[1/2] lib: Add library functions to check specified cgroup subsystem

1) Check if specified cgroup subsystem is supported.
2) Check if specified cgroup subsystem is mounted by default, and use
   custom mountpoint as the cgroup subsystem's mountpoint properly.
3) Add doc for these library functions.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
---
 doc/test-writing-guidelines.txt |  45 +++++++++++++++++
 include/tst_cgroups.h           |  13 +++++
 lib/tst_cgroups.c               | 107 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 165 insertions(+)
 create mode 100644 include/tst_cgroups.h
 create mode 100644 lib/tst_cgroups.c

Hi!
> 1) Check if specified cgroup subsystem is supported.
> 2) Check if specified cgroup subsystem is mounted by default, and use
>    custom mountpoint as the cgroup subsystem's mountpoint properly.
> 3) Add doc for these library functions.
> 
> Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
> ---
>  doc/test-writing-guidelines.txt |  45 +++++++++++++++++
>  include/tst_cgroups.h           |  13 +++++
>  lib/tst_cgroups.c               | 107 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 165 insertions(+)
>  create mode 100644 include/tst_cgroups.h
>  create mode 100644 lib/tst_cgroups.c
> 
> diff --git a/doc/test-writing-guidelines.txt b/doc/test-writing-guidelines.txt
> index 731be76..57ae6aa 100644
> --- a/doc/test-writing-guidelines.txt
> +++ b/doc/test-writing-guidelines.txt
> @@ -1539,6 +1539,51 @@ static struct tst_test test = {
>  };
>  -------------------------------------------------------------------------------
>  
> +2.2.29 Parsing specified cgroup subsystem
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +void tst_set_cgroup_mntpoint(char *subsys_name, char *spec_mntpoint);

This function name is a bit misleading as it actually mounts the cgroup.

> +void tst_umount_cgroup(char *spec_mntpoint);
> +
> +The 'tst_set_cgroup_mntpoint()' function checks if 'subsys_name' (i.e. specified
> +cgroup subsystem) is supported and mounted by default, and uses 'spec_mntpoint'
> +(custom mountpoint) as 'subsys_name' mountpoint to test.  For details:
> +1) It exits with TCONF if 'subsys_name' is not supported.
> +2) If 'subsys_name' is already mounted to a mountpoint, it creates a symlink
> +   named by 'spec_mntpoint' to the pre-existent mountpoint.
> +3) If 'subsys_name' is not mounted, it mounts 'subsys_name' to 'spec_mntpoint'.
> +
> +The 'tst_umount_cgroup()' function unmounts and removes 'spec_mntpoint'.
> +
> +Note:
> +We should set .needs_tmpdir flag because we create 'spec_mntpoint' in tmpdir.

Generally this is a great step forward but what this is missing:

* Automatic cleanup, if tests creates any subdirectories in the cgroup
  filesystem these will stay there until the machine is rebooted

  I guess that it will be much easier to create the interface so that it
  returns a path to a newly created subdirectory in the cgroup
  filesystem tree so that we can remove it recursively later on

* Interface in tst_test structure. I suppose that we may add something
  as const char *needs_cgroup; to the tst_test structure which would
  be set to subsystem name such as:

  static struct tst_test test = {
	...
	.needs_cgroup = "memory",
	...
  };

  And the test will then set const char *tst_cgroup_path pointer to the
  actuall path to the cgroup.

* I do wonder if we will nedd support for more than one cgroup type
  later on, but so far I do not see that need so keeping it as an
  interface for a single subsytem type will suffice.

