_ver_dir,
-                n::package_id() = std::static_pointer_cast<const ERepositoryID>(m.package_id())
-            ));
+    std::string config_protect, config_protect_mask;
+    if (! m.check())
+    {
+        WriteVDBEntryCommand write_vdb_entry_command(
+                make_named_values<WriteVDBEntryParams>(
+                    n::environment() = _imp->params.environment(),
+                    n::environment_file() = m.environment_file(),
+                    n::maybe_output_manager() = m.output_manager(),
+                    n::output_directory() = target_ver_dir,
+                    n::package_id() = std::static_pointer_cast<const ERepositoryID>(m.package_id())
+                ));
 
-    write_vdb_entry_command();
+        write_vdb_entry_command();
 
-    _imp->ndbam.add_entry(m.package_id()->name(), target_ver_dir);
+        _imp->ndbam.add_entry(m.package_id()->name(), target_ver_dir);
 
-    /* load CONFIG_PROTECT, CONFIG_PROTECT_MASK back */
-    std::string config_protect, config_protect_mask;
-    try
-    {
-        SafeIFStream c(target_ver_dir / "CONFIG_PROTECT");
-        config_protect = std::string((std::istreambuf_iterator<char>(c)), std::istreambuf_iterator<char>());
-    }
-    catch (const SafeIFStreamError &)
-    {
-    }
+        /* load CONFIG_PROTECT, CONFIG_PROTECT_MASK back */
+        try
+        {
+            SafeIFStream c(target_ver_dir / "CONFIG_PROTECT");
+            config_protect = std::string((std::istreambuf_iterator<char>(c)), std::istreambuf_iterator<char>());
+        }
+        catch (const SafeIFStreamError &)
+        {
+        }
 
-    try
-    {
-        SafeIFStream c_m(target_ver_dir / "CONFIG_PROTECT_MASK");
-        config_protect_mask = std::string((std::istreambuf_iterator<char>(c_m)), std::istreambuf_iterator<char>());
-    }
-    catch (const SafeIFStreamError &)
-    {
+        try
+        {
+            SafeIFStream c_m(target_ver_dir / "CONFIG_PROTECT_MASK");
+            config_protect_mask = std::string((std::istreambuf_iterator<char>(c_m)), std::istreambuf_iterator<char>());
+        }
+        catch (const SafeIFStreamError &)
+        {
+        }
     }
 
     bool fix_mtimes(std::static_pointer_cast<const ERepositoryID>(
@@ -427,12 +433,11 @@ ExndbamRepository::merge(const MergeParams & m)
 
     (m.used_this_for_config_protect())(config_protect);
 
-    if (! merger.check())
+    if (m.check())
     {
-        for (FSIterator d(target_ver_dir, { fsio_inode_sort, fsio_include_dotfiles }), d_end ; d != d_end ; ++d)
-            d->unlink();
-        target_ver_dir.rmdir();
-        throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        if (! merger.check())
+            throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        return;
     }
 
     merger.merge();
diff --git a/paludis/repositories/e/vdb_repository.cc b/paludis/repositories/e/vdb_repository.cc
index 9042468..c8e1954 100644
--- a/paludis/repositories/e/vdb_repository.cc
+++ b/paludis/repositories/e/vdb_repository.cc
@@ -893,44 +893,49 @@ VDBRepository::merge(const MergeParams & m)
 
     std::shared_ptr<const ERepositoryID> is_replace(package_id_if_exists(m.package_id()->name(), m.package_id()->version()));
 
+    std::string config_protect, config_protect_mask;
+
     FSPath tmp_vdb_dir(_imp->params.location());
-    if (! tmp_vdb_dir.stat().exists())
-        tmp_vdb_dir.mkdir(0755, { });
     tmp_vdb_dir /= stringify(m.package_id()->name().category());
-    if (! tmp_vdb_dir.stat().exists())
-        tmp_vdb_dir.mkdir(0755, { });
     tmp_vdb_dir /= ("-checking-" + stringify(m.package_id()->name().package()) + "-" + stringify(m.package_id()->version()));
-    tmp_vdb_dir.mkdir(0755, { });
 
-    WriteVDBEntryCommand write_vdb_entry_command(
-            make_named_values<WriteVDBEntryParams>(
-                n::environment() = _imp->params.environment(),
-                n::environment_file() = m.environment_file(),
-                n::maybe_output_manager() = m.output_manager(),
-                n::output_directory() = tmp_vdb_dir,
-                n::package_id() = std::static_pointer_cast<const ERepositoryID>(m.package_id())
-            ));
+    if (! m.check())
+    {
+        if (! tmp_vdb_dir.dirname().dirname().stat().exists())
+            tmp_vdb_dir.dirname().dirname().mkdir(0755, { });
+        if (! tmp_vdb_dir.dirname().stat().exists())
+            tmp_vdb_dir.dirname().mkdir(0755, { });
+        tmp_vdb_dir.mkdir(0755, { });
 
