nix copy: Add --profile flag

This allows `nix copy` to atomically copy a store path and point a
profile to it, without the risk that the store path might be GC'ed in
between. This is useful for instance when deploying a new NixOS system
profile from a remote store.

src/libcmd/command.cc

@@ -179,30 +179,34 @@ BuiltPathsCommand::BuiltPathsCommand(bool recursive)
 
 void BuiltPathsCommand::run(ref<Store> store, Installables && installables)
 {
-    BuiltPaths paths;
+    BuiltPaths rootPaths, allPaths;
+
     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())
-            paths.emplace_back(BuiltPath::Opaque{p});
+            rootPaths.emplace_back(BuiltPath::Opaque{p});
+        allPaths = rootPaths;
     } else {
-        paths = Installable::toBuiltPaths(getEvalStore(), store, realiseMode, operateOn, installables);
+        rootPaths = Installable::toBuiltPaths(getEvalStore(), store, realiseMode, operateOn, installables);
+        allPaths = rootPaths;
+
         if (recursive) {
             // XXX: This only computes the store path closure, ignoring
             // intermediate realisations
             StorePathSet pathsRoots, pathsClosure;
-            for (auto & root : paths) {
+            for (auto & root : rootPaths) {
                 auto rootFromThis = root.outPaths();
                 pathsRoots.insert(rootFromThis.begin(), rootFromThis.end());
             }
             store->computeFSClosure(pathsRoots, pathsClosure);
             for (auto & path : pathsClosure)
-                paths.emplace_back(BuiltPath::Opaque{path});
+                allPaths.emplace_back(BuiltPath::Opaque{path});
         }
     }
 
-    run(store, std::move(paths));
+    run(store, std::move(allPaths), std::move(rootPaths));
 }
 
 StorePathsCommand::StorePathsCommand(bool recursive)
@@ -210,10 +214,10 @@ StorePathsCommand::StorePathsCommand(bool recursive)
 {
 }
 
-void StorePathsCommand::run(ref<Store> store, BuiltPaths && paths)
+void StorePathsCommand::run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths)
 {
     StorePathSet storePaths;
-    for (auto & builtPath : paths)
+    for (auto & builtPath : allPaths)
         for (auto & p : builtPath.outPaths())
             storePaths.insert(p);
 
@@ -242,17 +246,21 @@ MixProfile::MixProfile()
     });
 }
 
-void MixProfile::updateProfile(const StorePath & storePath)
+void MixProfile::updateProfile(
+    const StorePath & storePath,
+    ref<Store> store_)
 {
     if (!profile) return;
-    auto store = getStore().dynamic_pointer_cast<LocalFSStore>();
+    auto store = store_.dynamic_pointer_cast<LocalFSStore>();
     if (!store) throw Error("'--profile' is not supported for this Nix store");
     auto profile2 = absPath(*profile);
     switchLink(profile2,
         createGeneration(*store, profile2, storePath));
 }
 
-void MixProfile::updateProfile(const BuiltPaths & buildables)
+void MixProfile::updateProfile(
+    const BuiltPaths & buildables,
+    ref<Store> store)
 {
     if (!profile) return;
 
@@ -274,7 +282,7 @@ void MixProfile::updateProfile(const BuiltPaths & buildables)
     if (result.size() != 1)
         throw UsageError("'--profile' requires that the arguments produce a single store path, but there are %d", result.size());
 
-    updateProfile(result[0]);
+    updateProfile(result[0], store);
 }
 
 MixDefaultProfile::MixDefaultProfile()
@@ -308,7 +316,8 @@ MixEnvironment::MixEnvironment() : ignoreEnvironment(false)
     });
 }
 
-void MixEnvironment::setEnviron() {
+void MixEnvironment::setEnviron()
+{
     if (ignoreEnvironment) {
         if (!unset.empty())
             throw UsageError("--unset does not make sense with --ignore-environment");

src/libcmd/command.hh

@@ -238,7 +238,7 @@ public:
 
     BuiltPathsCommand(bool recursive = false);
 
-    virtual void run(ref<Store> store, BuiltPaths && paths) = 0;
+    virtual void run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths) = 0;
 
     void run(ref<Store> store, Installables && installables) override;
 
@@ -251,7 +251,7 @@ struct StorePathsCommand : public BuiltPathsCommand
 
     virtual void run(ref<Store> store, StorePaths && storePaths) = 0;
 
-    void run(ref<Store> store, BuiltPaths && paths) override;
+    void run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths) override;
 };
 
 /**
@@ -301,11 +301,15 @@ struct MixProfile : virtual StoreCommand
     MixProfile();
 
     /* If 'profile' is set, make it point at 'storePath'. */
-    void updateProfile(const StorePath & storePath);
+    void updateProfile(
+        const StorePath & storePath,
+        ref<Store> store);
 
