shared_memory.py

"""Provides shared memory for direct access across processes.

The API of this package is currently provisional. Refer to the
documentation for details.
"""


__all__ = [ 'SharedMemory', 'ShareableList' ]


from functools import partial
import mmap
import os
import errno
import struct
import secrets

if os.name == "nt":
    import _winapi
    _USE_POSIX = False
else:
    import _posixshmem
    _USE_POSIX = True


_O_CREX = os.O_CREAT | os.O_EXCL

# FreeBSD (and perhaps other BSDs) limit names to 14 characters.
_SHM_SAFE_NAME_LENGTH = 14

# Shared memory block name prefix
if _USE_POSIX:
    _SHM_NAME_PREFIX = '/psm_'
else:
    _SHM_NAME_PREFIX = 'wnsm_'


def _make_filename():
    "Create a random filename for the shared memory object."
    # number of random bytes to use for name
    nbytes = (_SHM_SAFE_NAME_LENGTH - len(_SHM_NAME_PREFIX)) // 2
    assert nbytes >= 2, '_SHM_NAME_PREFIX too long'
    name = _SHM_NAME_PREFIX + secrets.token_hex(nbytes)
    assert len(name) <= _SHM_SAFE_NAME_LENGTH
    return name


class SharedMemory:
    """Creates a new shared memory block or attaches to an existing
    shared memory block.

    Every shared memory block is assigned a unique name.  This enables
    one process to create a shared memory block with a particular name
    so that a different process can attach to that same shared memory
    block using that same name.

    As a resource for sharing data across processes, shared memory blocks
    may outlive the original process that created them.  When one process
    no longer needs access to a shared memory block that might still be
    needed by other processes, the close() method should be called.
    When a shared memory block is no longer needed by any process, the
    unlink() method should be called to ensure proper cleanup."""

    # Defaults; enables close() and unlink() to run without errors.
    _name = None
    _fd = -1
    _mmap = None
    _buf = None
    _flags = os.O_RDWR
    _mode = 0o600
    _prepend_leading_slash = True if _USE_POSIX else False

    def __init__(self, name=None, create=False, size=0):
        if not size >= 0:
            raise ValueError("'size' must be a positive integer")
        if create:
            self._flags = _O_CREX | os.O_RDWR
        if name is None and not self._flags & os.O_EXCL:
            raise ValueError("'name' can only be None if create=True")

        if _USE_POSIX:

            # POSIX Shared Memory

            if name is None:
                while True:
                    name = _make_filename()
                    try:
                        self._fd = _posixshmem.shm_open(
                            name,
                            self._flags,
                            mode=self._mode
                        )
                    except FileExistsError:
                        continue
                    self._name = name
                    break
            else:
                name = "/" + name if self._prepend_leading_slash else name
                self._fd = _posixshmem.shm_open(
                    name,
                    self._flags,
                    mode=self._mode
                )
                self._name = name
            try:
                if create and size:
                    os.ftruncate(self._fd, size)
                stats = os.fstat(self._fd)
                size = stats.st_size
                self._mmap = mmap.mmap(self._fd, size)
            except OSError:
                self.unlink()
                raise

        else:

            # Windows Named Shared Memory

            if create:
                while True:
                    temp_name = _make_filename() if name is None else name
                    # Create and reserve shared memory block with this name
                    # until it can be attached to by mmap.
                    h_map = _winapi.CreateFileMapping(
                        _winapi.INVALID_HANDLE_VALUE,
                        _winapi.NULL,
                        _winapi.PAGE_READWRITE,
                        (size >> 32) & 0xFFFFFFFF,
                        size & 0xFFFFFFFF,
                        temp_name
                    )
                    try:
                        last_error_code = _winapi.GetLastError()
                        if last_error_code == _winapi.ERROR_ALREADY_EXISTS:
                            if name is not None:
                                raise FileExistsError(
                                    errno.EEXIST,
                                    os.strerror(errno.EEXIST),
                                    name,
                                    _winapi.ERROR_ALREADY_EXISTS
                                )
                            else:
                                continue
                        self._mmap = mmap.mmap(-1, size, tagname=temp_name)
                    finally:
                        _winapi.CloseHandle(h_map)
                    self._name = temp_name
                    break

            else:
                self._name = name
                # Dynamically determine the existing named shared memory
                # block's size which is likely a multiple of mmap.PAGESIZE.
                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)

        self._size = size
        self._buf = memoryview(self._mmap)

    def __del__(self):
        try:
            self.close()
        except OSError:
            pass

    def __reduce__(self):
        return (
            self.__class__,
            (
                self.name,
                False,
                self.size,
            ),
        )