-    write_vdb_entry_command();
+        WriteVDBEntryCommand write_vdb_entry_command(
+                make_named_values<WriteVDBEntryParams>(
+                    n::environment() = _imp->params.environment(),
+                    n::environment_file() = m.environment_file(),
+                    n::maybe_output_manager() = m.output_manager(),
+                    n::output_directory() = tmp_vdb_dir,
+                    n::package_id() = std::static_pointer_cast<const ERepositoryID>(m.package_id())
+                ));
 
-    /* load CONFIG_PROTECT, CONFIG_PROTECT_MASK from vdb */
-    std::string config_protect, config_protect_mask;
-    try
-    {
-        SafeIFStream c(tmp_vdb_dir / "CONFIG_PROTECT");
-        config_protect = std::string((std::istreambuf_iterator<char>(c)), std::istreambuf_iterator<char>());
-    }
-    catch (const SafeIFStreamError &)
-    {
-    }
+        write_vdb_entry_command();
 
-    try
-    {
-        SafeIFStream c_m(tmp_vdb_dir / "CONFIG_PROTECT_MASK");
-        config_protect_mask = std::string((std::istreambuf_iterator<char>(c_m)), std::istreambuf_iterator<char>());
-    }
-    catch (const SafeIFStreamError &)
-    {
+        /* load CONFIG_PROTECT, CONFIG_PROTECT_MASK from vdb */
+        try
+        {
+            SafeIFStream c(tmp_vdb_dir / "CONFIG_PROTECT");
+            config_protect = std::string((std::istreambuf_iterator<char>(c)), std::istreambuf_iterator<char>());
+        }
+        catch (const SafeIFStreamError &)
+        {
+        }
+
+        try
+        {
+            SafeIFStream c_m(tmp_vdb_dir / "CONFIG_PROTECT_MASK");
+            config_protect_mask = std::string((std::istreambuf_iterator<char>(c_m)), std::istreambuf_iterator<char>());
+        }
+        catch (const SafeIFStreamError &)
+        {
+        }
     }
 
     FSPath vdb_dir(_imp->params.location());
@@ -957,12 +962,11 @@ VDBRepository::merge(const MergeParams & m)
 
     (m.used_this_for_config_protect())(config_protect);
 
-    if (! merger.check())
+    if (m.check())
     {
-        for (FSIterator d(tmp_vdb_dir, { fsio_include_dotfiles, fsio_inode_sort }), d_end ; d != d_end ; ++d)
-            d->unlink();
-        tmp_vdb_dir.rmdir();
-        throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        if (! merger.check())
+            throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        return;
     }
 
     if (is_replace)
diff --git a/paludis/repositories/repository/repository_repository.cc b/paludis/repositories/repository/repository_repository.cc
index 3a59887..bd30f8a 100644
--- a/paludis/repositories/repository/repository_repository.cc
+++ b/paludis/repositories/repository/repository_repository.cc
@@ -442,6 +442,9 @@ RepositoryRepository::merge(const MergeParams & m)
     if (! is_suitable_destination_for(m.package_id()))
         throw ActionFailedError("Not a suitable destination for '" + stringify(*m.package_id()) + "'");
 
+    if (m.check())
+        return;
+
     std::string repo_sync(get_string_key(m.package_id(), "REPOSITORY_SYNC"));
     std::string repo_format(get_string_key(m.package_id(), "REPOSITORY_FORMAT"));
     std::string repo_name(stringify(m.package_id()->name().package()));
diff --git a/paludis/repositories/unavailable/unavailable_repository_id.cc b/paludis/repositories/unavailable/unavailable_repository_id.cc
index 7a1fc26..6875c5f 100644
--- a/paludis/repositories/unavailable/unavailable_repository_id.cc
+++ b/paludis/repositories/unavailable/unavailable_repository_id.cc
@@ -219,24 +219,46 @@ UnavailableRepositoryID::perform_action(Action & action) const
     std::shared_ptr<OutputManager> output_manager(install_action->options.make_output_manager()(*install_action));
     std::string used_config_protect;
 
