Blog posts for tags/socket

  1. Recognize connection errors

    Lately I've been dealing with an asynchronous TCP client app which sends messages to a remote server. Some of these messages are important, and cannot get lost. Because the connection may drop at any time, I had to implement a mechanism to resend the message once the client reconnects. As such, I needed a way to identify what constitutes a connection error.

    Python provides a builtin ConnectionError exception precisely for this purpose, but it turns out it's not enough. After observing logs in production, I found some errors that were not related to the socket connection per se, but rather to the system connectivity, like ENETUNREACH ("network unreachable") or ENETDOWN ("network down"). It's interesting to note how this distinction is reflected in the UNIX errno code prefixes: ECONN* (connection errors) vs. ENET* (network errors). I've noticed ENET* errors usually occur on a DHCP renewal, or more in general when the Wi-Fi signal is weak or absent. Because this code runs on a cleaning robot which constantly moves around the house, connection can become unstable when the robot gets far from the Wi-Fi Access Point, so it's pretty common to bump into errors like these:

    File "/usr/lib/python3.7/ssl.py", line 934, in send
    return self._sslobj.write(data)
OSError: [Errno 101] Network is unreachable

File "/usr/lib/python3.7/socket.py", line 222, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -3] Temporary failure in name resolution

File "/usr/lib/python3.7/ssl.py", line 934, in send
    return self._sslobj.write(data)
BrokenPipeError: [Errno 32] Broken pipe

File "/usr/lib/python3.7/ssl.py", line 934, in send
    return self._sslobj.write(data)
socket.timeout: The write operation timed out

    Production logs also revealed a considerable amount of SSL-related errors. I was uncertain what to do about those. The app is supposed to gracefully handle them, so theoretically they should represent a bug. Still, they are unequivocally related to the connection stream, and represent a failed attempt to send data, so we want to retry it. Example of logs I found:

    File "/usr/lib/python3.7/ssl.py", line 934, in send
    return self._sslobj.write(data)
ssl.SSLZeroReturnError: TLS/SSL connection has been closed (EOF)

File "/usr/lib/python3.7/ssl.py", line 934, in send
    return self._sslobj.write(data)
ssl.SSLError: [SSL: BAD_LENGTH] bad length

    Looking at production logs revealed what sort of brutal, rough and tumble place the Internet is, and how a network app must be ready to handle all sorts of unexpected error conditions which hardly show up during testing. To handle all of these cases I came up with this solution which I think is worth sharing, as it's generic enough to be reused in similar situations. If needed, this can be easily extended to include specific exceptions of third party libraries, like requests.exceptions.ConnectionError.

    import errno, socket, ssl

# Network errors, usually related to DHCP or wpa_supplicant (Wi-Fi).
NETWORK_ERRNOS = frozenset((
    errno.ENETUNREACH,  # "Network is unreachable"
    errno.ENETDOWN,  # "Network is down"
    errno.ENETRESET,  # "Network dropped connection on reset"
    errno.ENONET,  # "Machine is not on the network"
))

def is_connection_err(exc):
    """Return True if an exception is connection-related."""
    if isinstance(exc, ConnectionError):
        # https://docs.python.org/3/library/exceptions.html#ConnectionError
        # ConnectionError includes:
        # * BrokenPipeError (EPIPE, ESHUTDOWN)
        # * ConnectionAbortedError (ECONNABORTED)
        # * ConnectionRefusedError (ECONNREFUSED)
        # * ConnectionResetError (ECONNRESET)
        return True
    if isinstance(exc, socket.gaierror):
        # failed DNS resolution on connect()
        return True
    if isinstance(exc, (socket.timeout, TimeoutError)):
        # timeout on connect(), recv(), send()
        return True
    if isinstance(exc, OSError):
        # ENOTCONN == "Transport endpoint is not connected"
        return (exc.errno in NETWORK_ERRNOS) or (exc.errno == errno.ENOTCONN)
    if isinstance(exc, ssl.SSLError):
        # Let's consider any SSL error a connection error. Usually this is:
        # * ssl.SSLZeroReturnError: "TLS/SSL connection has been closed"
        # * ssl.SSLError: [SSL: BAD_LENGTH]
        return True
    return False

