16.1. "select" — Waiting for I/O completion
*******************************************

This module provides access to the "select()" and "poll()" functions
available in most operating systems, "epoll()" available on Linux 2.5+
and "kqueue()" available on most BSD. Note that on Windows, it only
works for sockets; on other operating systems, it also works for other
file types (in particular, on Unix, it works on pipes). It cannot be
used on regular files to determine whether a file has grown since it
was last read.

The module defines the following:

exception select.error

   The exception raised when an error occurs.  The accompanying value
   is a pair containing the numeric error code from "errno" and the
   corresponding string, as would be printed by the C function
   "perror()".

select.epoll([sizehint=-1])

   (Only supported on Linux 2.5.44 and newer.)  Returns an edge
   polling object, which can be used as Edge or Level Triggered
   interface for I/O events; see section Edge and Level Trigger
   Polling (epoll) Objects below for the methods supported by epolling
   objects.

   New in version 2.6.

select.poll()

   (Not supported by all operating systems.)  Returns a polling
   object, which supports registering and unregistering file
   descriptors, and then polling them for I/O events; see section
   Polling Objects below for the methods supported by polling objects.

select.kqueue()

   (Only supported on BSD.)  Returns a kernel queue object; see
   section Kqueue Objects below for the methods supported by kqueue
   objects.

   New in version 2.6.

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

   (Only supported on BSD.)  Returns a kernel event object; see
   section Kevent Objects below for the methods supported by kevent
   objects.

   New in version 2.6.

select.select(rlist, wlist, xlist[, timeout])

   This is a straightforward interface to the Unix "select()" system
   call. The first three arguments are sequences of ‘waitable
   objects’: either integers representing file descriptors or objects
   with a parameterless method named "fileno()" returning such an
   integer:

   * *rlist*: wait until ready for reading

   * *wlist*: wait until ready for writing

   * *xlist*: wait for an “exceptional condition” (see the manual
     page for what your system considers such a condition)

   Empty sequences are allowed, but acceptance of three empty
   sequences is platform-dependent. (It is known to work on Unix but
   not on Windows.)  The optional *timeout* argument specifies a time-
   out as a floating point number in seconds.  When the *timeout*
   argument is omitted the function blocks until at least one file
   descriptor is ready.  A time-out value of zero specifies a poll and
   never blocks.

   The return value is a triple of lists of objects that are ready:
   subsets of the first three arguments.  When the time-out is reached
   without a file descriptor becoming ready, three empty lists are
   returned.

   Among the acceptable object types in the sequences are Python file
   objects (e.g. "sys.stdin", or objects returned by "open()" or
   "os.popen()"), socket objects returned by "socket.socket()".  You
   may also define a *wrapper* class yourself, as long as it has an
   appropriate "fileno()" method (that really returns a file
   descriptor, not just a random integer).

   Note: File objects on Windows are not acceptable, but sockets
     are.  On Windows, the underlying "select()" function is provided
     by the WinSock library, and does not handle file descriptors that
     don’t originate from WinSock.

select.PIPE_BUF

   Files reported as ready for writing by "select()", "poll()" or
   similar interfaces in this module are guaranteed to not block on a
   write of up to "PIPE_BUF" bytes. This value is guaranteed by POSIX
   to be at least 512.  Availability: Unix.

   New in version 2.7.


