libutil/cgroup: destroy state record at destroy time

If state records are not destroyed at destroy time, this might confuse a
new build that thinks there's a remnant of a cgroup when actually it was
destroyed.

This fixes a bunch of inoffensive and noisy warnings about cgroups being
deleted by someone else.

Reported-by: Ramses <@rvdp:infosec.exchange>
Change-Id: Ib3d33f4ecd6143f33e032c5107b288b4ecabaee1
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libutil/cgroup.cc

@@ -311,6 +311,8 @@ void AutoDestroyCgroup::destroy()
         },
         cgroup_
     );
+
+    stateRecord.reset();
 }
 
 AutoDestroyCgroup::~AutoDestroyCgroup()
@@ -326,6 +328,9 @@ void AutoDestroyCgroup::cleansePreviousInstancesAndRecordOurself(
     const std::filesystem::path & cgroupRecordsDir
 )
 {
+    assert(
+        !stateRecord && "`stateRecord` cannot be created before the cleansing process takes place"
+    );
     createDirs(cgroupRecordsDir);
 
     auto cgroupFile = cgroupRecordsDir / name_;
@@ -337,7 +342,7 @@ void AutoDestroyCgroup::cleansePreviousInstancesAndRecordOurself(
     }
 
     writeFile(cgroupFile, std::get<std::filesystem::path>(cgroup_).string());
-    stateRecord.reset(cgroupFile, false);
+    stateRecord.emplace(cgroupFile, false);
 }
 
 void AutoDestroyCgroup::adoptProcess(int pid)

lix/libutil/cgroup.hh

@@ -138,8 +138,10 @@ private:
      * Path to the state record of this cgroup's existence.
      * This is used when the deletion process is interrupted
      * for the next run.
+     *
+     * This might be deleted earlier, in which case, this is left to none.
      */
-    AutoDelete stateRecord;
+    std::optional<AutoDelete> stateRecord;
 
     /*
      * Cleanse all previous instances of this cgroup where the deletion process