From 2c28502bc4363b6654b617afbaeecb1efdbb98b1 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Sun, 30 Nov 2025 19:09:21 +0100
Subject: [PATCH] Add getCgroupStats() function

---
 src/libutil/linux/cgroup.cc                  | 51 +++++++++++---------
 src/libutil/linux/include/nix/util/cgroup.hh |  6 +++
 2 files changed, 35 insertions(+), 22 deletions(-)

diff --git a/src/libutil/linux/cgroup.cc b/src/libutil/linux/cgroup.cc
index 9e78ac6d2..928b44d6c 100644
--- a/src/libutil/linux/cgroup.cc
+++ b/src/libutil/linux/cgroup.cc
@@ -49,6 +49,33 @@ StringMap getCgroups(const Path & cgroupFile)
     return cgroups;
 }
 
+CgroupStats getCgroupStats(const std::filesystem::path & cgroup)
+{
+    CgroupStats stats;
+
+    auto cpustatPath = cgroup / "cpu.stat";
+
+    if (pathExists(cpustatPath)) {
+        for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cpustatPath), "\n")) {
+            std::string_view userPrefix = "user_usec ";
+            if (hasPrefix(line, userPrefix)) {
+                auto n = string2Int<uint64_t>(line.substr(userPrefix.size()));
+                if (n)
+                    stats.cpuUser = std::chrono::microseconds(*n);
+            }
+
+            std::string_view systemPrefix = "system_usec ";
+            if (hasPrefix(line, systemPrefix)) {
+                auto n = string2Int<uint64_t>(line.substr(systemPrefix.size()));
+                if (n)
+                    stats.cpuSystem = std::chrono::microseconds(*n);
+            }
+        }
+    }
+
+    return stats;
+}
+
 static CgroupStats destroyCgroup(const std::filesystem::path & cgroup, bool returnStats)
 {
     if (!pathExists(cgroup))
@@ -114,28 +141,8 @@ static CgroupStats destroyCgroup(const std::filesystem::path & cgroup, bool retu
     }
 
     CgroupStats stats;
-
-    if (returnStats) {
-        auto cpustatPath = cgroup / "cpu.stat";
-
-        if (pathExists(cpustatPath)) {
-            for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cpustatPath), "\n")) {
-                std::string_view userPrefix = "user_usec ";
-                if (hasPrefix(line, userPrefix)) {
-                    auto n = string2Int<uint64_t>(line.substr(userPrefix.size()));
-                    if (n)
-                        stats.cpuUser = std::chrono::microseconds(*n);
-                }
-
-                std::string_view systemPrefix = "system_usec ";
-                if (hasPrefix(line, systemPrefix)) {
-                    auto n = string2Int<uint64_t>(line.substr(systemPrefix.size()));
-                    if (n)
-                        stats.cpuSystem = std::chrono::microseconds(*n);
-                }
-            }
-        }
-    }
+    if (returnStats)
+        stats = getCgroupStats(cgroup);
 
     if (rmdir(cgroup.c_str()) == -1)
         throw SysError("deleting cgroup %s", cgroup);
diff --git a/src/libutil/linux/include/nix/util/cgroup.hh b/src/libutil/linux/include/nix/util/cgroup.hh
index 59de13d46..a759bdd08 100644
--- a/src/libutil/linux/include/nix/util/cgroup.hh
+++ b/src/libutil/linux/include/nix/util/cgroup.hh
@@ -3,6 +3,7 @@
 
 #include <chrono>
 #include <optional>
+#include <filesystem>
 
 #include "nix/util/types.hh"
 
@@ -17,6 +18,11 @@ struct CgroupStats
     std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
 };
 
+/**
+ * Read statistics from the given cgroup.
+ */
+CgroupStats getCgroupStats(const std::filesystem::path & cgroup);
+
 /**
  * Destroy the cgroup denoted by 'path'. The postcondition is that
  * 'path' does not exist, and thus any processes in the cgroup have