    To use it:

    try:
    sock.sendall(b"hello there")
except Exception as err:
    if is_connection_err(err):
        schedule_on_reconnect(lambda: sock.sendall(b"hello there"))
    raise

  2. Python and sendfile

    sendfile(2) is a UNIX system call which provides a "zero-copy" way of copying data from one file descriptor (a file) to another (a socket). Because this copying is done entirely within the kernel, sendfile(2) is more efficient than the combination of file.read() and socket.send(), which requires transferring data to and from user space. This copying of the data twice imposes some performance and resource penalties which sendfile(2) syscall avoids; it also results in a single system call (and thus only one context switch), rather than the series of read(2) / write(2) system calls (each system call requiring a context switch) used internally for the data copying. A more exhaustive explanation of how sendfile(2) works is available here, but long story short is that sending a file with sendfile() is usually twice as fast than using plain socket.send(). Typical applications which can benefit from using sendfile() are FTP and HTTP servers.

    socket.sendfile()

    I recently contributed a patch for Python's socket module which adds a high-level socket.sendfile() method (see full discussion at BPO-17552). socket.sendfile() will transmit a file until EOF is reached by attempting to use os.sendfile(), if available, else it falls back on using plain socket.send(). Internally, it takes care of handling socket timeouts and provides two optional parameters to move the file offset or to send only a limited amount of bytes. I came up with this idea because getting all of that right is a bit tricky, so a generic wrapper seemed to be convenient to have. socket.sendfile() will make its appearance in Python 3.5.

    sendfile and Python

    sendfile(2) made its first appearance into the Python stdlib kind of late: Python 3.3. It was contributed by Ross Lagerwall and me in BPO-10882. Since the patch didn't make it into python 2.X and I wanted to use sendfile() in pyftpdlib I later decided to release it as a stand alone module working with older (2.5+) Python versions (see pysendfile project). Starting with version 3.5, Python will hopefully start using sendfile() more extensively, in details:

    Also, Windows provides something similar to sendfile(2): TransmitFile. Now that socket.sendfile() is in place it seems natural to add support for it as well (see BPO-21721).

    Backport to Python 2.6 and 2.7

    For those of you who are interested in using socket.sendfile() with older Python 2.6 and 2.7 versions here's a backport. It requires pysendfile module to be installed. Full code including tests is hosted here.

    #!/usr/bin/env python

"""
This is a backport of socket.sendfile() for Python 2.6 and 2.7.
socket.sendfile() will be included in Python 3.5:
http://bugs.python.org/issue17552
Usage:

>>> import socket
>>> file = open("somefile.bin", "rb")
>>> sock = socket.create_connection(("localhost", 8021))
>>> sendfile(sock, file)
42319283
>>>
"""

import errno
import io
import os
import select
import socket
try:
    memoryview  # py 2.7 only
except NameError:
    memoryview = lambda x: x

if os.name == 'posix':
    import sendfile as pysendfile  # requires "pip install pysendfile"
else:
    pysendfile = None


_RETRY = frozenset((errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK,
                    errno.EINPROGRESS))


class _GiveupOnSendfile(Exception):
    pass


if pysendfile is not None:

    def _sendfile_use_sendfile(sock, file, offset=0, count=None):
        _check_sendfile_params(sock, file, offset, count)
        sockno = sock.fileno()
        try:
            fileno = file.fileno()
        except (AttributeError, io.UnsupportedOperation) as err:
            raise _GiveupOnSendfile(err)  # not a regular file
        try:
            fsize = os.fstat(fileno).st_size
        except OSError:
            raise _GiveupOnSendfile(err)  # not a regular file
        if not fsize:
            return 0  # empty file
        blocksize = fsize if not count else count

        timeout = sock.gettimeout()
        if timeout == 0:
            raise ValueError("non-blocking sockets are not supported")
        # poll/select have the advantage of not requiring any
        # extra file descriptor, contrarily to epoll/kqueue
        # (also, they require a single syscall).
        if hasattr(select, 'poll'):
            if timeout is not None:
                timeout *= 1000
            pollster = select.poll()
            pollster.register(sockno, select.POLLOUT)