+    MergeParams merge_params(make_named_values<MergeParams>(
+                n::build_start_time() = build_start_time,
+                n::check() = true,
+                n::environment_file() = FSPath("/dev/null"),
+                n::image_dir() = FSPath("/dev/null"),
+                n::merged_entries() = std::make_shared<FSPathSet>(),
+                n::options() = MergerOptions(),
+                n::output_manager() = output_manager,
+                n::package_id() = shared_from_this(),
+                n::perform_uninstall() = install_action->options.perform_uninstall(),
+                n::replacing() = install_action->options.replacing(),
+                n::used_this_for_config_protect() = std::bind(
+                    &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
+                ));
+
+    switch (install_action->options.want_phase()("check_merge"))
+    {
+        case wp_yes:
+            {
+                merge_params.check() = true;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
+            }
+            break;
+
+        case wp_skip:
+            break;
+
+        case wp_abort:
+            throw ActionAbortedError("Told to abort install");
+
+        case last_wp:
+            throw InternalError(PALUDIS_HERE, "bad WantPhase");
+    }
+
     switch (install_action->options.want_phase()("merge"))
     {
         case wp_yes:
             {
-                (*install_action->options.destination()).destination_interface()->merge(
-                        make_named_values<MergeParams>(
-                            n::build_start_time() = build_start_time,
-                            n::environment_file() = FSPath("/dev/null"),
-                            n::image_dir() = FSPath("/dev/null"),
-                            n::merged_entries() = std::make_shared<FSPathSet>(),
-                            n::options() = MergerOptions(),
-                            n::output_manager() = output_manager,
-                            n::package_id() = shared_from_this(),
-                            n::perform_uninstall() = install_action->options.perform_uninstall(),
-                            n::replacing() = install_action->options.replacing(),
-                            n::used_this_for_config_protect() = std::bind(
-                                    &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
-                            ));
+                merge_params.check() = false;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
             }
             break;
 
diff --git a/paludis/repositories/unpackaged/installed_repository.cc b/paludis/repositories/unpackaged/installed_repository.cc
index dd76324..66a8bd1 100644
--- a/paludis/repositories/unpackaged/installed_repository.cc
+++ b/paludis/repositories/unpackaged/installed_repository.cc
@@ -317,10 +317,14 @@ InstalledUnpackagedRepository::merge(const MergeParams & m)
     else
     {
         std::string uid(stringify(m.package_id()->name().category()) + "---" + stringify(m.package_id()->name().package()));
+
         uid_dir /= "data";
-        uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
+        if (! m.check())
+            uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
+
         uid_dir /= uid;
-        uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
+        if (! m.check())
+            uid_dir.mkdir(0755, { fspmkdo_ok_if_exists });
     }
 
     FSPath target_ver_dir(uid_dir);
@@ -329,29 +333,34 @@ InstalledUnpackagedRepository::merge(const MergeParams & m)
     if (target_ver_dir.stat().exists())
         throw ActionFailedError("Temporary merge directory '" + stringify(target_ver_dir) + "' already exists, probably "
                 "due to a previous failed install. If it is safe to do so, please remove this directory and try again.");
-    target_ver_dir.mkdir(0755, { });
 
-    {
-        SafeOFStream source_repository_file(target_ver_dir / "source_repository", -1, true);
-        source_repository_file << m.package_id()->repository_name() << std::endl;
-    }
+    if (! m.check())
+        target_ver_dir.mkdir(0755, { });
 
-    if (m.package_id()->short_description_key())
+    if (! m.check())
     {
-        SafeOFStream description_file(target_ver_dir / "description", -1, true);
-        description_file << m.package_id()->short_description_key()->value() << std::endl;
-    }
+        {
+            SafeOFStream source_repository_file(target_ver_dir / "source_repository", -1, true);
+            source_repository_file << m.package_id()->repository_name() << std::endl;
+        }
 
-    if (m.package_id()->build_dependencies_key())
-    {
-        SafeOFStream build_dependencies_file(target_ver_dir / "build_dependencies", -1, true);
-        build_dependencies_file << m.package_id()->build_dependencies_key()->pretty_print_value(UnformattedPrettyPrinter(), { }) << std::endl;
-    }
+        if (m.package_id()->short_description_key())
+        {
+            SafeOFStream description_file(target_ver_dir / "description", -1, true);
+            description_file << m.package_id()->short_description_key()->value() << std::endl;
+        }
 
-    if (m.package_id()->run_dependencies_key())
-    {
-        SafeOFStream run_dependencies_file(target_ver_dir / "run_dependencies", -1, true);
-        run_dependencies_file << m.package_id()->run_dependencies_key()->pretty_print_value(UnformattedPrettyPrinter(), { }) << std::endl;
+        if (m.package_id()->build_dependencies_key())
+        {
+            SafeOFStream build_dependencies_file(target_ver_dir / "build_dependencies", -1, true);
+            build_dependencies_file << m.package_id()->build_dependencies_key()->pretty_print_value(UnformattedPrettyPrinter(), { }) << std::endl;
+        }
+
+        if (m.package_id()->run_dependencies_key())
+        {
+            SafeOFStream run_dependencies_file(target_ver_dir / "run_dependencies", -1, true);
+            run_dependencies_file << m.package_id()->run_dependencies_key()->pretty_print_value(UnformattedPrettyPrinter(), { }) << std::endl;
+        }
     }
 