16.1.1. Edge and Level Trigger Polling (epoll) Objects
======================================================

   http://linux.die.net/man/4/epoll

   *eventmask*

   +-------------------------+-------------------------------------------------+
   | Constant                | Meaning                                         |
   |=========================|=================================================|
   | "EPOLLIN"               | Available for read                              |
   +-------------------------+-------------------------------------------------+
   | "EPOLLOUT"              | Available for write                             |
   +-------------------------+-------------------------------------------------+
   | "EPOLLPRI"              | Urgent data for read                            |
   +-------------------------+-------------------------------------------------+
   | "EPOLLERR"              | Error condition happened on the assoc. fd       |
   +-------------------------+-------------------------------------------------+
   | "EPOLLHUP"              | Hang up happened on the assoc. fd               |
   +-------------------------+-------------------------------------------------+
   | "EPOLLET"               | Set Edge Trigger behavior, the default is Level |
   |                         | Trigger behavior                                |
   +-------------------------+-------------------------------------------------+
   | "EPOLLONESHOT"          | Set one-shot behavior. After one event is       |
   |                         | pulled out, the fd is internally disabled       |
   +-------------------------+-------------------------------------------------+
   | "EPOLLRDNORM"           | Equivalent to "EPOLLIN"                         |
   +-------------------------+-------------------------------------------------+
   | "EPOLLRDBAND"           | Priority data band can be read.                 |
   +-------------------------+-------------------------------------------------+
   | "EPOLLWRNORM"           | Equivalent to "EPOLLOUT"                        |
   +-------------------------+-------------------------------------------------+
   | "EPOLLWRBAND"           | Priority data may be written.                   |
   +-------------------------+-------------------------------------------------+
   | "EPOLLMSG"              | Ignored.                                        |
   +-------------------------+-------------------------------------------------+

epoll.close()

   Close the control file descriptor of the epoll object.

epoll.fileno()

   Return the file descriptor number of the control fd.

epoll.fromfd(fd)

   Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

   Register a fd descriptor with the epoll object.

   Note: Registering a file descriptor that’s already registered
     raises an IOError – contrary to Polling Objects’s register.

epoll.modify(fd, eventmask)

   Modify a register file descriptor.

epoll.unregister(fd)

   Remove a registered file descriptor from the epoll object.

epoll.poll([timeout=-1[, maxevents=-1]])

   Wait for events. timeout in seconds (float)


16.1.2. Polling Objects
=======================

The "poll()" system call, supported on most Unix systems, provides
better scalability for network servers that service many, many clients
at the same time. "poll()" scales better because the system call only
requires listing the file descriptors of interest, while "select()"
builds a bitmap, turns on bits for the fds of interest, and then
afterward the whole bitmap has to be linearly scanned again.
"select()" is O(highest file descriptor), while "poll()" is O(number
of file descriptors).

poll.register(fd[, eventmask])

   Register a file descriptor with the polling object.  Future calls
   to the "poll()" method will then check whether the file descriptor
   has any pending I/O events.  *fd* can be either an integer, or an
   object with a "fileno()" method that returns an integer.  File
   objects implement "fileno()", so they can also be used as the
   argument.

   *eventmask* is an optional bitmask describing the type of events
   you want to check for, and can be a combination of the constants
   "POLLIN", "POLLPRI", and "POLLOUT", described in the table below.
   If not specified, the default value used will check for all 3 types
   of events.

   +---------------------+--------------------------------------------+
   | Constant            | Meaning                                    |
   |=====================|============================================|
   | "POLLIN"            | There is data to read                      |
   +---------------------+--------------------------------------------+
   | "POLLPRI"           | There is urgent data to read               |
   +---------------------+--------------------------------------------+
   | "POLLOUT"           | Ready for output: writing will not block   |
   +---------------------+--------------------------------------------+
   | "POLLERR"           | Error condition of some sort               |
   +---------------------+--------------------------------------------+
   | "POLLHUP"           | Hung up                                    |
   +---------------------+--------------------------------------------+
   | "POLLNVAL"          | Invalid request: descriptor not open       |
   +---------------------+--------------------------------------------+

   Registering a file descriptor that’s already registered is not an
   error, and has the same effect as registering the descriptor
   exactly once.

poll.modify(fd, eventmask)

   Modifies an already registered fd. This has the same effect as
   "register(fd, eventmask)".  Attempting to modify a file descriptor
   that was never registered causes an "IOError" exception with errno
   "ENOENT" to be raised.

   New in version 2.6.

poll.unregister(fd)

   Remove a file descriptor being tracked by a polling object.  Just
   like the "register()" method, *fd* can be an integer or an object
   with a "fileno()" method that returns an integer.

   Attempting to remove a file descriptor that was never registered
   causes a "KeyError" exception to be raised.

poll.poll([timeout])

   Polls the set of registered file descriptors, and returns a
   possibly-empty list containing "(fd, event)" 2-tuples for the
   descriptors that have events or errors to report. *fd* is the file
   descriptor, and *event* is a bitmask with bits set for the reported
   events for that descriptor — "POLLIN" for waiting input, "POLLOUT"
   to indicate that the descriptor can be written to, and so forth. An
   empty list indicates that the call timed out and no file
   descriptors had any events to report. If *timeout* is given, it
   specifies the length of time in milliseconds which the system will
   wait for events before returning. If *timeout* is omitted,
   negative, or "None", the call will block until there is an event
   for this poll object.


