_pyio.py

"""
Python implementation of the io module.
"""

from __future__ import (print_function, unicode_literals)

import os
import abc
import codecs
import sys
import warnings
import errno
# Import thread instead of threading to reduce startup cost
try:
    from thread import allocate_lock as Lock
except ImportError:
    from dummy_thread import allocate_lock as Lock

import io
from io import (__all__, SEEK_SET, SEEK_CUR, SEEK_END)
from errno import EINTR

__metaclass__ = type

# open() uses st_blksize whenever we can
DEFAULT_BUFFER_SIZE = 8 * 1024  # bytes

# NOTE: Base classes defined here are registered with the "official" ABCs
# defined in io.py. We don't use real inheritance though, because we don't want
# to inherit the C implementations.


class BlockingIOError(IOError):

    """Exception raised when I/O would block on a non-blocking I/O stream."""

    def __init__(self, errno, strerror, characters_written=0):
        super(IOError, self).__init__(errno, strerror)
        if not isinstance(characters_written, (int, long)):
            raise TypeError("characters_written must be a integer")
        self.characters_written = characters_written


def open(file, mode="r", buffering=-1,
         encoding=None, errors=None,
         newline=None, closefd=True):

    r"""Open file and return a stream.  Raise IOError upon failure.

    file is either a text or byte string giving the name (and the path
    if the file isn't in the current working directory) of the file to
    be opened or an integer file descriptor of the file to be
    wrapped. (If a file descriptor is given, it is closed when the
    returned I/O object is closed, unless closefd is set to False.)

    mode is an optional string that specifies the mode in which the file
    is opened. It defaults to 'r' which means open for reading in text
    mode.  Other common values are 'w' for writing (truncating the file if
    it already exists), and 'a' for appending (which on some Unix systems,
    means that all writes append to the end of the file regardless of the
    current seek position). In text mode, if encoding is not specified the
    encoding used is platform dependent. (For reading and writing raw
    bytes use binary mode and leave encoding unspecified.) The available
    modes are:

    ========= ===============================================================
    Character Meaning
    --------- ---------------------------------------------------------------
    'r'       open for reading (default)
    'w'       open for writing, truncating the file first
    'a'       open for writing, appending to the end of the file if it exists
    'b'       binary mode
    't'       text mode (default)
    '+'       open a disk file for updating (reading and writing)
    'U'       universal newline mode (for backwards compatibility; unneeded
              for new code)
    ========= ===============================================================

    The default mode is 'rt' (open for reading text). For binary random
    access, the mode 'w+b' opens and truncates the file to 0 bytes, while
    'r+b' opens the file without truncation.

    Python distinguishes between files opened in binary and text modes,
    even when the underlying operating system doesn't. Files opened in
    binary mode (appending 'b' to the mode argument) return contents as
    bytes objects without any decoding. In text mode (the default, or when
    't' is appended to the mode argument), the contents of the file are
    returned as strings, the bytes having been first decoded using a
    platform-dependent encoding or using the specified encoding if given.

    buffering is an optional integer used to set the buffering policy.
    Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
    line buffering (only usable in text mode), and an integer > 1 to indicate
    the size of a fixed-size chunk buffer.  When no buffering argument is
    given, the default buffering policy works as follows:

    * Binary files are buffered in fixed-size chunks; the size of the buffer
      is chosen using a heuristic trying to determine the underlying device's
      "block size" and falling back on `io.DEFAULT_BUFFER_SIZE`.
      On many systems, the buffer will typically be 4096 or 8192 bytes long.