     NDBAMMerger merger(
@@ -372,12 +381,11 @@ InstalledUnpackagedRepository::merge(const MergeParams & m)
                 n::root() = installed_root_key()->value()
             ));
 
-    if (! merger.check())
+    if (m.check())
     {
-        for (FSIterator d(target_ver_dir, { fsio_include_dotfiles, fsio_inode_sort }), d_end ; d != d_end ; ++d)
-            d->unlink();
-        target_ver_dir.rmdir();
-        throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        if (! merger.check())
+            throw ActionFailedError("Not proceeding with install due to merge sanity check failing");
+        return;
     }
 
     merger.merge();
diff --git a/paludis/repositories/unpackaged/unpackaged_id.cc b/paludis/repositories/unpackaged/unpackaged_id.cc
index 0379f89..12db7bc 100644
--- a/paludis/repositories/unpackaged/unpackaged_id.cc
+++ b/paludis/repositories/unpackaged/unpackaged_id.cc
@@ -433,28 +433,51 @@ UnpackagedID::perform_action(Action & action) const
             throw InternalError(PALUDIS_HERE, "bad WantPhase");
     }
 
+    MergerOptions extra_merger_options;
+    if (preserve_work_choice && preserve_work_choice->enabled())
+        extra_merger_options += mo_nondestructive;
+
+    MergeParams merge_params(make_named_values<MergeParams>(
+                n::build_start_time() = build_start_time,
+                n::check() = true,
+                n::environment_file() = FSPath("/dev/null"),
+                n::image_dir() = fs_location_key()->value(),
+                n::merged_entries() = std::make_shared<FSPathSet>(),
+                n::options() = (MergerOptions() + mo_rewrite_symlinks + mo_allow_empty_dirs)
+                | extra_merger_options,
+                n::output_manager() = output_manager,
+                n::package_id() = shared_from_this(),
+                n::perform_uninstall() = install_action->options.perform_uninstall(),
+                n::replacing() = install_action->options.replacing(),
+                n::used_this_for_config_protect() = std::bind(
+                    &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
+                ));
+
+    switch (install_action->options.want_phase()("check_merge"))
+    {
+        case wp_yes:
+            {
+                merge_params.check() = true;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
+            }
+            break;
+
+        case wp_skip:
+            break;
+
+        case wp_abort:
+            throw ActionAbortedError("Told to abort install");
+
+        case last_wp:
+            throw InternalError(PALUDIS_HERE, "bad WantPhase");
+    }
+
     switch (install_action->options.want_phase()("merge"))
     {
         case wp_yes:
             {
-                MergerOptions extra_merger_options;
-                if (preserve_work_choice && preserve_work_choice->enabled())
-                    extra_merger_options += mo_nondestructive;
-                (*install_action->options.destination()).destination_interface()->merge(
-                        make_named_values<MergeParams>(
-                            n::build_start_time() = build_start_time,
-                            n::environment_file() = FSPath("/dev/null"),
-                            n::image_dir() = fs_location_key()->value(),
-                            n::merged_entries() = std::make_shared<FSPathSet>(),
-                            n::options() = (MergerOptions() + mo_rewrite_symlinks + mo_allow_empty_dirs)
-                                | extra_merger_options,
-                            n::output_manager() = output_manager,
-                            n::package_id() = shared_from_this(),
-                            n::perform_uninstall() = install_action->options.perform_uninstall(),
-                            n::replacing() = install_action->options.replacing(),
-                            n::used_this_for_config_protect() = std::bind(
-                                    &used_this_for_config_protect, std::ref(used_config_protect), std::placeholders::_1)
-                            ));
+                merge_params.check() = false;
+                (*install_action->options.destination()).destination_interface()->merge(merge_params);
             }
             break;
 
diff --git a/paludis/repository.hh b/paludis/repository.hh
index d667bbe..f99e1ca 100644
--- a/paludis/repository.hh
+++ b/paludis/repository.hh
@@ -60,6 +60,7 @@ namespace paludis
     {
         typedef Name<struct name_arch> arch;
         typedef Name<struct name_build_start_time> build_start_time;
+        typedef Name<struct name_check> check;
         typedef Name<struct name_destination_interface> destination_interface;
         typedef Name<struct name_environment_file> environment_file;
         typedef Name<struct name_environment_variable_interface> environment_variable_interface;
@@ -145,6 +146,15 @@ namespace paludis
          */
         NamedValue<n::build_start_time, Timestamp> build_start_time;
 
+        /**
+         * Whether to check or perform the merge.
+         *
+         * A check must be performed before a merge.
+         *
+         * \since 0.59
+         */
+        NamedValue<n::check, bool> check;
+
         NamedValue<n::environment_file, FSPath> environment_file;
         NamedValue<n::image_dir, FSPath> image_dir;