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

/*
 * Copyright (C) 2020 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 runinhibit contains operations for establishing, removing and
// querying snap run inhibition lock.
package runinhibit

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"time"

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

// defaultInhibitDir is the directory where inhibition files are stored.
const defaultInhibitDir = "/var/lib/snapd/inhibit"

// InhibitDir is the directory where inhibition files are stored.
// This value can be changed by calling dirs.SetRootDir.
var InhibitDir = defaultInhibitDir

func init() {
	dirs.AddRootDirCallback(func(root string) {
		InhibitDir = filepath.Join(root, defaultInhibitDir)
	})
}

// Hint is a string representing reason for the inhibition of "snap run".
type Hint string

const (
	// HintNotInhibited is used when "snap run" is not inhibited.
	HintNotInhibited Hint = ""
	// HintInhibitedGateRefresh represents inhibition of a "snap run" while gate-auto-refresh hook is run.
	HintInhibitedGateRefresh Hint = "gate-refresh"
	// HintInhibitedForRefresh represents inhibition of a "snap run" while a refresh change is being performed.
	HintInhibitedForRefresh Hint = "refresh"
	// HintInhibitedForPreDownload represents inhibition of a "snap run" while a
	// pre-download is triggering a refresh.
	HintInhibitedForPreDownload Hint = "pre-download"
	// HintInhibitedForRemove represents inhibition of a "snap run" while a remove change is being performed.
	HintInhibitedForRemove Hint = "remove"
)

const hintFilePostfix = "lock"

func (hint Hint) validate() error {
	if len(hint) == 0 {
		return fmt.Errorf("lock hint cannot be empty")
	}
	if string(hint) == hintFilePostfix {
		return fmt.Errorf("hint cannot have value %q", hintFilePostfix)
	}
	return nil
}

// HintFile returns the full path of the run inhibition lock file for the given snap.
func HintFile(snapName string) string {
	return filepath.Join(InhibitDir, fmt.Sprintf("%s.%s", snapName, hintFilePostfix))
}

func InhibitInfoFile(snapName string, hint Hint) string {
	return filepath.Join(InhibitDir, fmt.Sprintf("%s.%s", snapName, hint))
}

func openHintFileLock(snapName string) (*osutil.FileLock, error) {
	return osutil.NewFileLockWithMode(HintFile(snapName), 0644)
}

// InhibitInfo holds data of the previous snap revision that will be needed by
// "snap run" while the snap is unlinked (i.e. the current symlink is removed).
type InhibitInfo struct {
	// Previous is the previous revision for the snap being refreshed.
	Previous snap.Revision `json:"previous"`
}

func (info InhibitInfo) validate() error {
	if info.Previous.Unset() {
		return fmt.Errorf("snap revision cannot be unset")
	}
	return nil
}

func removeInhibitInfoFiles(snapName string) error {
	infoGlob := filepath.Join(InhibitDir, snapName+".*")
	// There should be one file only, but just in case
	files, err := filepath.Glob(infoGlob)
	if err != nil {
		return err
	}
	hintFile := filepath.Base(HintFile(snapName))
	for _, f := range files {
		// Don't remove hint
		if filepath.Base(f) == hintFile {
			continue
		}
		if err := os.Remove(f); err != nil {
			return err
		}
	}
	return nil
}

// Unlocker functions are passed from code using runinhibit to indicate that global
// state should be unlocked during file lock operations to avoid having deadlocks where
// both inhibition hint file and state are locked and waiting for each other. Unlocker
// being nil indicates not to do this.
type Unlocker func() (relock func())

// LockWithHint sets a persistent "snap run" inhibition lock, for the given snap, with a given hint
// and saves given info that will be needed by "snap run" during inhibition (e.g. snap revision).
//
// The hint cannot be empty. It should be one of the Hint constants defined in
// this package. With the hint in place "snap run" will not allow the snap to
// start and will block, presenting a user interface if possible. Also
// info.Previous corresponding to the snap revision that was installed must be
// provided and cannot be unset.
//
// If unlocker is passed it indicates that the global state needs to be unlocked
// before taking the inhibition hint file lock. It is the responsibility of the
// caller to make sure state is locked if a non-nil unlocker is passed.
func LockWithHint(snapName string, hint Hint, info InhibitInfo, unlocker Unlocker) error {
	if unlocker != nil {
		// unlock/relock global state
		relock := unlocker()
		defer relock()
	}

	if err := hint.validate(); err != nil {
		return err
	}
	if err := info.validate(); err != nil {
		return err
	}
	if err := os.MkdirAll(InhibitDir, 0755); err != nil {
		return err
	}
	flock, err := openHintFileLock(snapName)
	if err != nil {
		return err
	}
	defer flock.Close()