    * "Interactive" text files (files for which isatty() returns True)
      use line buffering.  Other text files use the policy described above
      for binary files.

    encoding is the name of the encoding used to decode or encode the
    file. This should only be used in text mode. The default encoding is
    platform dependent, but any encoding supported by Python can be
    passed.  See the codecs module for the list of supported encodings.

    errors is an optional string that specifies how encoding errors are to
    be handled---this argument should not be used in binary mode. Pass
    'strict' to raise a ValueError exception if there is an encoding error
    (the default of None has the same effect), or pass 'ignore' to ignore
    errors. (Note that ignoring encoding errors can lead to data loss.)
    See the documentation for codecs.register for a list of the permitted
    encoding error strings.

    newline controls how universal newlines works (it only applies to text
    mode). It can be None, '', '\n', '\r', and '\r\n'.  It works as
    follows:

    * On input, if newline is None, universal newlines mode is
      enabled. Lines in the input can end in '\n', '\r', or '\r\n', and
      these are translated into '\n' before being returned to the
      caller. If it is '', universal newline mode is enabled, but line
      endings are returned to the caller untranslated. If it has any of
      the other legal values, input lines are only terminated by the given
      string, and the line ending is returned to the caller untranslated.

    * On output, if newline is None, any '\n' characters written are
      translated to the system default line separator, os.linesep. If
      newline is '', no translation takes place. If newline is any of the
      other legal values, any '\n' characters written are translated to
      the given string.

    If closefd is False, the underlying file descriptor will be kept open
    when the file is closed. This does not work when a file name is given
    and must be True in that case.

    open() returns a file object whose type depends on the mode, and
    through which the standard file operations such as reading and writing
    are performed. When open() is used to open a file in a text mode ('w',
    'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open
    a file in a binary mode, the returned class varies: in read binary
    mode, it returns a BufferedReader; in write binary and append binary
    modes, it returns a BufferedWriter, and in read/write mode, it returns
    a BufferedRandom.

    It is also possible to use a string or bytearray as a file for both
    reading and writing. For strings StringIO can be used like a file
    opened in a text mode, and for bytes a BytesIO can be used like a file
    opened in a binary mode.
    """
    if not isinstance(file, (basestring, int, long)):
        raise TypeError("invalid file: %r" % file)
    if not isinstance(mode, basestring):
        raise TypeError("invalid mode: %r" % mode)
    if not isinstance(buffering, (int, long)):
        raise TypeError("invalid buffering: %r" % buffering)
    if encoding is not None and not isinstance(encoding, basestring):
        raise TypeError("invalid encoding: %r" % encoding)
    if errors is not None and not isinstance(errors, basestring):
        raise TypeError("invalid errors: %r" % errors)
    modes = set(mode)
    if modes - set("arwb+tU") or len(mode) > len(modes):
        raise ValueError("invalid mode: %r" % mode)
    reading = "r" in modes
    writing = "w" in modes
    appending = "a" in modes
    updating = "+" in modes
    text = "t" in modes
    binary = "b" in modes
    if "U" in modes:
        if writing or appending:
            raise ValueError("can't use U and writing mode at once")
        reading = True
    if text and binary:
        raise ValueError("can't have text and binary mode at once")
    if reading + writing + appending > 1:
        raise ValueError("can't have read/write/append mode at once")
    if not (reading or writing or appending):
        raise ValueError("must have exactly one of read/write/append mode")
    if binary and encoding is not None:
        raise ValueError("binary mode doesn't take an encoding argument")
    if binary and errors is not None:
        raise ValueError("binary mode doesn't take an errors argument")
    if binary and newline is not None:
        raise ValueError("binary mode doesn't take a newline argument")
    raw = FileIO(file,
                 (reading and "r" or "") +
                 (writing and "w" or "") +
                 (appending and "a" or "") +
                 (updating and "+" or ""),
                 closefd)
    result = raw
    try:
        line_buffering = False
        if buffering == 1 or buffering < 0 and raw.isatty():
            buffering = -1
            line_buffering = True
        if buffering < 0:
            buffering = DEFAULT_BUFFER_SIZE
            try:
                bs = os.fstat(raw.fileno()).st_blksize
            except (os.error, AttributeError):
                pass
            else:
                if bs > 1:
                    buffering = bs
        if buffering < 0:
            raise ValueError("invalid buffering size")
        if buffering == 0:
            if binary:
                return result
            raise ValueError("can't have unbuffered text I/O")
        if updating:
            buffer = BufferedRandom(raw, buffering)
        elif writing or appending:
            buffer = BufferedWriter(raw, buffering)
        elif reading:
            buffer = BufferedReader(raw, buffering)
        else:
            raise ValueError("unknown mode: %r" % mode)
        result = buffer
        if binary:
            return result
        text = TextIOWrapper(buffer, encoding, errors, newline, line_buffering)
        result = text
        text.mode = mode
        return result
    except:
        result.close()
        raise


