Add a setting for enabling cgroups

doc/manual/src/release-notes/rl-next.md

@@ -43,10 +43,11 @@
 
   ```
   extra-experimental-features = cgroups
+  use-cgroups = true
   ```
 
-  to `nix.conf`. It is also automatically enabled for builds that
-  require the `uid-range` system feature.
+  to `nix.conf`. Cgroups are required for derivations that require the
+  `uid-range` system feature.
 
 * `nix build --json` now prints some statistics about top-level
   derivations, such as CPU statistics when cgroups are enabled.

src/libstore/build/local-derivation-goal.cc

@@ -401,9 +401,14 @@ static void linkOrCopy(const Path & from, const Path & to)
 void LocalDerivationGoal::startBuilder()
 {
     if ((buildUser && buildUser->getUIDCount() != 1)
-        || settings.isExperimentalFeatureEnabled(Xp::Cgroups))
+        #if __linux__
+        || settings.useCgroups
+        #endif
+        )
     {
         #if __linux__
+        settings.requireExperimentalFeature(Xp::Cgroups);
+
         auto ourCgroups = getCgroups("/proc/self/cgroup");
         auto ourCgroup = ourCgroups[""];
         if (ourCgroup == "")

src/libstore/globals.hh

@@ -304,6 +304,17 @@ public:
         "id-count",
         "The number of UIDs/GIDs to use for dynamic ID allocation."};
 
+    #if __linux__
+    Setting<bool> useCgroups{
+        this, false, "use-cgroups",
+        R"(
+          Whether to execute builds inside cgroups. Cgroups are
+          enabled automatically for derivations that require the
+          `uid-range` system feature.
+        )"
+    };
+    #endif
+
     Setting<bool> impersonateLinux26{this, false, "impersonate-linux-26",
         "Whether to impersonate a Linux 2.6 machine on newer kernels.",
         {"build-impersonate-linux-26"}};

tests/containers.nix

@@ -23,7 +23,7 @@ makeTest ({
           nix.binaryCaches = lib.mkForce [ ];
           nix.extraOptions =
             ''
-              extra-experimental-features = nix-command auto-allocate-uids
+              extra-experimental-features = nix-command auto-allocate-uids cgroups
               extra-system-features = uid-range
             '';
           nix.nixPath = [ "nixpkgs=${nixpkgs}" ];