// -*- Mode: Go; indent-tabs-mode: t -*-

/*
 * Copyright (C) 2017 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package cgroup

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
	"time"

	"github.com/snapcore/snapd/dirs"
	"github.com/snapcore/snapd/logger"
	"github.com/snapcore/snapd/osutil"
)

const defaultFreezerCgroupV1Dir = "/sys/fs/cgroup/freezer"
const maxFreezeTimeout = 3000 * time.Millisecond
const freezePulseDelay = 100 * time.Millisecond

var freezerCgroupV1Dir = defaultFreezerCgroupV1Dir

var osReadFile = os.ReadFile

func init() {
	dirs.AddRootDirCallback(func(root string) {
		freezerCgroupV1Dir = filepath.Join(root, defaultFreezerCgroupV1Dir)
	})
}

// FreezeSnapProcesses suspends execution of all the processes belonging to
// a given snap. Processes remain frozen until ThawSnapProcesses is called,
// care must be taken not to freezer processes indefinitely.
//
// The freeze operation is not instant. Once commenced it proceeds
// asynchronously. Internally the function waits for the freezing to complete
// in at most maxFreezeTimeout or if the passed context is cancelled. If this
// time is insufficient then the processes are thawed and an error is returned.
//
// A correct implementation is picked depending on cgroup v1 or v2 use in the
// system. When cgroup v1 is detected, the call will directly act on the freezer
// group created when a snap process was started, while with v2 the call will
// act on all tracking groups of a snap.
//
// This operation can be mocked with MockFreezing
var FreezeSnapProcesses = func(ctx context.Context, snapName string) error {
	return errors.New("FreezeSnapProcesses not implemented")
}

// ThawSnapProcesses resumes execution of all processes belonging to a given snap.
//
// A correct implementation is picked depending on cgroup v1 or v2 use in the
// system. When cgroup v1 is detected, the call will directly act on the freezer
// group created when a snap process was started, while with v2 the call will
// act on all tracking groups of a snap.
//
// This operation can be mocked with MockFreezing
var ThawSnapProcesses = func(snapName string) error {
	return errors.New("ThawSnapProcesses not implemented")
}

// freezeSnapProcessesImplV1 freezes all the processes originating from the given snap.
// Processes are frozen regardless of which particular snap application they
// originate from.
var freezeSnapProcessesImplV1 = func(ctx context.Context, snapName string) error {
	fname := filepath.Join(freezerCgroupV1Dir, fmt.Sprintf("snap.%s", snapName), "freezer.state")
	if err := os.WriteFile(fname, []byte("FROZEN"), 0644); errors.Is(err, fs.ErrNotExist) {
		// When there's no freezer cgroup we don't have to freeze anything.
		// This can happen when no process belonging to a given snap has been
		// started yet.
		return nil
	} else if err != nil {
		return fmt.Errorf("cannot freeze processes of snap %q, %w", snapName, err)
	}

	// Add an upper bound to the timeout if cgroup is stuck at freeze state
	ctxWithTimeout, cancel := context.WithTimeout(ctx, maxFreezeTimeout)
	defer cancel()
	ticker := time.NewTicker(freezePulseDelay)
	// The ticker.Stop() can be removed starting 1.23 where the garbage collector can recover
	// unreferenced tickers, even if they haven't been stopped.
	defer ticker.Stop()
	for {
		data, err := osReadFile(fname)
		if err != nil {
			return fmt.Errorf("cannot determine the freeze state of processes of snap %q, %w", snapName, err)
		}
		// If the cgroup is frozen then we are done
		if bytes.Equal(bytes.TrimSpace(data), []byte("FROZEN")) {
			return nil
		}
		// Timeout or add a bit of delay
		select {
		case <-ctxWithTimeout.Done():
			// If we got here then we timed out after seeing FREEZING for too long.
			ThawSnapProcesses(snapName) // ignore the error, this is best-effort.
			return fmt.Errorf("cannot finish freezing processes of snap %q (freezer state: %s): %w", snapName, data, ctxWithTimeout.Err())
		case <-ticker.C:
		}
	}
}

var thawSnapProcessesImplV1 = func(snapName string) error {
	fname := filepath.Join(freezerCgroupV1Dir, fmt.Sprintf("snap.%s", snapName), "freezer.state")
	if err := os.WriteFile(fname, []byte("THAWED"), 0644); err != nil && errors.Is(err, fs.ErrNotExist) {
		// When there's no freezer cgroup we don't have to thaw anything.
		// This can happen when no process belonging to a given snap has been
		// started yet.
		return nil
	} else if err != nil {
		return fmt.Errorf("cannot thaw processes of snap %q", snapName)
	}
	return nil
}

func applyToSnap(snapName string, action func(groupName string) error, skipError func(err error) bool) error {
	if action == nil {
		return fmt.Errorf("internal error: action is nil")
	}
	if skipError == nil {
		return fmt.Errorf("internal error: skip error is nil")
	}
	canary := fmt.Sprintf("snap.%s.", snapName)
	cgroupRoot := filepath.Join(rootPath, cgroupMountPoint)
	if _, dir, _ := osutil.DirExists(cgroupRoot); !dir {
		return nil
	}
	return filepath.Walk(filepath.Join(rootPath, cgroupMountPoint), func(name string, info os.FileInfo, err error) error {
		if err != nil {
			if skipError(err) {
				// we don't know whether it's a file or
				// directory, so just return nil instead
				return nil
			}
			return err
		}
		if !info.IsDir() {
			return nil
		}
		if !strings.HasPrefix(info.Name(), canary) {
			return nil
		}
		// snap applications end up inside a cgroup related to a
		// service, or when ran standalone, a scope
		if ext := filepath.Ext(info.Name()); ext != ".scope" && ext != ".service" {
			return nil
		}
		// found a group
		if err := action(name); err != nil && !skipError(err) {
			return err
		}
		return filepath.SkipDir
	})
}

