Move daemon process into sub-cgroup

The daemon process is now moved into a new sub-cgroup called nix-daemon when the
daemon starts. This is necessary to abide by the no-processes-in-inner-nodes
rule, because the service cgroup becomes an inner node when the child cgroups
for the build are created (see LocalDerivationGoal::startBuilder()).

See #9675

src/libutil/linux/cgroup.cc

@@ -6,6 +6,7 @@
 
 #include <chrono>
 #include <cmath>
+#include <mutex>
 #include <regex>
 #include <unordered_set>
 #include <thread>
@@ -144,4 +145,35 @@ CgroupStats destroyCgroup(const Path & cgroup)
     return destroyCgroup(cgroup, true);
 }
 
+std::string getCurrentCgroup()
+{
+    auto cgroupFS = getCgroupFS();
+    if (!cgroupFS)
+        throw Error("cannot determine the cgroups file system");
+
+    auto ourCgroups = getCgroups("/proc/self/cgroup");
+    auto ourCgroup = ourCgroups[""];
+    if (ourCgroup == "")
+        throw Error("cannot determine cgroup name from /proc/self/cgroup");
+    return ourCgroup;
+}
+
+static std::optional<std::string> rootCgroup;
+static std::mutex rootCgroupMutex;
+
+std::string getRootCgroup()
+{
+    {
+        std::lock_guard<std::mutex> guard(rootCgroupMutex);
+        if (rootCgroup)
+            return *rootCgroup;
+    }
+    auto current = getCurrentCgroup();
+    std::lock_guard<std::mutex> guard(rootCgroupMutex);
+    if (rootCgroup)
+        return *rootCgroup;
+    rootCgroup = current;
+    return current;
+}
+
 }

src/libutil/linux/cgroup.hh

@@ -25,4 +25,13 @@ struct CgroupStats
  */
 CgroupStats destroyCgroup(const Path & cgroup);
 
+std::string getCurrentCgroup();
+
+/**
+ * Get the cgroup that should be used as the parent when creating new
+ * sub-cgroups. The first time this is called, the current cgroup will be
+ * returned, and then all subsequent calls will return the original cgroup.
+ */
+std::string getRootCgroup();
+
 }