class DocDescriptor:
    """Helper for builtins.open.__doc__
    """
    def __get__(self, obj, typ):
        return (
            "open(file, mode='r', buffering=-1, encoding=None, "
                 "errors=None, newline=None, closefd=True)\n\n" +
            open.__doc__)

class OpenWrapper:
    """Wrapper for builtins.open

    Trick so that open won't become a bound method when stored
    as a class variable (as dbm.dumb does).

    See initstdio() in Python/pythonrun.c.
    """
    __doc__ = DocDescriptor()

    def __new__(cls, *args, **kwargs):
        return open(*args, **kwargs)


class UnsupportedOperation(ValueError, IOError):
    pass


class IOBase:
    __metaclass__ = abc.ABCMeta

    """The abstract base class for all I/O classes, acting on streams of
    bytes. There is no public constructor.

    This class provides dummy implementations for many methods that
    derived classes can override selectively; the default implementations
    represent a file that cannot be read, written or seeked.

    Even though IOBase does not declare read, readinto, or write because
    their signatures will vary, implementations and clients should
    consider those methods part of the interface. Also, implementations
    may raise an IOError when operations they do not support are called.

    The basic type used for binary data read from or written to a file is
    the bytes type. Method arguments may also be bytearray or memoryview of
    arrays of bytes. In some cases, such as readinto, a writable object such
    as bytearray is required. Text I/O classes work with unicode data.

    Note that calling any method (even inquiries) on a closed stream is
    undefined. Implementations may raise IOError in this case.

    IOBase (and its subclasses) support the iterator protocol, meaning
    that an IOBase object can be iterated over yielding the lines in a
    stream.

    IOBase also supports the :keyword:`with` statement. In this example,
    fp is closed after the suite of the with statement is complete:

    with open('spam.txt', 'r') as fp:
        fp.write('Spam and eggs!')
    """

    ### Internal ###

    def _unsupported(self, name):
        """Internal: raise an exception for unsupported operations."""
        raise UnsupportedOperation("%s.%s() not supported" %
                                   (self.__class__.__name__, name))

    ### Positioning ###

    def seek(self, pos, whence=0):
        """Change stream position.

        Change the stream position to byte offset pos. Argument pos is
        interpreted relative to the position indicated by whence.  Values
        for whence are:

        * 0 -- start of stream (the default); offset should be zero or positive
        * 1 -- current stream position; offset may be negative
        * 2 -- end of stream; offset is usually negative

        Return the new absolute position.
        """
        self._unsupported("seek")

    def tell(self):
        """Return current stream position."""
        return self.seek(0, 1)

    def truncate(self, pos=None):
        """Truncate file to size bytes.

        Size defaults to the current IO position as reported by tell().  Return
        the new size.
        """
        self._unsupported("truncate")

    ### Flush and close ###

    def flush(self):
        """Flush write buffers, if applicable.

        This is not implemented for read-only and non-blocking streams.
        """
        self._checkClosed()
        # XXX Should this return the number of bytes written???

    __closed = False

    def close(self):
        """Flush and close the IO object.

        This method has no effect if the file is already closed.
        """
        if not self.__closed:
            try:
                self.flush()
            finally:
                self.__closed = True

    def __del__(self):
        """Destructor.  Calls close()."""
        # The try/except block is in case this is called at program
        # exit time, when it's possible that globals have already been
        # deleted, and then the close() call might fail.  Since
        # there's nothing we can do about such failures and they annoy
        # the end users, we suppress the traceback.
        try:
            self.close()
        except:
            pass

    ### Inquiries ###

    def seekable(self):
        """Return whether object supports random access.