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

    @property
    def buf(self):
        "A memoryview of contents of the shared memory block."
        return self._buf

    @property
    def name(self):
        "Unique name that identifies the shared memory block."
        reported_name = self._name
        if _USE_POSIX and self._prepend_leading_slash:
            if self._name.startswith("/"):
                reported_name = self._name[1:]
        return reported_name

    @property
    def size(self):
        "Size in bytes."
        return self._size

    def close(self):
        """Closes access to the shared memory from this instance but does
        not destroy the shared memory block."""
        if self._buf is not None:
            self._buf.release()
            self._buf = None
        if self._mmap is not None:
            self._mmap.close()
            self._mmap = None
        if _USE_POSIX and self._fd >= 0:
            os.close(self._fd)
            self._fd = -1

    def unlink(self):
        """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 _USE_POSIX and self._name:
            _posixshmem.shm_unlink(self._name)


_encoding = "utf8"

class ShareableList:
    """Pattern for a mutable list-like object shareable via a shared
    memory block.  It differs from the built-in list type in that these
    lists can not change their overall length (i.e. no append, insert,
    etc.)

    Because values are packed into a memoryview as bytes, the struct
    packing format for any storable value must require no more than 8
    characters to describe its format."""

    _types_mapping = {
        int: "q",
        float: "d",
        bool: "xxxxxxx?",
        str: "%ds",
        bytes: "%ds",
        None.__class__: "xxxxxx?x",
    }
    _alignment = 8
    _back_transforms_mapping = {
        0: lambda value: value,                   # int, float, bool
        1: lambda value: value.rstrip(b'\x00').decode(_encoding),  # str
        2: lambda value: value.rstrip(b'\x00'),   # bytes
        3: lambda _value: None,                   # None
    }

    @staticmethod
    def _extract_recreation_code(value):
        """Used in concert with _back_transforms_mapping to convert values
        into the appropriate Python objects when retrieving them from
        the list as well as when storing them."""
        if not isinstance(value, (str, bytes, None.__class__)):
            return 0
        elif isinstance(value, str):
            return 1
        elif isinstance(value, bytes):
            return 2
        else:
            return 3  # NoneType

    def __init__(self, sequence=None, *, name=None):
        if sequence is not None:
            _formats = [
                self._types_mapping[type(item)]
                    if not isinstance(item, (str, bytes))
                    else self._types_mapping[type(item)] % (
                        self._alignment * (len(item) // self._alignment + 1),
                    )
                for item in sequence
            ]
            self._list_len = len(_formats)
            assert sum(len(fmt) <= 8 for fmt in _formats) == self._list_len
            self._allocated_bytes = tuple(
                    self._alignment if fmt[-1] != "s" else int(fmt[:-1])
                    for fmt in _formats
            )
            _recreation_codes = [
                self._extract_recreation_code(item) for item in sequence
            ]
            requested_size = struct.calcsize(
                "q" + self._format_size_metainfo +
                "".join(_formats) +
                self._format_packing_metainfo +
                self._format_back_transform_codes
            )

        else:
            requested_size = 8  # Some platforms require > 0.

        if name is not None and sequence is None:
            self.shm = SharedMemory(name)
        else:
            self.shm = SharedMemory(name, create=True, size=requested_size)

        if sequence is not None:
            _enc = _encoding
            struct.pack_into(
                "q" + self._format_size_metainfo,
                self.shm.buf,
                0,
                self._list_len,
                *(self._allocated_bytes)
            )
            struct.pack_into(
                "".join(_formats),
                self.shm.buf,
                self._offset_data_start,
                *(v.encode(_enc) if isinstance(v, str) else v for v in sequence)
            )
            struct.pack_into(
                self._format_packing_metainfo,
                self.shm.buf,
                self._offset_packing_formats,
                *(v.encode(_enc) for v in _formats)
            )
            struct.pack_into(
                self._format_back_transform_codes,
                self.shm.buf,
                self._offset_back_transform_codes,
                *(_recreation_codes)
            )

        else:
            self._list_len = len(self)  # Obtains size from offset 0 in buffer.
            self._allocated_bytes = struct.unpack_from(
                self._format_size_metainfo,
                self.shm.buf,
                1 * 8
            )

    def _get_packing_format(self, position):
        "Gets the packing format for a single value stored in the list."
        position = position if position >= 0 else position + self._list_len
        if (position >= self._list_len) or (self._list_len < 0):
            raise IndexError("Requested position out of range.")

