attempt to provide only healthy connections from the pool

Adds redis.selector, a module that provides the best selector strategy
available on the current platform. A redis.selector polls a socket to
provide two pieces of functionality:

1. Check whether data can be read from the socket. Prior versions of redis-py
provided this behavior with just select.select(). select() has lots of
limitations, most notably a limit of ~1024 file descriptors. Now that
better selectors are available, this should make can_read() faster and
able to accomodate more clients. See #1115 and #486

2. Check whether a socket is ready for a command to be sent. This doubles
as a health check. It ensures that the socket is available for writing,
has no data to read and has no known errors. Anytime a socket is
disconnected or hung up, data is available to be read, typically zero bytes.

ConnectionPool.get_connection has been modified to ensure that connections
it returns are connected and are ready for a command to be sent. If
get_connection encounters a case where a socket isn't ready for a command
the connection is reconnected and checked again.

TODO: more tests for this stuff. implement EPoll and KQueue selectors.

Fixes #1115
Fixes #486

1 files changed, 0 insertions, 13 deletions

diff --git a/redis/_compat.py b/redis/_compat.py
index c9213a6..bde6fb6 100644
--- a/redis/_compat.py
+++ b/redis/_compat.py
@@ -9,17 +9,6 @@ if sys.version_info[0] < 3 or (sys.version_info[0] == 3 and
     import socket
     import time
 
-    from select import select as _select, error as select_error
-
-    def select(rlist, wlist, xlist, timeout):
-        while True:
-            try:
-                return _select(rlist, wlist, xlist, timeout)
-            except select_error as e:
-                if e.args[0] == errno.EINTR:
-                    continue
-                raise
-
     # Wrapper for handling interruptable system calls.
     def _retryable_call(s, func, *args, **kwargs):
         # Some modules (SSL) use the _fileobject wrapper directly and
@@ -65,8 +54,6 @@ if sys.version_info[0] < 3 or (sys.version_info[0] == 3 and
         return _retryable_call(sock, sock.recv_into, *args, **kwargs)
 
 else:  # Python 3.5 and above automatically retry EINTR
-    from select import select
-
     def recv(sock, *args, **kwargs):
         return sock.recv(*args, **kwargs)