        If False, seek(), tell() and truncate() will raise IOError.
        This method may need to do a test seek().
        """
        return False

    def _checkSeekable(self, msg=None):
        """Internal: raise an IOError if file is not seekable
        """
        if not self.seekable():
            raise IOError("File or stream is not seekable."
                          if msg is None else msg)


    def readable(self):
        """Return whether object was opened for reading.

        If False, read() will raise IOError.
        """
        return False

    def _checkReadable(self, msg=None):
        """Internal: raise an IOError if file is not readable
        """
        if not self.readable():
            raise IOError("File or stream is not readable."
                          if msg is None else msg)

    def writable(self):
        """Return whether object was opened for writing.

        If False, write() and truncate() will raise IOError.
        """
        return False

    def _checkWritable(self, msg=None):
        """Internal: raise an IOError if file is not writable
        """
        if not self.writable():
            raise IOError("File or stream is not writable."
                          if msg is None else msg)

    @property
    def closed(self):
        """closed: bool.  True iff the file has been closed.

        For backwards compatibility, this is a property, not a predicate.
        """
        return self.__closed

    def _checkClosed(self, msg=None):
        """Internal: raise a ValueError if file is closed
        """
        if self.closed:
            raise ValueError("I/O operation on closed file."
                             if msg is None else msg)

    ### Context manager ###

    def __enter__(self):
        """Context management protocol.  Returns self."""
        self._checkClosed()
        return self

    def __exit__(self, *args):
        """Context management protocol.  Calls close()"""
        self.close()

    ### Lower-level APIs ###

    # XXX Should these be present even if unimplemented?

    def fileno(self):
        """Returns underlying file descriptor if one exists.

        An IOError is raised if the IO object does not use a file descriptor.
        """
        self._unsupported("fileno")

    def isatty(self):
        """Return whether this is an 'interactive' stream.

        Return False if it can't be determined.
        """
        self._checkClosed()
        return False

    ### Readline[s] and writelines ###

    def readline(self, limit=-1):
        r"""Read and return a line from the stream.

        If limit is specified, at most limit bytes will be read.

        The line terminator is always b'\n' for binary files; for text
        files, the newlines argument to open can be used to select the line
        terminator(s) recognized.
        """
        # For backwards compatibility, a (slowish) readline().
        if hasattr(self, "peek"):
            def nreadahead():
                readahead = self.peek(1)
                if not readahead:
                    return 1
                n = (readahead.find(b"\n") + 1) or len(readahead)
                if limit >= 0:
                    n = min(n, limit)
                return n
        else:
            def nreadahead():
                return 1
        if limit is None:
            limit = -1
        elif not isinstance(limit, (int, long)):
            raise TypeError("limit must be an integer")
        res = bytearray()
        while limit < 0 or len(res) < limit:
            b = self.read(nreadahead())
            if not b:
                break
            res += b
            if res.endswith(b"\n"):
                break
        return bytes(res)

    def __iter__(self):
        self._checkClosed()
        return self

    def next(self):
        line = self.readline()
        if not line:
            raise StopIteration
        return line

    def readlines(self, hint=None):
        """Return a list of lines from the stream.

        hint can be specified to control the number of lines read: no more
        lines will be read if the total size (in bytes/characters) of all
        lines so far exceeds hint.
        """
        if hint is not None and not isinstance(hint, (int, long)):
            raise TypeError("integer or None expected")
        if hint is None or hint <= 0:
            return list(self)
        n = 0
        lines = []
        for line in self:
            lines.append(line)
            n += len(line)
            if n >= hint:
                break
        return lines

    def writelines(self, lines):
        self._checkClosed()
        for line in lines:
            self.write(line)

io.IOBase.register(IOBase)


class RawIOBase(IOBase):

    """Base class for raw binary I/O."""

