//go:build linux

package authconfig

import (
	"context"
	"errors"
	"os"
	"sync"
	"time"

	"golang.org/x/sys/unix"
)

type transactionLock struct {
	directoryFD int
	fileFD      int
	closed      bool
}

type projectionLockHooks struct {
	beforeCreate func()
	afterOpen    func()
}

var projectionLockHookState struct {
	sync.Mutex
	hooks projectionLockHooks
}

func setProjectionLockHooksForTest(hooks projectionLockHooks) func() {
	projectionLockHookState.Lock()
	previous := projectionLockHookState.hooks
	projectionLockHookState.hooks = hooks
	projectionLockHookState.Unlock()
	return func() {
		projectionLockHookState.Lock()
		projectionLockHookState.hooks = previous
		projectionLockHookState.Unlock()
	}
}

func projectionLockHooksForTest() projectionLockHooks {
	projectionLockHookState.Lock()
	defer projectionLockHookState.Unlock()
	return projectionLockHookState.hooks
}

func acquireTransactionLock(ctx context.Context, directory string) (*transactionLock, error) {
	if ctx == nil || ctx.Err() != nil || requirePrivateDirectory(directory) != nil {
		return nil, errProjectionIntegrity
	}
	directoryFD, err := unix.Open(directory, unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY|unix.O_NOFOLLOW, 0)
	if err != nil {
		return nil, errProjectionIntegrity
	}
	lock := &transactionLock{directoryFD: directoryFD, fileFD: -1}
	fail := func() (*transactionLock, error) {
		_ = lock.Close()
		return nil, errProjectionIntegrity
	}
	if !validTransactionDirectory(directoryFD) {
		return fail()
	}
	fileFD, err := openTransactionLockFile(directoryFD)
	if err != nil {
		return fail()
	}
	lock.fileFD = fileFD
	if hook := projectionLockHooksForTest().afterOpen; hook != nil {
		hook()
	}
	var opened unix.Stat_t
	if unix.Fstat(fileFD, &opened) != nil || !validTransactionLockFile(opened) || !sameTransactionLockName(directoryFD, opened) {
		return fail()
	}
	if err := flockTransactionFile(ctx, fileFD); err != nil {
		_ = lock.Close()
		return nil, err
	}
	if !sameTransactionLockName(directoryFD, opened) {
		return fail()
	}
	return lock, nil
}

func openTransactionLockFile(directoryFD int) (int, error) {
	for attempt := 0; attempt < 2; attempt++ {
		fd, err := unix.Openat(directoryFD, transactionLockFileName, unix.O_RDWR|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
		if err == nil {
			return fd, nil
		}
		if !errors.Is(err, unix.ENOENT) {
			return -1, errProjectionIntegrity
		}
		if hook := projectionLockHooksForTest().beforeCreate; hook != nil {
			hook()
		}
		fd, err = unix.Openat(directoryFD, transactionLockFileName, unix.O_RDWR|unix.O_CREAT|unix.O_EXCL|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0o600)
		if err == nil {
			if unix.Fchmod(fd, 0o600) != nil || unix.Fsync(fd) != nil || unix.Fsync(directoryFD) != nil {
				unix.Close(fd)
				return -1, errProjectionIntegrity
			}
			return fd, nil
		}
		if !errors.Is(err, unix.EEXIST) {
			return -1, errProjectionIntegrity
		}
	}
	return -1, errProjectionIntegrity
}

func validTransactionDirectory(fd int) bool {
	var stat unix.Stat_t
	return unix.Fstat(fd, &stat) == nil && stat.Mode&unix.S_IFMT == unix.S_IFDIR && stat.Uid == uint32(os.Geteuid()) && stat.Mode&0o077 == 0
}

func validTransactionLockFile(stat unix.Stat_t) bool {
	return stat.Mode&unix.S_IFMT == unix.S_IFREG && stat.Nlink == 1 && stat.Uid == uint32(os.Geteuid()) && stat.Mode&0o7777 == 0o600
}

func sameTransactionLockName(directoryFD int, opened unix.Stat_t) bool {
	var named unix.Stat_t
	return unix.Fstatat(directoryFD, transactionLockFileName, &named, unix.AT_SYMLINK_NOFOLLOW) == nil && validTransactionLockFile(named) && named.Dev == opened.Dev && named.Ino == opened.Ino
}

func flockTransactionFile(ctx context.Context, fd int) error {
	for {
		if err := ctx.Err(); err != nil {
			return err
		}
		err := unix.Flock(fd, unix.LOCK_EX|unix.LOCK_NB)
		if err == nil {
			return nil
		}
		if !errors.Is(err, unix.EWOULDBLOCK) {
			return errProjectionIntegrity
		}
		if hook := coordinatorHooks().onOuterLockContention; hook != nil {
			hook()
		}
		timer := time.NewTimer(10 * time.Millisecond)
		select {
		case <-ctx.Done():
			if !timer.Stop() {
				<-timer.C
			}
			return ctx.Err()
		case <-timer.C:
		}
	}
}

func (lock *transactionLock) Close() error {
	if lock == nil || lock.closed {
		return nil
	}
	lock.closed = true
	var err error
	if lock.fileFD >= 0 {
		if unlockErr := unix.Flock(lock.fileFD, unix.LOCK_UN); unlockErr != nil {
			err = errors.Join(err, errProjectionCleanup)
		}
		if closeErr := unix.Close(lock.fileFD); closeErr != nil {
			err = errors.Join(err, errProjectionCleanup)
		}
		lock.fileFD = -1
	}
	if lock.directoryFD >= 0 {
		if closeErr := unix.Close(lock.directoryFD); closeErr != nil {
			err = errors.Join(err, errProjectionCleanup)
		}
		lock.directoryFD = -1
	}
	return err
}