// writeExistingFile can be used as a drop-in replacement for os.WriteFile,
// but does not create a file when it does not exist
func writeExistingFile(where string, data []byte) error {
	f, err := os.OpenFile(where, os.O_WRONLY|os.O_TRUNC, 0644)
	if err != nil {
		return err
	}
	_, errW := f.Write(data)
	errC := f.Close()
	// pick the right error
	if errW != nil {
		return errW
	}
	return errC
}

func freezeOneV2(ctx context.Context, dir string) error {
	groupName := filepath.Base(dir)
	fname := filepath.Join(dir, "cgroup.freeze")
	if err := writeExistingFile(fname, []byte("1")); err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			//  the group may be gone already
			return nil
		}
		return fmt.Errorf("cannot freeze processes in group %q: %w", groupName, err)
	}

	// Add an upper bound to the timeout if cgroup is stuck at freeze state
	ctxWithTimeout, cancel := context.WithTimeout(ctx, maxFreezeTimeout)
	defer cancel()
	ticker := time.NewTicker(freezePulseDelay)
	// ticker.Stop() can be removed starting 1.23 where the garbage collector can recover
	// unreferenced tickers, even if they haven't been stopped.
	defer ticker.Stop()
	for {
		data, err := os.ReadFile(fname)
		if err != nil {
			if errors.Is(err, fs.ErrNotExist) {
				// group may be gone
				return nil
			}
			return fmt.Errorf("cannot determine the freeze state of processes in group %q: %w", groupName, err)
		}
		// If the cgroup is still freezing then wait a moment and try again.
		if bytes.Equal(bytes.TrimSpace(data), []byte("1")) {
			// we're done
			return nil
		}
		// timeout or add a bit of delay
		select {
		case <-ctxWithTimeout.Done():
			return fmt.Errorf("cannot freeze processes in group %q: %w", groupName, ctxWithTimeout.Err())
		case <-ticker.C:
		}
	}
}

func skipErrNotExist(err error) bool {
	return errors.Is(err, fs.ErrNotExist)
}

// freezeSnapProcessesImplV2 freezes all the processes originating from the
// given snap. Processes are frozen regardless of which particular snap
// application they originate from.
func freezeSnapProcessesImplV2(ctx context.Context, snapName string) error {
	// in case of v2, the process calling this code, (eg. snap-update-ns)
	// may already be part of the trackign cgroup for particular snap, care
	// must be taken to not freeze ourselves
	ownGroup, err := cgroupProcessPathInTrackingCgroup(os.Getpid())
	if err != nil {
		return err
	}
	ownGroupDir := filepath.Join(rootPath, cgroupMountPoint, ownGroup)
	freezeOne := func(dir string) error {
		if dir == ownGroupDir {
			// let's not freeze ourselves
			logger.Debugf("freeze, skipping own group %v", dir)
			return nil
		}
		return freezeOneV2(ctx, dir)
	}
	// freeze, skipping ENOENT errors
	err = applyToSnap(snapName, freezeOne, skipErrNotExist)
	if err == nil {
		return nil
	}
	// we either got here because we hit a timeout freezing snap processes
	// or some other error

	// ignore errors when thawing processes, this is best-effort.
	alwaysSkipError := func(_ error) bool { return true }
	thawSnapProcessesV2(snapName, alwaysSkipError)
	return fmt.Errorf("cannot finish freezing processes of snap %q: %w", snapName, err)
}

func thawOneV2(dir string) error {
	fname := filepath.Join(dir, "cgroup.freeze")
	if err := writeExistingFile(fname, []byte("0")); err != nil && !errors.Is(err, fs.ErrNotExist) {
		return err
	}
	return nil
}

func thawSnapProcessesV2(snapName string, skipError func(error) bool) error {
	if skipError == nil {
		return fmt.Errorf("internal error: skip error is nil")
	}
	thawOne := func(dir string) error {
		if err := thawOneV2(dir); err != nil && !skipError(err) {
			return fmt.Errorf("cannot thaw processes of snap %q, %w", snapName, err)
		}
		return nil
	}
	return applyToSnap(snapName, thawOne, skipError)
}

func thawSnapProcessesImplV2(snapName string) error {
	// thaw skipping ENOENT errors
	return thawSnapProcessesV2(snapName, skipErrNotExist)
}

// MockFreezing replaces the real implementation of freeze and thaw.
func MockFreezing(freeze func(ctx context.Context, snapName string) error, thaw func(snapName string) error) (restore func()) {
	oldFreeze := FreezeSnapProcesses
	oldThaw := ThawSnapProcesses

	FreezeSnapProcesses = freeze
	ThawSnapProcesses = thaw

	return func() {
		FreezeSnapProcesses = oldFreeze
		ThawSnapProcesses = oldThaw
	}
}