    # The read() method is implemented by calling readinto(); derived
    # classes that want to support read() only need to implement
    # readinto() as a primitive operation.  In general, readinto() can be
    # more efficient than read().

    # (It would be tempting to also provide an implementation of
    # readinto() in terms of read(), in case the latter is a more suitable
    # primitive operation, but that would lead to nasty recursion in case
    # a subclass doesn't implement either.)

    def read(self, n=-1):
        """Read and return up to n bytes.

        Returns an empty bytes object on EOF, or None if the object is
        set not to block and has no data to read.
        """
        if n is None:
            n = -1
        if n < 0:
            return self.readall()
        b = bytearray(n.__index__())
        n = self.readinto(b)
        if n is None:
            return None
        del b[n:]
        return bytes(b)

    def readall(self):
        """Read until EOF, using multiple read() call."""
        res = bytearray()
        while True:
            data = self.read(DEFAULT_BUFFER_SIZE)
            if not data:
                break
            res += data
        if res:
            return bytes(res)
        else:
            # b'' or None
            return data

    def readinto(self, b):
        """Read up to len(b) bytes into b.

        Returns number of bytes read (0 for EOF), or None if the object
        is set not to block and has no data to read.
        """
        self._unsupported("readinto")

    def write(self, b):
        """Write the given buffer to the IO stream.

        Returns the number of bytes written, which may be less than len(b).
        """
        self._unsupported("write")

io.RawIOBase.register(RawIOBase)
from _io import FileIO
RawIOBase.register(FileIO)


class BufferedIOBase(IOBase):

    """Base class for buffered IO objects.

    The main difference with RawIOBase is that the read() method
    supports omitting the size argument, and does not have a default
    implementation that defers to readinto().

    In addition, read(), readinto() and write() may raise
    BlockingIOError if the underlying raw stream is in non-blocking
    mode and not ready; unlike their raw counterparts, they will never
    return None.

    A typical implementation should not inherit from a RawIOBase
    implementation, but wrap one.
    """

    def read(self, n=None):
        """Read and return up to n bytes.

        If the argument is omitted, None, or negative, reads and
        returns all data until EOF.

        If the argument is positive, and the underlying raw stream is
        not 'interactive', multiple raw reads may be issued to satisfy
        the byte count (unless EOF is reached first).  But for
        interactive raw streams (XXX and for pipes?), at most one raw
        read will be issued, and a short result does not imply that
        EOF is imminent.

        Returns an empty bytes array on EOF.

        Raises BlockingIOError if the underlying raw stream has no
        data at the moment.
        """
        self._unsupported("read")

    def read1(self, n=None):
        """Read up to n bytes with at most one read() system call."""
        self._unsupported("read1")

    def readinto(self, b):
        """Read up to len(b) bytes into b.

        Like read(), this may issue multiple reads to the underlying raw
        stream, unless the latter is 'interactive'.

        Returns the number of bytes read (0 for EOF).

        Raises BlockingIOError if the underlying raw stream has no
        data at the moment.
        """
        data = self.read(len(b))
        n = len(data)
        try:
            b[:n] = data
        except TypeError as err:
            import array
            if not isinstance(b, array.array):
                raise err
            b[:n] = array.array(b'b', data)
        return n

    def write(self, b):
        """Write the given buffer to the IO stream.

        Return the number of bytes written, which is always len(b).

        Raises BlockingIOError if the buffer is full and the
        underlying raw stream cannot accept more data at the moment.
        """
        self._unsupported("write")

    def detach(self):
        """
        Separate the underlying raw stream from the buffer and return it.

        After the raw stream has been detached, the buffer is in an unusable
        state.
        """
        self._unsupported("detach")

io.BufferedIOBase.register(BufferedIOBase)


class _BufferedIOMixin(BufferedIOBase):

    """A mixin implementation of BufferedIOBase with an underlying raw stream.