16.1.3. Kqueue Objects
======================

kqueue.close()

   Close the control file descriptor of the kqueue object.

kqueue.fileno()

   Return the file descriptor number of the control fd.

kqueue.fromfd(fd)

   Create a kqueue object from a given file descriptor.

kqueue.control(changelist, max_events[, timeout]) -> eventlist

   Low level interface to kevent

   * changelist must be an iterable of kevent objects or "None"

   * max_events must be 0 or a positive integer

   * timeout in seconds (floats possible); the default is "None", to
     wait forever


16.1.4. Kevent Objects
======================

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

kevent.ident

   Value used to identify the event. The interpretation depends on the
   filter but it’s usually the file descriptor. In the constructor
   ident can either be an int or an object with a fileno() function.
   kevent stores the integer internally.

kevent.filter

   Name of the kernel filter.

   +-----------------------------+-----------------------------------------------+
   | Constant                    | Meaning                                       |
   |=============================|===============================================|
   | "KQ_FILTER_READ"            | Takes a descriptor and returns whenever there |
   |                             | is data available to read                     |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_WRITE"           | Takes a descriptor and returns whenever there |
   |                             | is data available to write                    |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_AIO"             | AIO requests                                  |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_VNODE"           | Returns when one or more of the requested     |
   |                             | events watched in *fflag* occurs              |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_PROC"            | Watch for events on a process id              |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_NETDEV"          | Watch for events on a network device [not     |
   |                             | available on Mac OS X]                        |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_SIGNAL"          | Returns whenever the watched signal is        |
   |                             | delivered to the process                      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_TIMER"           | Establishes an arbitrary timer                |
   +-----------------------------+-----------------------------------------------+

kevent.flags

   Filter action.

   +-----------------------------+-----------------------------------------------+
   | Constant                    | Meaning                                       |
   |=============================|===============================================|
   | "KQ_EV_ADD"                 | Adds or modifies an event                     |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_DELETE"              | Removes an event from the queue               |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ENABLE"              | Permitscontrol() to returns the event         |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_DISABLE"             | Disablesevent                                 |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ONESHOT"             | Removes event after first occurrence          |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_CLEAR"               | Reset the state after an event is retrieved   |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_SYSFLAGS"            | internal event                                |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_FLAG1"               | internal event                                |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_EOF"                 | Filter specific EOF condition                 |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ERROR"               | See return values                             |
   +-----------------------------+-----------------------------------------------+

kevent.fflags

   Filter specific flags.

   "KQ_FILTER_READ" and  "KQ_FILTER_WRITE" filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   |==============================|==============================================|
   | "KQ_NOTE_LOWAT"              | low water mark of a socket buffer            |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_VNODE" filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   |==============================|==============================================|
   | "KQ_NOTE_DELETE"             | *unlink()* was called                        |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_WRITE"              | a write occurred                             |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_EXTEND"             | the file was extended                        |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_ATTRIB"             | an attribute was changed                     |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINK"               | the link count has changed                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_RENAME"             | the file was renamed                         |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_REVOKE"             | access to the file was revoked               |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_PROC" filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   |==============================|==============================================|
   | "KQ_NOTE_EXIT"               | the process has exited                       |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_FORK"               | the process has called *fork()*              |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_EXEC"               | the process has executed a new process       |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_PCTRLMASK"          | internal filter flag                         |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_PDATAMASK"          | internal filter flag                         |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_TRACK"              | follow a process across *fork()*             |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_CHILD"              | returned on the child process for            |
   |                              | *NOTE_TRACK*                                 |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_TRACKERR"           | unable to attach to a child                  |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_NETDEV" filter flags (not available on Mac OS X):

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   |==============================|==============================================|
   | "KQ_NOTE_LINKUP"             | link is up                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINKDOWN"           | link is down                                 |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINKINV"            | link state is invalid                        |
   +------------------------------+----------------------------------------------+

kevent.data

   Filter specific data.

kevent.udata

   User defined value.
