Add a new Cmd type working on RealisedPaths

Where a `RealisedPath` is a store path with its history, meaning either
an opaque path for stuff that has been directly added to the store, or a
`Realisation` for stuff that has been built by a derivation

This is a low-level refactoring that doesn't bring anything by itself
(except a few dozen extra lines of code :/ ), but raising the
abstraction level a bit is important on a number of levels:

- Commands like `nix build` have to query for the realisations after the
  build is finished which is fragile (see
  27905f12e4a7207450abe37c9ed78e31603b67e1 for example). Having them
  oprate directly at the realisation level would avoid that
- Others like `nix copy` currently operate directly on (built) store
  paths, but need a bit more information as they will need to register
  the realisations on the remote side

3 files changed, 85 insertions, 28 deletions

diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index 614dee788..efdc98d5a 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -54,7 +54,7 @@ void StoreCommand::run()
     run(getStore());
 }
 
-StorePathsCommand::StorePathsCommand(bool recursive)
+RealisedPathsCommand::RealisedPathsCommand(bool recursive)
     : recursive(recursive)
 {
     if (recursive)
@@ -81,30 +81,40 @@ StorePathsCommand::StorePathsCommand(bool recursive)
     });
 }
 
-void StorePathsCommand::run(ref<Store> store)
+void RealisedPathsCommand::run(ref<Store> store)
 {
-    StorePaths storePaths;
-
+    std::vector<RealisedPath> paths;
     if (all) {
         if (installables.size())
             throw UsageError("'--all' does not expect arguments");
+        // XXX: Only uses opaque paths, ignores all the realisations
         for (auto & p : store->queryAllValidPaths())
-            storePaths.push_back(p);
-    }
-
-    else {
-        for (auto & p : toStorePaths(store, realiseMode, operateOn, installables))
-            storePaths.push_back(p);
-
+            paths.push_back(p);
+    } else {
+        auto pathSet = toRealisedPaths(store, realiseMode, operateOn, installables);
         if (recursive) {
-            StorePathSet closure;
-            store->computeFSClosure(StorePathSet(storePaths.begin(), storePaths.end()), closure, false, false);
-            storePaths.clear();
-            for (auto & p : closure)
-                storePaths.push_back(p);
+            auto roots = std::move(pathSet);
+            pathSet = {};
+            RealisedPath::closure(*store, roots, pathSet);
         }
+        for (auto & path : pathSet)
+            paths.push_back(path);
     }
 
+    run(store, std::move(paths));
+}
+
+StorePathsCommand::StorePathsCommand(bool recursive)
+    : RealisedPathsCommand(recursive)
+{
+}
+
+void StorePathsCommand::run(ref<Store> store, std::vector<RealisedPath> paths)
+{
+    StorePaths storePaths;
+    for (auto & p : paths)
+        storePaths.push_back(p.path());
+
     run(store, std::move(storePaths));
 }
 
diff --git a/src/libcmd/command.hh b/src/libcmd/command.hh
index ed6980075..8c0b3a94a 100644
--- a/src/libcmd/command.hh
+++ b/src/libcmd/command.hh
@@ -141,7 +141,7 @@ private:
 };
 
 /* A command that operates on zero or more store paths. */
-struct StorePathsCommand : public InstallablesCommand
+struct RealisedPathsCommand : public InstallablesCommand
 {
 private:
 
@@ -154,17 +154,28 @@ protected:
 
 public:
 
-    StorePathsCommand(bool recursive = false);
+    RealisedPathsCommand(bool recursive = false);
 
     using StoreCommand::run;
 
-    virtual void run(ref<Store> store, std::vector<StorePath> storePaths) = 0;
+    virtual void run(ref<Store> store, std::vector<RealisedPath> paths) = 0;
 
     void run(ref<Store> store) override;
 
     bool useDefaultInstallables() override { return !all; }
 };
 
+struct StorePathsCommand : public RealisedPathsCommand
+{
+    StorePathsCommand(bool recursive = false);
+
+    using RealisedPathsCommand::run;
+
+    virtual void run(ref<Store> store, std::vector<StorePath> storePaths) = 0;
+
+    void run(ref<Store> store, std::vector<RealisedPath> paths) override;
+};
+
 /* A command that operates on exactly one store path. */
 struct StorePathCommand : public InstallablesCommand
 {
@@ -218,6 +229,12 @@ std::set<StorePath> toDerivations(ref<Store> store,
     std::vector<std::shared_ptr<Installable>> installables,
     bool useDeriver = false);
 
+std::set<RealisedPath> toRealisedPaths(
+    ref<Store> store,
+    Realise mode,
+    OperateOn operateOn,
+    std::vector<std::shared_ptr<Installable>> installables);
+
 /* Helper function to generate args that invoke $EDITOR on
    filename:lineno. */
 Strings editorFor(const Pos & pos);
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 4e6bf4a9a..98a27ded9 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -704,23 +704,43 @@ Buildables build(ref<Store> store, Realise mode,
     return buildables;
 }
 
-StorePathSet toStorePaths(ref<Store> store,
-    Realise mode, OperateOn operateOn,
+std::set<RealisedPath> toRealisedPaths(
+    ref<Store> store,
+    Realise mode,
+    OperateOn operateOn,
     std::vector<std::shared_ptr<Installable>> installables)
 {
-    StorePathSet outPaths;
-
+    std::set<RealisedPath> res;
     if (operateOn == OperateOn::Output) {
         for (auto & b : build(store, mode, installables))
             std::visit(overloaded {
                 [&](BuildableOpaque bo) {
-                    outPaths.insert(bo.path);
+                    res.insert(bo.path);
                 },
                 [&](BuildableFromDrv bfd) {
+                    auto drv = store->readDerivation(bfd.drvPath);
+                    auto outputHashes = staticOutputHashes(*store, drv);
                     for (auto & output : bfd.outputs) {
-                        if (!output.second)
-                            throw Error("Cannot operate on output of unbuilt CA drv");
-                        outPaths.insert(*output.second);
+                        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+                            if (!outputHashes.count(output.first))
+                                throw Error(
+                                    "The derivation %s doesn't have an output "
+                                    "named %s",
+                                    store->printStorePath(bfd.drvPath),
+                                    output.first);
+                            auto outputId = DrvOutput{outputHashes.at(output.first), output.first};
+                            auto realisation = store->queryRealisation(outputId);
+                            if (!realisation)
+                                throw Error("Cannot operate on output of unbuilt CA drv %s", outputId.to_string());
+                            res.insert(RealisedPath{*realisation});
+                        }
+                        else {
+                            // If ca-derivations isn't enabled, behave as if
+                            // all the paths are opaque to keep the default
+                            // behavior
+                            assert(output.second);
+                            res.insert(*output.second);
+                        }
                     }
                 },
             }, b);
@@ -731,9 +751,19 @@ StorePathSet toStorePaths(ref<Store> store,
         for (auto & i : installables)
             for (auto & b : i->toBuildables())
                 if (auto bfd = std::get_if<BuildableFromDrv>(&b))
-                    outPaths.insert(bfd->drvPath);
+                    res.insert(bfd->drvPath);
     }
 
+    return res;
+}
+
+StorePathSet toStorePaths(ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    std::vector<std::shared_ptr<Installable>> installables)
+{
+    StorePathSet outPaths;
+    for (auto & path : toRealisedPaths(store, mode, operateOn, installables))
+            outPaths.insert(path.path());
     return outPaths;
 }