> +[source,c]
> +-------------------------------------------------------------------------------
> +#include "tst_test.h"
> +#include "tst_cgroups.h"
> +
> +static void setup(void)
> +{
> +	...
> +	tst_set_cgroups_mntpoint("memory", "mntpoint");
> +	...
> +}
> +
> +static void cleanup(void)
> +{
> +	...
> +	tst_umount_cgroup("mntpoint");
> +	...
> +}
> +
> +sturct tst_test test = {
> +        ...
> +        .setup = setup,
> +        .cleanup = cleanup,
> +        ...
> +};
> +-------------------------------------------------------------------------------
>  
>  2.3 Writing a testcase in shell
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> diff --git a/include/tst_cgroups.h b/include/tst_cgroups.h
> new file mode 100644
> index 0000000..70f5d18
> --- /dev/null
> +++ b/include/tst_cgroups.h
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (c) 2019 FUJITSU LIMITED. All rights reserved.
> + * Author: Xiao Yang <yangx.jy@cn.fujitsu.com>
> + */
> +
> +#ifndef TST_CGROUPS_H__
> +#define TST_CGROUPS_H__
> +
> +void tst_set_cgroup_mntpoint(char *subsys_name, char *spec_mntpoint);
> +void tst_umount_cgroup(char *spec_mntpoint);
> +
> +#endif	/* TST_CGROUPS_H__ */
> diff --git a/lib/tst_cgroups.c b/lib/tst_cgroups.c
> new file mode 100644
> index 0000000..dd027c5
> --- /dev/null
> +++ b/lib/tst_cgroups.c
> @@ -0,0 +1,107 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (c) 2019 FUJITSU LIMITED. All rights reserved.
> + * Author: Xiao Yang <yangx.jy@cn.fujitsu.com>
> + */
> +
> +#include <errno.h>
> +#include <unistd.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <mntent.h>
> +
> +#define TST_NO_DEFAULT_MAIN
> +#include "tst_test.h"
> +#include "tst_safe_stdio.h"
> +
> +#define CGROUPS_PATH	"/proc/cgroups"
> +#define MOUNTS_PATH	"/proc/mounts"
> +
> +static char *cgroup_mntpoint;
> +static int cgroup_mounted;
> +
> +void tst_umount_cgroup(char *spec_mntpoint)
> +{
> +	char buf[1024];
> +
> +	if (cgroup_mntpoint) {
> +		if (!access(spec_mntpoint, F_OK))
> +			SAFE_UNLINK(spec_mntpoint);
> +		return;
> +	}
> +
> +	if (cgroup_mounted) {
> +		/* 'spec_mntpoint' may be treated as source instead
> +		 * of target, so append '/' to 'spec_mntpoint'.  See
> +		 * ltp commit 4617249 for details.
> +		 */
> +		sprintf(buf, "%s/", spec_mntpoint);
> +		SAFE_UMOUNT(buf);
> +		cgroup_mounted = 0;
> +	}
> +
> +	if (!access(spec_mntpoint, F_OK))
> +		SAFE_RMDIR(spec_mntpoint);
> +}
> +
> +void tst_set_cgroup_mntpoint(char *subsys_name, char *spec_mntpoint)
> +{
> +	FILE *fp = NULL, *mp = NULL;
> +	struct mntent *mnt;
> +	char buf[1024], name[64];
> +	int enabled = 0, supported = 0;
> +
> +	if (access(CGROUPS_PATH, F_OK))
> +		tst_brk(TCONF, "Kernel didn't support for control groups");
> +
> +	fp = SAFE_FOPEN(CGROUPS_PATH, "r");
> +
> +	while (fgets(buf, 1024, fp)) {
> +		if (sscanf(buf, "%s %*d %*d %d", name, &enabled) == -1) {
> +			SAFE_FCLOSE(fp);
> +			tst_brk(TBROK | TERRNO,
> +				"sscanf() failed to read %s info",
> +				subsys_name);
> +		}
> +
> +		if (!strcmp(name, subsys_name)) {
> +			supported++;
> +			break;
> +		}
> +	}
> +
> +	SAFE_FCLOSE(fp);
> +
> +	if (!supported || !enabled) {
> +		tst_brk(TCONF,
> +			"%s wasn't supported by kernel or it wasn't enabled",
> +			subsys_name);
> +	}
> +
> +	mp = setmntent(MOUNTS_PATH, "r");
> +	if (mp == NULL) {
> +		tst_brk(TBROK | TERRNO,
> +			"setmntent() failed to open /proc/mounts");
> +	}
> +
> +	while ((mnt = getmntent(mp))) {
> +		if (hasmntopt(mnt, subsys_name)) {
> +			cgroup_mntpoint = mnt->mnt_dir;
> +			break;
> +		}
> +	}
> +
> +	endmntent(mp);
> +
> +	if (cgroup_mntpoint) {
> +		/* Use pre-existent subsys mountpoint and creat a symlink
> +		 * named by spec_mntpoint to it.
> +		 */
> +		SAFE_SYMLINK(cgroup_mntpoint, spec_mntpoint);
> +	} else {
> +		/* Mount subsys to spec_mntpoint. */
> +		SAFE_MKDIR(spec_mntpoint, 0777);
> +		SAFE_MOUNT("none", spec_mntpoint, "cgroup", 0, subsys_name);
> +		cgroup_mounted = 1;
> +	}
> +}
> -- 
> 1.8.3.1
> 
> 
>