func SubsystemMountpoints()

in metric/system/cgroup/util.go [205:278]
49 lines of code
16 McCabe index (conditional complexity)

func SubsystemMountpoints(rootfs resolve.Resolver, subsystems map[string]struct{}) (Mountpoints, error) {
	// TODO: will we run into mount namespace issues if we use /proc/self/mountinfo?
	mountinfo, err := os.Open(rootfs.ResolveHostFS("/proc/self/mountinfo"))
	if err != nil {
		return Mountpoints{}, err
	}
	defer mountinfo.Close()

	mounts := map[string]string{}
	mountInfo := Mountpoints{}
	sc := bufio.NewScanner(mountinfo)
	possibleV2Paths := []string{}
	for sc.Scan() {
		// https://www.kernel.org/doc/Documentation/filesystems/proc.txt
		// Example:
		// 25 21 0:20 / /cgroup/cpu rw,relatime - cgroup cgroup rw,cpu
		line := strings.TrimSpace(sc.Text())
		if line == "" {
			continue
		}

		mount, err := parseMountinfoLine(line)
		if err != nil {
			return Mountpoints{}, err
		}

		// if the mountpoint from the subsystem has a different root than ours, it probably belongs to something else.
		if !strings.HasPrefix(mount.mountpoint, rootfs.ResolveHostFS("")) {
			continue
		}

		// cgroupv1 option
		if mount.filesystemType == "cgroup" {
			for _, opt := range mount.superOptions {
				// Sometimes the subsystem name is written like "name=blkio".
				fields := strings.SplitN(opt, "=", 2)
				if len(fields) > 1 {
					opt = fields[1]
				}

				// Test if option is a subsystem name.
				if _, found := subsystems[opt]; found {
					// Add the subsystem mount if it does not already exist.
					if _, exists := mounts[opt]; !exists {
						mounts[opt] = mount.mountpoint
					}
				}
			}
		}

		// V2 option
		if mount.filesystemType == "cgroup2" {
			possibleV2Paths = append(possibleV2Paths, mount.mountpoint)
		}

	}

	mountInfo.V2Loc = getProperV2Paths(rootfs, possibleV2Paths)
	mountInfo.V1Mounts = mounts

	// we only care about a contanerized root path if we're trying to monitor a host system
	// from inside a container
	// This logic helps us proper fetch the cgroup path when we're running inside a container
	// with a private namespace
	if mountInfo.V2Loc != "" && rootfs.IsSet() && cgroupNSStateFetch() {
		mountInfo.ContainerizedRootMount, err = guessContainerCgroupPath(mountInfo.V2Loc, os.Getpid())
		// treat this as a non-fatal error. If we end up needing this value, the lookups will fail down the line
		if err != nil {
			logp.L().Debugf("could not fetch cgroup path inside container: %w", err)
		}
	}

	return mountInfo, sc.Err()
}