	// The following order of execution is important to avoid race conditions.
	// Take the lock
	if err := flock.Lock(); err != nil {
		return err
	}
	// Write inhibit info
	buf, err := json.Marshal(info)
	if err != nil {
		return err
	}
	if err := os.WriteFile(InhibitInfoFile(snapName, hint), buf, 0644); err != nil {
		return err
	}
	// Write hint
	f := flock.File()
	if err := f.Truncate(0); err != nil {
		return err
	}
	if _, err = f.WriteString(string(hint)); err != nil {
		return err
	}
	if err := f.Sync(); err != nil {
		return err
	}

	return nil
}

// Unlock truncates the run inhibition lock, for the given snap.
//
// An empty inhibition lock means uninhibited "snap run".
//
// If unlocker is passed it indicates that the global state needs to be unlocked
// before taking the inhibition hint file lock. It is the responsibility of the
// caller to make sure state is locked if a non-nil unlocker is passed.
func Unlock(snapName string, unlocker Unlocker) error {
	if unlocker != nil {
		// unlock/relock global state
		relock := unlocker()
		defer relock()
	}

	flock, err := openHintFileLock(snapName)
	if os.IsNotExist(err) {
		return nil
	}
	if err != nil {
		return err
	}
	defer flock.Close()

	// The following order of execution is important to avoid race conditions.
	// Take the lock
	if err := flock.Lock(); err != nil {
		return err
	}
	// Write HintNotInhibited
	f := flock.File()
	if err := f.Truncate(0); err != nil {
		return err
	}
	if err := f.Sync(); err != nil {
		return err
	}
	// Remove inhibit info file
	if err := removeInhibitInfoFiles(snapName); err != nil {
		return err
	}

	return nil
}

// IsLocked returns the state of the run inhibition lock for the given snap.
//
// It returns the current, non-empty hint if inhibition is in place. Otherwise
// it returns an empty hint.
//
// If unlocker is passed it indicates that the global state needs to be unlocked
// before taking the inhibition hint file lock. It is the responsibility of the
// caller to make sure state is locked if a non-nil unlocker is passed.
func IsLocked(snapName string, unlocker Unlocker) (Hint, InhibitInfo, error) {
	if unlocker != nil {
		// unlock/relock global state
		relock := unlocker()
		defer relock()
	}

	hintFlock, err := osutil.OpenExistingLockForReading(HintFile(snapName))
	if os.IsNotExist(err) {
		return "", InhibitInfo{}, nil
	}
	if err != nil {
		return "", InhibitInfo{}, err
	}
	defer hintFlock.Close()

	// The following order of execution is important to avoid race conditions.
	// Take the lock
	if err := hintFlock.ReadLock(); err != nil {
		return "", InhibitInfo{}, err
	}
	// Read hint
	hint, err := hintFromFile(hintFlock.File())
	if err != nil {
		return "", InhibitInfo{}, err
	}
	if hint == HintNotInhibited {
		return hint, InhibitInfo{}, nil
	}
	// Read inhibit info
	info, err := readInhibitInfo(snapName, hint)
	if err != nil {
		return "", InhibitInfo{}, err
	}

	return hint, info, nil
}

// RemoveLockFile removes the run inhibition lock for the given snap.
//
// This function should not be used as a substitute of Unlock, as race-free
// ability to inspect the inhibition state relies on flock(2) which requires the
// file to exist in the first place and non-privileged processes cannot create
// it.
//
// The function does not fail if the inhibition lock does not exist.
//
// If unlocker is passed it indicates that the global state needs to be unlocked
// before taking the inhibition hint file lock. It is the responsibility of the
// caller to make sure state is locked if a non-nil unlocker is passed.
func RemoveLockFile(snapName string, unlocker Unlocker) error {
	if unlocker != nil {
		// unlock/relock global state
		relock := unlocker()
		defer relock()
	}

	hintFlock, err := osutil.OpenExistingLockForReading(HintFile(snapName))
	if os.IsNotExist(err) {
		return nil
	}
	if err != nil {
		return err
	}
	defer hintFlock.Close()

