#!/usr/local/bin/python3.12
# vim:fileencoding=utf-8
# License: GPL v3 Copyright: 2022, Kovid Goyal <kovid at kovidgoyal.net>


import errno
import mmap
import os
import secrets
import stat
import struct
from typing import Optional, Union
from calibre.constants import iswindows, ismacos

SHM_NAME_MAX = 30 if ismacos else 254
if iswindows:
    import _winapi
else:
    import _posixshmem


def make_filename(prefix: str) -> str:
    "Create a random filename for the shared memory object."
    # number of random bytes to use for name. Use a largeish value
    # to make double unlink safe.
    if not iswindows and not prefix.startswith('/'):
        # FreeBSD requires name to start with /
        prefix = '/' + prefix
    plen = len(prefix.encode('utf-8'))
    safe_length = min(plen + 64, SHM_NAME_MAX)
    if safe_length - plen < 2:
        raise OSError(errno.ENAMETOOLONG, f'SHM filename prefix {prefix} is too long')
    nbytes = (safe_length - plen) // 2
    name = prefix + secrets.token_hex(nbytes)
    return name


class SharedMemory:
    '''
    Create or access randomly named shared memory. To create call with empty name and specific size.
    To access call with name only.

    WARNING: The actual size of the shared memory may be larger than the requested size.
    '''
    _fd: int = -1
    _name: str = ''
    _mmap: Optional[mmap.mmap] = None
    _size: int = 0
    size_fmt = '!I'
    num_bytes_for_size = struct.calcsize(size_fmt)

    def __init__(
            self, name: str = '', size: int = 0, readonly: bool = False,
            mode: int = stat.S_IREAD | stat.S_IWRITE,
            prefix: str = 'calibre-'
    ):
        if size < 0:
            raise TypeError("'size' must be a non-negative integer")
        if size and name:
            raise TypeError('Cannot specify both name and size')
        if not name:
            flags = os.O_CREAT | os.O_EXCL
            if not size:
                raise TypeError("'size' must be > 0")
        else:
            flags = 0
        flags |= os.O_RDONLY if readonly else os.O_RDWR
        access = mmap.ACCESS_READ if readonly else mmap.ACCESS_WRITE

        create = not name
        tries = 30
        while not name and tries > 0:
            tries -= 1
            q = make_filename(prefix)
            if iswindows:
                h_map = _winapi.CreateFileMapping(
                    _winapi.INVALID_HANDLE_VALUE,
                    _winapi.NULL,
                    _winapi.PAGE_READONLY if readonly else _winapi.PAGE_READWRITE,
                    (size >> 32) & 0xFFFFFFFF,
                    size & 0xFFFFFFFF,
                    q
                )
                try:
                    last_error_code = _winapi.GetLastError()
                    if last_error_code == _winapi.ERROR_ALREADY_EXISTS:
                        continue
                    self._mmap = mmap.mmap(-1, size, tagname=q, access=access)
                    name = q
                finally:
                    _winapi.CloseHandle(h_map)
            else:
                try:
                    self._fd = _posixshmem.shm_open(q, flags, mode=mode)
                    name = q
                except FileExistsError:
                    continue
        if tries <= 0:
            raise OSError(f'Failed to create a uniquely named SHM file, try shortening the prefix from: {prefix}')
        self._name = name

        if not create and iswindows:
            h_map = _winapi.OpenFileMapping(
                _winapi.FILE_MAP_READ,
                False,
                name
            )
            try:
                p_buf = _winapi.MapViewOfFile(
                    h_map,
                    _winapi.FILE_MAP_READ,
                    0,
                    0,
                    0
                )
            finally:
                _winapi.CloseHandle(h_map)
            size = _winapi.VirtualQuerySize(p_buf)
            self._mmap = mmap.mmap(-1, size, tagname=name)
        if not iswindows:
            if not create:
                self._fd = _posixshmem.shm_open(name, flags, mode)
            try:
                if flags & os.O_CREAT and size:
                    os.ftruncate(self._fd, size)
                self.stats = os.fstat(self._fd)
                size = self.stats.st_size
                self._mmap = mmap.mmap(self._fd, size, access=access)
            except OSError:
                self.unlink()
                raise

        self._size = size

    def read(self, sz: int = 0) -> bytes:
        if sz <= 0:
            sz = self.size
        return self.mmap.read(sz)

    def write(self, data: bytes) -> None:
        self.mmap.write(data)

    def tell(self) -> int:
        return self.mmap.tell()

    def seek(self, pos: int, whence: int = os.SEEK_SET) -> None:
        self.mmap.seek(pos, whence)

    def flush(self) -> None:
        self.mmap.flush()

    def write_data_with_size(self, data: Union[str, bytes]) -> None:
        if isinstance(data, str):
            data = data.encode('utf-8')
        sz = struct.pack(self.size_fmt, len(data))
        self.write(sz)
        self.write(data)

    def read_data_with_size(self) -> bytes:
        sz = struct.unpack(self.size_fmt, self.read(self.num_bytes_for_size))[0]
        return self.read(sz)

    def __del__(self) -> None:
        try:
            self.close()
        except OSError:
            pass

    def __enter__(self) -> 'SharedMemory':
        return self

    def __exit__(self, *a: object) -> None:
        self.close()

    @property
    def size(self) -> int:
        return self._size

    @property
    def name(self) -> str:
        return self._name

    @property
    def mmap(self) -> mmap.mmap:
        ans = self._mmap
        if ans is None:
            raise RuntimeError('Cannot access the mmap of a closed shared memory object')
        return ans

    def fileno(self) -> int:
        return self._fd

    def __repr__(self) -> str:
        return f'{self.__class__.__name__}({self.name!r}, size={self.size})'

    def close(self) -> None:
        """Closes access to the shared memory from this instance but does
        not destroy the shared memory block."""
        if self._mmap is not None:
            self._mmap.close()
            self._mmap = None
        if self._fd > -1:
            os.close(self._fd)
            self._fd = -1
        self.unlink()

    def unlink(self) -> None:
        """Requests that the underlying shared memory block be destroyed.

        In order to ensure proper cleanup of resources, unlink should be
        called once (and only once) across all processes which have access
        to the shared memory block."""
        if self._name:
            if not iswindows:
                try:
                    _posixshmem.shm_unlink(self._name)
                except FileNotFoundError:
                    pass
            self._name = ''


def find_tests():
    import unittest

    class TestSHM(unittest.TestCase):
        ae = unittest.TestCase.assertEqual

        def test_shm(self):
            with SharedMemory(size=64) as shm:
                q = b'test'
                shm.write_data_with_size(q)
                self.ae(shm.tell(), shm.num_bytes_for_size + len(q))
                shm.flush()
                with SharedMemory(shm.name, readonly=True) as s2:
                    self.ae(s2.read_data_with_size(), q)
                    shm.write(b'ABCD')
                    shm.flush()
                    self.ae(s2.read(4), b'ABCD')
                self.assertTrue(shm.name)
            self.assertFalse(shm.name)

    return unittest.defaultTestLoader.loadTestsFromTestCase(TestSHM)
