nix build: Add '--profile' flag

This replaces 'nix-env --set'. For example:

  $ nix build --profile /nix/var/nix/profiles/system \
      ~/Misc/eelco-configurations:nixosConfigurations.vyr.config.system.build.toplevel

updates the NixOS system profile from a flake.

This could have been a separate command (e.g. 'nix set-profile') but
1) '--profile' is pretty similar to '--out-link'; and 2) '--profile'
could be useful for other command (like 'nix dev-shell').

src/nix/build.cc

@@ -6,7 +6,7 @@
 
 using namespace nix;
 
-struct CmdBuild : MixDryRun, InstallablesCommand
+struct CmdBuild : MixDryRun, MixProfile, InstallablesCommand
 {
     Path outLink = "result";
 
@@ -41,6 +41,10 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                 "To build the build.x86_64-linux attribute from release.nix:",
                 "nix build -f release.nix build.x86_64-linux"
             },
+            Example{
+                "To make a profile point at GNU Hello:",
+                "nix build --profile /tmp/profile nixpkgs:hello"
+            },
         };
     }
 
@@ -52,11 +56,9 @@ struct CmdBuild : MixDryRun, InstallablesCommand
         evalState->addRegistryOverrides(registryOverrides);
         if (dryRun) return;
 
+        if (outLink != "") {
             for (size_t i = 0; i < buildables.size(); ++i) {
-            auto & b(buildables[i]);
-
-            if (outLink != "")
-                for (auto & output : b.outputs)
+                for (auto & output : buildables[i].outputs)
                     if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
                         std::string symlink = outLink;
                         if (i) symlink += fmt("-%d", i);
@@ -65,6 +67,9 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                     }
             }
         }
+
+        updateProfile(buildables);
+    }
 };
 
 static auto r1 = registerCommand<CmdBuild>("build");

src/nix/command.cc

@@ -1,6 +1,7 @@
 #include "command.hh"
 #include "store-api.hh"
 #include "derivations.hh"
+#include "profiles.hh"
 
 namespace nix {
 
@@ -81,4 +82,44 @@ void StorePathCommand::run(ref<Store> store)
     run(store, *storePaths.begin());
 }
 
+MixProfile::MixProfile()
+{
+    mkFlag()
+        .longName("profile")
+        .description("profile to update")
+        .labels({"path"})
+        .dest(&profile);
+}
+
+void MixProfile::updateProfile(const Path & storePath)
+{
+    if (!profile) return;
+    auto store = getStore().dynamic_pointer_cast<LocalFSStore>();
+    if (!store) throw Error("'--profile' is not supported for this Nix store");
+    switchLink(*profile,
+        createGeneration(
+            ref<LocalFSStore>(store),
+            *profile, storePath));
+}
+
+void MixProfile::updateProfile(const Buildables & buildables)
+{
+    if (!profile) return;
+
+    std::optional<Path> result;
+
+    for (auto & buildable : buildables) {
+        for (auto & output : buildable.outputs) {
+            if (result)
+                throw Error("'--profile' requires that the arguments produce a single store path, but there are multiple");
+            result = output.second;
+        }
+    }
+
+    if (!result)
+        throw Error("'--profile' requires that the arguments produce a single store path, but there are none");
+
+    updateProfile(*result);
+}
+
 }

src/nix/command.hh

@@ -219,4 +219,18 @@ PathSet toDerivations(ref<Store> store,
     std::vector<std::shared_ptr<Installable>> installables,
     bool useDeriver = false);
 
+struct MixProfile : virtual Args, virtual StoreCommand
+{
+    std::optional<Path> profile;
+
+    MixProfile();
+
+    /* If 'profile' is set, make it point at 'storePath'. */
+    void updateProfile(const Path & storePath);
+
+    /* If 'profile' is set, make it point at the store path produced
+       by 'buildables'. */
+    void updateProfile(const Buildables & buildables);
+};
+
 }