    This passes most requests on to the underlying raw stream.  It
    does *not* provide implementations of read(), readinto() or
    write().
    """

    def __init__(self, raw):
        self._raw = raw

    ### Positioning ###

    def seek(self, pos, whence=0):
        new_position = self.raw.seek(pos, whence)
        if new_position < 0:
            raise IOError("seek() returned an invalid position")
        return new_position

    def tell(self):
        pos = self.raw.tell()
        if pos < 0:
            raise IOError("tell() returned an invalid position")
        return pos

    def truncate(self, pos=None):
        # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
        # and a flush may be necessary to synch both views of the current
        # file state.
        self.flush()

        if pos is None:
            pos = self.tell()
        # XXX: Should seek() be used, instead of passing the position
        # XXX  directly to truncate?
        return self.raw.truncate(pos)

    ### Flush and close ###

    def flush(self):
        if self.closed:
            raise ValueError("flush of closed file")
        self.raw.flush()

    def close(self):
        if self.raw is not None and not self.closed:
            try:
                # may raise BlockingIOError or BrokenPipeError etc
                self.flush()
            finally:
                self.raw.close()

    def detach(self):
        if self.raw is None:
            raise ValueError("raw stream already detached")
        self.flush()
        raw = self._raw
        self._raw = None
        return raw

    ### Inquiries ###

    def seekable(self):
        return self.raw.seekable()

    def readable(self):
        return self.raw.readable()

    def writable(self):
        return self.raw.writable()

    @property
    def raw(self):
        return self._raw

    @property
    def closed(self):
        return self.raw.closed

    @property
    def name(self):
        return self.raw.name

    @property
    def mode(self):
        return self.raw.mode

    def __repr__(self):
        clsname = self.__class__.__name__
        try:
            name = self.name
        except Exception:
            return "<_pyio.{0}>".format(clsname)
        else:
            return "<_pyio.{0} name={1!r}>".format(clsname, name)

    ### Lower-level APIs ###

    def fileno(self):
        return self.raw.fileno()

    def isatty(self):
        return self.raw.isatty()


class BytesIO(BufferedIOBase):

    """Buffered I/O implementation using an in-memory bytes buffer."""

    def __init__(self, initial_bytes=None):
        buf = bytearray()
        if initial_bytes is not None:
            buf.extend(initial_bytes)
        self._buffer = buf
        self._pos = 0

    def __getstate__(self):
        if self.closed:
            raise ValueError("__getstate__ on closed file")
        return self.__dict__.copy()

    def getvalue(self):
        """Return the bytes value (contents) of the buffer
        """
        if self.closed:
            raise ValueError("getvalue on closed file")
        return bytes(self._buffer)

    def read(self, n=None):
        if self.closed:
            raise ValueError("read from closed file")
        if n is None:
            n = -1
        if not isinstance(n, (int, long)):
            raise TypeError("integer argument expected, got {0!r}".format(
                type(n)))
        if n < 0:
            n = len(self._buffer)
        if len(self._buffer) <= self._pos:
            return b""
        newpos = min(len(self._buffer), self._pos + n)
        b = self._buffer[self._pos : newpos]
        self._pos = newpos
        return bytes(b)

    def read1(self, n):
        """This is the same as read.
        """
        return self.read(n)

    def write(self, b):
        if self.closed:
            raise ValueError("write to closed file")
        if isinstance(b, unicode):
            raise TypeError("can't write unicode to binary stream")
        n = len(b)
        if n == 0:
            return 0
        pos = self._pos
        if pos > len(self._buffer):
            # Inserts null bytes between the current end of the file
            # and the new write position.
            padding = b'\x00' * (pos - len(self._buffer))
            self._buffer += padding
        self._buffer[pos:pos + n] = b
        self._pos += n
        return n