        v = struct.unpack_from(
            "8s",
            self.shm.buf,
            self._offset_packing_formats + position * 8
        )[0]
        fmt = v.rstrip(b'\x00')
        fmt_as_str = fmt.decode(_encoding)

        return fmt_as_str

    def _get_back_transform(self, position):
        "Gets the back transformation function for a single value."

        position = position if position >= 0 else position + self._list_len
        if (position >= self._list_len) or (self._list_len < 0):
            raise IndexError("Requested position out of range.")

        transform_code = struct.unpack_from(
            "b",
            self.shm.buf,
            self._offset_back_transform_codes + position
        )[0]
        transform_function = self._back_transforms_mapping[transform_code]

        return transform_function

    def _set_packing_format_and_transform(self, position, fmt_as_str, value):
        """Sets the packing format and back transformation code for a
        single value in the list at the specified position."""

        position = position if position >= 0 else position + self._list_len
        if (position >= self._list_len) or (self._list_len < 0):
            raise IndexError("Requested position out of range.")

        struct.pack_into(
            "8s",
            self.shm.buf,
            self._offset_packing_formats + position * 8,
            fmt_as_str.encode(_encoding)
        )

        transform_code = self._extract_recreation_code(value)
        struct.pack_into(
            "b",
            self.shm.buf,
            self._offset_back_transform_codes + position,
            transform_code
        )

    def __getitem__(self, position):
        try:
            offset = self._offset_data_start \
                     + sum(self._allocated_bytes[:position])
            (v,) = struct.unpack_from(
                self._get_packing_format(position),
                self.shm.buf,
                offset
            )
        except IndexError:
            raise IndexError("index out of range")

        back_transform = self._get_back_transform(position)
        v = back_transform(v)

        return v

    def __setitem__(self, position, value):
        try:
            offset = self._offset_data_start \
                     + sum(self._allocated_bytes[:position])
            current_format = self._get_packing_format(position)
        except IndexError:
            raise IndexError("assignment index out of range")

        if not isinstance(value, (str, bytes)):
            new_format = self._types_mapping[type(value)]
        else:
            if len(value) > self._allocated_bytes[position]:
                raise ValueError("exceeds available storage for existing str")
            if current_format[-1] == "s":
                new_format = current_format
            else:
                new_format = self._types_mapping[str] % (
                    self._allocated_bytes[position],
                )

        self._set_packing_format_and_transform(
            position,
            new_format,
            value
        )
        value = value.encode(_encoding) if isinstance(value, str) else value
        struct.pack_into(new_format, self.shm.buf, offset, value)

    def __reduce__(self):
        return partial(self.__class__, name=self.shm.name), ()

    def __len__(self):
        return struct.unpack_from("q", self.shm.buf, 0)[0]

    def __repr__(self):
        return f'{self.__class__.__name__}({list(self)}, name={self.shm.name!r})'

    @property
    def format(self):
        "The struct packing format used by all currently stored values."
        return "".join(
            self._get_packing_format(i) for i in range(self._list_len)
        )

    @property
    def _format_size_metainfo(self):
        "The struct packing format used for metainfo on storage sizes."
        return f"{self._list_len}q"

    @property
    def _format_packing_metainfo(self):
        "The struct packing format used for the values' packing formats."
        return "8s" * self._list_len

    @property
    def _format_back_transform_codes(self):
        "The struct packing format used for the values' back transforms."
        return "b" * self._list_len

    @property
    def _offset_data_start(self):
        return (self._list_len + 1) * 8  # 8 bytes per "q"

    @property
    def _offset_packing_formats(self):
        return self._offset_data_start + sum(self._allocated_bytes)

    @property
    def _offset_back_transform_codes(self):
        return self._offset_packing_formats + self._list_len * 8

    def count(self, value):
        "L.count(value) -> integer -- return number of occurrences of value."

        return sum(value == entry for entry in self)

    def index(self, value):
        """L.index(value) -> integer -- return first index of value.
        Raises ValueError if the value is not present."""

        for position, entry in enumerate(self):
            if value == entry:
                return position
        else:
            raise ValueError(f"{value!r} not in this container")