Add an 'add-root' comamnd to manually add indirect roots

src/nix/add-root.cc

@@ -0,0 +1,61 @@
+#include "command.hh"
+#include "args.hh"
+#include "shared.hh"
+#include "store-cast.hh"
+#include "indirect-root-store.hh"
+#include "common-args.hh"
+#include "strings.hh"
+#include "installable-derived-path.hh"
+
+using namespace nix;
+
+struct CmdAddRoot: StoreCommand
+{
+    std::vector<std::string> links;
+    bool checkResults = true;
+
+    CmdAddRoot()
+    {
+        expectArgs({
+            .label = "indirect-roots",
+            .handler = {&links},
+            .completer = completePath,
+        });
+    }
+
+    std::string description() override
+    {
+        return "Add indirect gc-roots through the symlink arguments";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "add-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 the path is safe from concurrent GC of fail.
+            }
+
+            indirectRootStore.addIndirectRoot(indirectPath);
+        }
+    }
+};
+
+static auto rCmdAddRoot = registerCommand<CmdAddRoot>("add-root");

src/nix/add-root.md

@@ -0,0 +1,18 @@
+R""(
+
+# Examples
+
+  ```console
+  $ readlink foo
+  /nix/store/xxx
+  $ nix add-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.
+
+)""

src/nix/meson.build

@@ -68,6 +68,7 @@ subdir('build-utils-meson/diagnostics')
 subdir('build-utils-meson/generate-header')
 
 nix_sources = [config_h] + files(
+  'add-root.cc',
   'add-to-store.cc',
   'app.cc',
   'self-exe.cc',