	// The following order of execution is important to avoid race conditions.
	// Take the lock
	if err := hintFlock.Lock(); err != nil {
		return err
	}
	// Remove inhibit info files
	if err := removeInhibitInfoFiles(snapName); err != nil {
		return err
	}
	// Remove hint file
	err = os.Remove(HintFile(snapName))
	if err != nil && !os.IsNotExist(err) {
		return err
	}

	return nil
}

// needed for better mocking
type ticker interface {
	Wait() <-chan time.Time
}

type timeTicker struct {
	interval time.Duration
}

func (t *timeTicker) Wait() <-chan time.Time {
	return time.After(t.interval)
}

var newTicker = func(interval time.Duration) ticker {
	return &timeTicker{interval}
}

// WaitWhileInhibited blocks until snap is not inhibited anymore and then returns
// a locked hint file lock.
//
// The `inhibited` callback is run if the snap is initially inhibited, otherwise the `notInhibited`
// callback is run. In either case, this callback runs with the hint file lock held`.
//
// If inhibited callback returns true, WaitWhileInhibited returns instantly without
// waiting for the snap not being inhibited.
//
// NOTE: A snap without a hint file is considered not inhibited and a nil FileLock is returned.
//
// NOTE: It is the caller's responsibility to release the returned file lock.
var WaitWhileInhibited = func(ctx context.Context, snapName string, notInhibited func(ctx context.Context) error, inhibited func(ctx context.Context, hint Hint, inhibitInfo *InhibitInfo) (cont bool, err error), interval time.Duration) (flock *osutil.FileLock, err error) {
	ticker := newTicker(interval)

	// Release lock if we return early with an error
	defer func() {
		// Keep lock if no errors occur
		if err != nil && flock != nil {
			flock.Close()
			flock = nil
		}
	}()

	for {
		flock, err = osutil.OpenExistingLockForReading(HintFile(snapName))
		// We must return flock alongside errors so that cleanup defer can close it.
		if os.IsNotExist(err) {
			if notInhibited != nil {
				if err := notInhibited(ctx); err != nil {
					// No flock opened, it is okay to return nil here
					return nil, err
				}
			}
			return nil, nil
		}
		if err != nil {
			// No flock opened, it is okay to return nil here
			return nil, err
		}

		// Hold read lock
		if err := flock.ReadLock(); err != nil {
			return flock, err
		}

		// Read inhibition hint
		hint, err := hintFromFile(flock.File())
		if err != nil {
			return flock, err
		}

		if hint == HintNotInhibited {
			if notInhibited != nil {
				if err := notInhibited(ctx); err != nil {
					return flock, err
				}
			}
			return flock, nil
		} else {
			if inhibited != nil {
				inhibitInfo, err := readInhibitInfo(snapName, hint)
				if err != nil {
					return flock, err
				}
				cont, err := inhibited(ctx, hint, &inhibitInfo)
				if err != nil {
					return flock, err
				}
				if cont {
					return flock, nil
				}
			}
		}

		// Close the lock file explicitly so we can try to lock again after waiting for the interval.
		flock.Close()
		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case <-ticker.Wait():
		}
	}
}

func hintFromFile(hintFile *os.File) (Hint, error) {
	buf, err := io.ReadAll(hintFile)
	if err != nil {
		return "", err
	}
	return Hint(string(buf)), nil
}

func readInhibitInfo(snapName string, hint Hint) (InhibitInfo, error) {
	buf, err := os.ReadFile(InhibitInfoFile(snapName, hint))
	if err != nil {
		return InhibitInfo{}, err
	}
	var info InhibitInfo
	err = json.Unmarshal(buf, &info)
	if err != nil {
		return InhibitInfo{}, err
	}
	return info, nil
}
