tests: when running a manager object in a test, migrate to private cgroup subroot first (#6576)

Without this "meson test" will end up running all tests in the same
cgroup root, and they all will try to manage it. Which usually isn't too
bad, except when they end up clearing up each other's cgroups. This race
is hard to trigger but has caused various CI runs to fail spuriously.

With this change we simply move every test that runs a manager object
into their own private cgroup. Note that we don't clean up the cgroup at
the end, we leave that to the cgroup manager around it.

This fixes races that become visible by test runs throwing out errors
like this:

```
exec-systemcallfilter-failing.service: Passing 0 fds to service
exec-systemcallfilter-failing.service: About to execute: /bin/echo 'This should not be seen'
exec-systemcallfilter-failing.service: Forked /bin/echo as 5693
exec-systemcallfilter-failing.service: Changed dead -> start
exec-systemcallfilter-failing.service: Failed to attach to cgroup /exec-systemcallfilter-failing.service: No such file or directory
Received SIGCHLD from PID 5693 ((echo)).
Child 5693 ((echo)) died (code=exited, status=219/CGROUP)
exec-systemcallfilter-failing.service: Child 5693 belongs to exec-systemcallfilter-failing.service
exec-systemcallfilter-failing.service: Main process exited, code=exited, status=219/CGROUP
exec-systemcallfilter-failing.service: Changed start -> failed
exec-systemcallfilter-failing.service: Unit entered failed state.
exec-systemcallfilter-failing.service: Failed with result 'exit-code'.
exec-systemcallfilter-failing.service: cgroup is empty
Assertion 'service->main_exec_status.status == status_expected' failed at ../src/src/test/test-execute.c:71, function check(). Aborting.
```

BTW, I tracked this race down by using perf:

```
        # perf record -e cgroup:cgroup_mkdir,cgroup_rmdir
        …
        # perf script
```

Thanks a lot @iaguis, @alban for helping me how to use perf for this.

Fixes #5895.

src/test/meson.build

@@ -42,7 +42,8 @@ tests += [
          [],
          []],
 
-        [['src/test/test-engine.c'],
+        [['src/test/test-engine.c',
+          'src/test/test-helper.c'],
          [libcore,
           libudev,
           libsystemd_internal],
@@ -105,7 +106,8 @@ tests += [
          [],
          'ENABLE_EFI'],
 
-        [['src/test/test-unit-name.c'],
+        [['src/test/test-unit-name.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,
@@ -115,7 +117,8 @@ tests += [
           libmount,
           libblkid]],
 
-        [['src/test/test-unit-file.c'],
+        [['src/test/test-unit-file.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,
@@ -456,7 +459,8 @@ tests += [
          '', 'manual'],
 
 
-        [['src/test/test-cgroup-mask.c'],
+        [['src/test/test-cgroup-mask.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,
@@ -486,7 +490,8 @@ tests += [
          [],
          []],
 
-        [['src/test/test-path.c'],
+        [['src/test/test-path.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,
@@ -496,7 +501,8 @@ tests += [
           libmount,
           libblkid]],
 
-        [['src/test/test-execute.c'],
+        [['src/test/test-execute.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,
@@ -524,7 +530,8 @@ tests += [
          [],
          []],
 
-        [['src/test/test-sched-prio.c'],
+        [['src/test/test-sched-prio.c',
+          'src/test/test-helper.c'],
          [libcore,
           libshared],
          [threads,

src/test/test-cgroup-mask.c

@@ -34,6 +34,8 @@ static int test_cgroup_mask(void) {
         FDSet *fdset = NULL;
         int r;
 
+        enter_cgroup_subroot();
+
         /* Prepare the manager. */
         assert_se(set_unit_path(get_testdata_dir("")) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());

src/test/test-engine.c

@@ -37,6 +37,8 @@ int main(int argc, char *argv[]) {
         Job *j;
         int r;
 
+        enter_cgroup_subroot();
+
         /* prepare the test */
         assert_se(set_unit_path(get_testdata_dir("")) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());

src/test/test-execute.c

@@ -526,6 +526,8 @@ int main(int argc, char *argv[]) {
                 return EXIT_TEST_SKIP;
         }
 
+        enter_cgroup_subroot();
+
         assert_se(setenv("XDG_RUNTIME_DIR", "/tmp/", 1) == 0);
         assert_se(set_unit_path(get_testdata_dir("/test-execute")) >= 0);
 

src/test/test-helper.c

@@ -0,0 +1,41 @@
+/***
+  This file is part of systemd.
+
+  Copyright 2017 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "test-helper.h"
+#include "random-util.h"
+#include "alloc-util.h"
+#include "cgroup-util.h"
+
+void enter_cgroup_subroot(void) {
+        _cleanup_free_ char *cgroup_root = NULL, *cgroup_subroot = NULL;
+        CGroupMask supported;
+
+        assert_se(cg_pid_get_path(NULL, 0, &cgroup_root) >= 0);
+        assert_se(asprintf(&cgroup_subroot, "%s/%" PRIx64, cgroup_root, random_u64()) >= 0);
+        assert_se(cg_mask_supported(&supported) >= 0);
+
+        /* If this fails, then we don't mind as the later cgroup operations will fail too, and it's fine if we handle
+         * any errors at that point. */
+
+        if (cg_create_everywhere(supported, _CGROUP_MASK_ALL, cgroup_subroot) < 0)
+                return;
+
+        if (cg_attach_everywhere(supported, cgroup_subroot, 0, NULL, NULL) < 0)
+                return;
+}

src/test/test-helper.h

@@ -39,3 +39,5 @@
                -ENOENT,                                         \
                -ENOMEDIUM /* cannot determine cgroup */         \
                )
+
+void enter_cgroup_subroot(void);

src/test/test-path.c

@@ -45,6 +45,8 @@ static int setup_test(Manager **m) {
 
         assert_se(m);
 
+        enter_cgroup_subroot();
+
         r = manager_new(UNIT_FILE_USER, true, &tmp);
         if (MANAGER_SKIP_TEST(r)) {
                 log_notice_errno(r, "Skipping test: manager_new: %m");

src/test/test-sched-prio.c

@@ -34,6 +34,8 @@ int main(int argc, char *argv[]) {
         FDSet *fdset = NULL;
         int r;
 
+        enter_cgroup_subroot();
+
         /* prepare the test */
         assert_se(set_unit_path(get_testdata_dir("")) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());

src/test/test-unit-file.c

@@ -848,6 +848,8 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
+        enter_cgroup_subroot();
+
         assert_se(runtime_dir = setup_fake_runtime_dir());
 
         r = test_unit_file_get_set();

src/test/test-unit-name.c

@@ -469,6 +469,8 @@ int main(int argc, char* argv[]) {
         log_parse_environment();
         log_open();
 
+        enter_cgroup_subroot();
+
         assert_se(runtime_dir = setup_fake_runtime_dir());
 
         test_unit_name_is_valid();