    def seek(self, pos, whence=0):
        if self.closed:
            raise ValueError("seek on closed file")
        try:
            pos.__index__
        except AttributeError:
            raise TypeError("an integer is required")
        if whence == 0:
            if pos < 0:
                raise ValueError("negative seek position %r" % (pos,))
            self._pos = pos
        elif whence == 1:
            self._pos = max(0, self._pos + pos)
        elif whence == 2:
            self._pos = max(0, len(self._buffer) + pos)
        else:
            raise ValueError("invalid whence value")
        return self._pos

    def tell(self):
        if self.closed:
            raise ValueError("tell on closed file")
        return self._pos

    def truncate(self, pos=None):
        if self.closed:
            raise ValueError("truncate on closed file")
        if pos is None:
            pos = self._pos
        else:
            try:
                pos.__index__
            except AttributeError:
                raise TypeError("an integer is required")
            if pos < 0:
                raise ValueError("negative truncate position %r" % (pos,))
        del self._buffer[pos:]
        return pos

    def readable(self):
        if self.closed:
            raise ValueError("I/O operation on closed file.")
        return True

    def writable(self):
        if self.closed:
            raise ValueError("I/O operation on closed file.")
        return True

    def seekable(self):
        if self.closed:
            raise ValueError("I/O operation on closed file.")
        return True


class BufferedReader(_BufferedIOMixin):

    """BufferedReader(raw[, buffer_size])

    A buffer for a readable, sequential BaseRawIO object.

    The constructor creates a BufferedReader for the given readable raw
    stream and buffer_size. If buffer_size is omitted, DEFAULT_BUFFER_SIZE
    is used.
    """

    def __init__(self, raw, buffer_size=DEFAULT_BUFFER_SIZE):
        """Create a new buffered reader using the given readable raw IO object.
        """
        if not raw.readable():
            raise IOError('"raw" argument must be readable.')

        _BufferedIOMixin.__init__(self, raw)
        if buffer_size <= 0:
            raise ValueError("invalid buffer size")
        self.buffer_size = buffer_size
        self._reset_read_buf()
        self._read_lock = Lock()

    def _reset_read_buf(self):
        self._read_buf = b""
        self._read_pos = 0

    def read(self, n=None):
        """Read n bytes.

        Returns exactly n bytes of data unless the underlying raw IO
        stream reaches EOF or if the call would block in non-blocking
        mode. If n is negative, read until EOF or until read() would
        block.
        """
        if n is not None and n < -1:
            raise ValueError("invalid number of bytes to read")
        with self._read_lock:
            return self._read_unlocked(n)

    def _read_unlocked(self, n=None):
        nodata_val = b""
        empty_values = (b"", None)
        buf = self._read_buf
        pos = self._read_pos

        # Special case for when the number of bytes to read is unspecified.
        if n is None or n == -1:
            self._reset_read_buf()
            chunks = [buf[pos:]]  # Strip the consumed bytes.
            current_size = 0
            while True:
                # Read until EOF or until read() would block.
                try:
                    chunk = self.raw.read()
                except IOError as e:
                    if e.errno != EINTR:
                        raise
                    continue
                if chunk in empty_values:
                    nodata_val = chunk
                    break
                current_size += len(chunk)
                chunks.append(chunk)
            return b"".join(chunks) or nodata_val

        # The number of bytes to read is specified, return at most n bytes.
        avail = len(buf) - pos  # Length of the available buffered data.
        if n <= avail:
            # Fast path: the data to read is fully buffered.
            self._read_pos += n
            return buf[pos:pos+n]
        # Slow path: read from the stream until enough bytes are read,
        # or until an EOF occurs or until read() would block.
        chunks = [buf[pos:]]
        wanted = max(self.buffer_size, n)
        while avail < n:
            try:
                chunk = self.raw.read(wanted)
            except IOError as e:
                if e.errno != EINTR:
                    raise
                continue
            if chunk in empty_values:
                nodata_val = chunk
                break
            avail += len(chunk)
            chunks.append(chunk)
        # n is more than avail only when an EOF occurred or when
        # read() would have blocked.