Add 'store add-gc-root' command to manually add indirect roots

src/nix/meson.build

@@ -105,6 +105,7 @@ nix_sources = [config_h] + files(
   'run.cc',
   'search.cc',
   'sigs.cc',
+  'store-add-gc-root.cc',
   'store-copy-log.cc',
   'store-delete.cc',
   'store-gc.cc',

src/nix/store-add-gc-root.cc

@@ -0,0 +1,68 @@
+#include "command.hh"
+#include "args.hh"
+#include "shared.hh"
+#include "store-cast.hh"
+#include "indirect-root-store.hh"
+#include "common-args.hh"
+
+using namespace nix;
+
+struct CmdAddGCRoot : StoreCommand
+{
+    std::vector<std::string> links;
+    bool checkResults = true;
+
+    CmdAddGCRoot()
+    {
+        expectArgs({
+            .label = "indirect-roots",
+            .handler = {&links},
+            .completer = completePath,
+        });
+
+        addFlag({
+            .longName = "no-check",
+            .description = "Do not test the validity of created roots.",
+            .handler = {&checkResults, false},
+        });
+    }
+
+    std::string description() override
+    {
+        return "Add indirect gc roots through the symlink arguments";
+    }
+
+    std::string doc() override
+    {
+        return
+#include "store-add-gc-root.md"
+            ;
+    }
+
+    Category category() override
+    {
+        return catSecondary;
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto & indirectRootStore = require<IndirectRootStore>(*store);
+
+        for (auto & link : links) {
+            auto indirectPath = absPath(link);
+            if (indirectRootStore.isInStore(indirectPath)) {
+                throw Error("Indirect root '%1%' must not be in the Nix store", link);
+            }
+
+            if (checkResults) {
+                auto path = indirectRootStore.followLinksToStorePath(indirectPath);
+                indirectRootStore.addTempRoot(path);
+                // TODO: ensure `path` is safe from concurrent GC or fail.
+            }
+
+            indirectRootStore.addIndirectRoot(indirectPath);
+        }
+    }
+};
+
+static auto rCmdAddGCRoot = registerCommand2<CmdAddGCRoot>({"store", "add-gc-root"});

src/nix/store-add-gc-root.md

@@ -0,0 +1,17 @@
+R""(
+
+# Examples
+
+  ```console
+  $ ln -s /nix/store/xxx foo
+  $ nix store add-gc-root foo
+  $ nix-store -q --roots /nix/store/xxx
+  .../foo -> /nix/store/xxx
+  ```
+
+# Description
+
+This command adds garbage collector root to the paths referenced by the symlinks passed as arguments.
+These are called indirect roots, as the root will disappear as soon as the intermediate symlink gets deleted.
+
+)""

tests/functional/meson.build

@@ -184,6 +184,7 @@ suites = [
       'debugger.sh',
       'extra-sandbox-profile.sh',
       'help.sh',
+      'store-add-gc-root.sh',
     ],
     'workdir': meson.current_build_dir(),
   },