            def wait_for_fd():
                if pollster.poll(timeout) == []:
                    raise socket._socket.timeout('timed out')
        else:
            # call select() once in order to solicit ValueError in
            # case we run out of fds
            try:
                select.select([], [sockno], [], 0)
            except ValueError:
                raise _GiveupOnSendfile(err)

            def wait_for_fd():
                fds = select.select([], [sockno], [], timeout)
                if fds == ([], [], []):
                    raise socket._socket.timeout('timed out')

        total_sent = 0
        # localize variable access to minimize overhead
        os_sendfile = pysendfile.sendfile
        try:
            while True:
                if timeout:
                    wait_for_fd()
                if count:
                    blocksize = count - total_sent
                    if blocksize <= 0:
                        break
                try:
                    sent = os_sendfile(sockno, fileno, offset, blocksize)
                except OSError as err:
                    if err.errno in _RETRY:
                        # Block until the socket is ready to send some
                        # data; avoids hogging CPU resources.
                        wait_for_fd()
                    else:
                        if total_sent == 0:
                            # We can get here for different reasons, the main
                            # one being 'file' is not a regular mmap(2)-like
                            # file, in which case we'll fall back on using
                            # plain send().
                            raise _GiveupOnSendfile(err)
                        raise err
                else:
                    if sent == 0:
                        break  # EOF
                    offset += sent
                    total_sent += sent
            return total_sent
        finally:
            if total_sent > 0 and hasattr(file, 'seek'):
                file.seek(offset)
else:
    def _sendfile_use_sendfile(sock, file, offset=0, count=None):
        raise _GiveupOnSendfile(
            "sendfile() not available on this platform")


def _sendfile_use_send(sock, file, offset=0, count=None):
    _check_sendfile_params(sock, file, offset, count)
    if sock.gettimeout() == 0:
        raise ValueError("non-blocking sockets are not supported")
    if offset:
        file.seek(offset)
    blocksize = min(count, 8192) if count else 8192
    total_sent = 0
    # localize variable access to minimize overhead
    file_read = file.read
    sock_send = sock.send
    try:
        while True:
            if count:
                blocksize = min(count - total_sent, blocksize)
                if blocksize <= 0:
                    break
            data = memoryview(file_read(blocksize))
            if not data:
                break  # EOF
            while True:
                try:
                    sent = sock_send(data)
                except OSError as err:
                    if err.errno in _RETRY:
                        continue
                    raise
                else:
                    total_sent += sent
                    if sent < len(data):
                        data = data[sent:]
                    else:
                        break
        return total_sent
    finally:
        if total_sent > 0 and hasattr(file, 'seek'):
            file.seek(offset + total_sent)


def _check_sendfile_params(sock, file, offset, count):
    if 'b' not in getattr(file, 'mode', 'b'):
        raise ValueError("file should be opened in binary mode")
    if not sock.type & socket.SOCK_STREAM:
        raise ValueError("only SOCK_STREAM type sockets are supported")
    if count is not None:
        if not isinstance(count, int):
            raise TypeError(
                "count must be a positive integer (got %s)" % repr(count))
        if count <= 0:
            raise ValueError(
                "count must be a positive integer (got %s)" % repr(count))


def sendfile(sock, file, offset=0, count=None):
    """sendfile(sock, file[, offset[, count]]) -> sent

    Send a *file* over a connected socket *sock* until EOF is
    reached by using high-performance sendfile(2) and return the
    total number of bytes which were sent.
    *file* must be a regular file object opened in binary mode.
    If sendfile() is not available (e.g. Windows) or file is
    not a regular file socket.send() will be used instead.
    *offset* tells from where to start reading the file.
    If specified, *count* is the total number of bytes to transmit
    as opposed to sending the file until EOF is reached.
    File position is updated on return or also in case of error in
    which case file.tell() can be used to figure out the number of
    bytes which were sent.
    The socket must be of SOCK_STREAM type.
    Non-blocking sockets are not supported.
    """
    try:
        return _sendfile_use_sendfile(sock, file, offset, count)
    except _GiveupOnSendfile:
        return _sendfile_use_send(sock, file, offset, count)

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