     /* If 'profile' is set, make it point at the store path produced
        by 'buildables'. */
-    void updateProfile(const BuiltPaths & buildables);
+    void updateProfile(
+        const BuiltPaths & buildables,
+        ref<Store> store);
 };
 
 struct MixDefaultProfile : MixProfile

src/nix/build.cc

@@ -161,7 +161,7 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
         BuiltPaths buildables2;
         for (auto & b : buildables)
             buildables2.push_back(b.path);
-        updateProfile(buildables2);
+        updateProfile(buildables2, store);
     }
 };
 

src/nix/copy.cc

@@ -4,7 +4,7 @@
 
 using namespace nix;
 
-struct CmdCopy : virtual CopyCommand, virtual BuiltPathsCommand
+struct CmdCopy : virtual CopyCommand, virtual BuiltPathsCommand, MixProfile
 {
     CheckSigsFlag checkSigs = CheckSigs;
 
@@ -43,19 +43,21 @@ struct CmdCopy : virtual CopyCommand, virtual BuiltPathsCommand
 
     Category category() override { return catSecondary; }
 
-    void run(ref<Store> srcStore, BuiltPaths && paths) override
+    void run(ref<Store> srcStore, BuiltPaths && allPaths, BuiltPaths && rootPaths) override
     {
         auto dstStore = getDstStore();
 
         RealisedPath::Set stuffToCopy;
 
-        for (auto & builtPath : paths) {
+        for (auto & builtPath : allPaths) {
             auto theseRealisations = builtPath.toRealisedPaths(*srcStore);
             stuffToCopy.insert(theseRealisations.begin(), theseRealisations.end());
         }
 
         copyPaths(
             *srcStore, *dstStore, stuffToCopy, NoRepair, checkSigs, substitute);
+
+        updateProfile(rootPaths, dstStore);
     }
 };
 

src/nix/copy.md

@@ -55,6 +55,15 @@ R""(
   # nix copy --to /tmp/nix nixpkgs#hello --no-check-sigs
   ```
 
+* Update the NixOS system profile to point to a closure copied from a
+  remote machine:
+
+  ```console
+  # nix copy --from ssh://server \
+      --profile /nix/var/nix/profiles/system \
+      /nix/store/r14v3km89zm3prwsa521fab5kgzvfbw4-nixos-system-foobar-24.05.20240925.759537f
+  ```
+
 # Description
 
 `nix copy` copies store path closures between two Nix stores. The

src/nix/develop.cc

@@ -502,7 +502,7 @@ struct Common : InstallableCommand, MixProfile
 
         auto strPath = store->printStorePath(shellOutPath);
 
-        updateProfile(shellOutPath);
+        updateProfile(shellOutPath, store);
 
         debug("reading environment file '%s'", strPath);
 

src/nix/profile.cc

@@ -424,7 +424,7 @@ struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
         }
 
         try {
-            updateProfile(manifest.build(store));
+            updateProfile(manifest.build(store), store);
         } catch (BuildEnvFileConflictError & conflictError) {
             // FIXME use C++20 std::ranges once macOS has it
             //       See https://github.com/NixOS/nix/compare/3efa476c5439f8f6c1968a6ba20a31d1239c2f04..1fe5d172ece51a619e879c4b86f603d9495cc102
@@ -669,7 +669,7 @@ struct CmdProfileRemove : virtual EvalCommand, MixDefaultProfile, MixProfileElem
             removedCount,
             newManifest.elements.size());
 
-        updateProfile(newManifest.build(store));
+        updateProfile(newManifest.build(store), store);
     }
 };
 
@@ -779,7 +779,7 @@ struct CmdProfileUpgrade : virtual SourceExprCommand, MixDefaultProfile, MixProf
                 builtPaths.find(&*installable)->second.first);
         }
 
-        updateProfile(manifest.build(store));
+        updateProfile(manifest.build(store), store);
     }
 };
 

src/nix/realisation.cc

@@ -36,7 +36,7 @@ struct CmdRealisationInfo : BuiltPathsCommand, MixJSON
 
     Category category() override { return catSecondary; }
 
-    void run(ref<Store> store, BuiltPaths && paths) override
+    void run(ref<Store> store, BuiltPaths && paths, BuiltPaths && rootPaths) override
     {
         experimentalFeatureSettings.require(Xp::CaDerivations);
         RealisedPath::Set realisations;

tests/functional/zstd.sh

@@ -18,7 +18,9 @@ HASH=$(nix hash path $outPath)
 clearStore
 clearCacheCache
 
-nix copy --from $cacheURI $outPath --no-check-sigs
+nix copy --from $cacheURI $outPath --no-check-sigs --profile $TEST_ROOT/profile
+
+[[ -e $TEST_ROOT/profile ]]
 
 if ls $cacheDir/nar/*.zst &> /dev/null; then
     echo "files do exist"