//go:build linux
// +build linux

package memlimit

import (
	"path/filepath"

	"github.com/containerd/cgroups/v3"
	"github.com/containerd/cgroups/v3/cgroup1"
	"github.com/containerd/cgroups/v3/cgroup2"
)

const (
	cgroupMountPoint = "/sys/fs/cgroup"
)

// FromCgroup returns the memory limit based on the cgroups version on this system.
func FromCgroup() (uint64, error) {
	switch cgroups.Mode() {
	case cgroups.Legacy:
		return FromCgroupV1()
	case cgroups.Hybrid:
		return FromCgroupHybrid()
	case cgroups.Unified:
		return FromCgroupV2()
	}
	return 0, ErrNoCgroup
}

// FromCgroupV1 returns the memory limit from the cgroup v1.
func FromCgroupV1() (uint64, error) {
	cg, err := cgroup1.Load(cgroup1.RootPath, cgroup1.WithHiearchy(
		cgroup1.SingleSubsystem(cgroup1.Default, cgroup1.Memory),
	))
	if err != nil {
		return 0, err
	}

	metrics, err := cg.Stat(cgroup1.IgnoreNotExist)
	if err != nil {
		return 0, err
	}

	if limit := metrics.GetMemory().GetHierarchicalMemoryLimit(); limit != 0 {
		return limit, nil
	}

	return 0, ErrNoLimit
}

// FromCgroupHybrid returns the memory limit from the cgroup v1 or v2.
// It checks the cgroup v2 first, and if it fails, it falls back to cgroup v1.
func FromCgroupHybrid() (uint64, error) {
	limit, err := fromCgroupV2(filepath.Join(cgroupMountPoint, "unified"))
	if err == nil {
		return limit, nil
	} else if err != ErrNoLimit {
		return 0, err
	}

	return FromCgroupV1()
}

// FromCgroupV2 returns the memory limit from the cgroup v2.
func FromCgroupV2() (uint64, error) {
	return fromCgroupV2(cgroupMountPoint)
}

func fromCgroupV2(mountPoint string) (uint64, error) {
	path, err := cgroup2.NestedGroupPath("")
	if err != nil {
		return 0, err
	}

	m, err := cgroup2.Load(path, cgroup2.WithMountpoint(mountPoint))
	if err != nil {
		return 0, err
	}

	stats, err := m.Stat()
	if err != nil {
		return 0, err
	}

	if limit := stats.GetMemory().GetUsageLimit(); limit != 0 {
		return limit, nil
	}

	return 0, ErrNoLimit
}