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Mocking ideally slow network that only allows reading and/or writing one byte at a time

branch: master
README.markdown

Description

This tool emulates an ideally slow network by mocking the polling and read/write syscalls exposed by glibc via the LD_PRELOAD technique.

By preloading this dynamic library to your network server process, it will intercept the "poll", "close", "send", and "writev" syscalls, only allow the writing syscalls to actually write a single byte at a time (without flushing), and returns EAGAIN until another "poll" called on the current socket fd.

Similarly, one can configure this library to intercept the "read", "recv", "recvfrom" calls on the C level to emulate extremely slow reading operations either with or without slow writes at the same time.

The socket fd must be called first by a "poll" call to mark itself to this tool as an "active fd" and trigger subsequence the writing syscalls to behave differently.

With this tool, one can emulate extreme network conditions even locally (with the loopback device).

This tool is intended to supersed the good old etcproxy tool.

Build

Just issue the following command to build the file mockeagain.so

make

The Gnu make and gcc compiler are required.

On *BSD, it's often required to run the command "gmake".

Usage

For now, only "poll" syscall and slow writing mocking are supported.

Here we take Nginx as an example:

  1. Ensure that you have passed the --with-poll_module option while invoking nginx's configure script to build nginx.

  2. Ensure that you've enabled the "poll" event model in your nginx.conf:

    events { use poll; worker_connections 1024; }

  3. Ensure that you've disabled nginx's own write buffer:

    postpone_output 1; # only postpone a single byte, default 1460 bytes

and also have the following line in your nginx.conf:

env LD_PRELOAD;
  1. Run your Nginx this way:

    MOCKEAGAIN=w LD_PRELOAD=/path/to/mockeagain.so /path/to/nginx ...

Environments

The following system environment variables can be used to control the behavior of this tool. You should use Nginx's standard "env" directive to enable the environments that you use in your nginx.conf file, as in

env MOCKEAGAIN_VERBOSE;
env MOCKEAGAIN;
env MOCKEAGAIN_WRITE_TIMEOUT_PATTERN;

If you're using the Test::Nginx test scaffold, then these directives are automatically configured for you.

MOCKEAGAIN

This environment controls whether to mocking reads or writes or both.

When the letter "r" or "R" appear in the value string, then reads will be mocked.

When the letter "w" or "W" appear in the value string, then writes will be mocked.

When this environment is either not set or set to something unrecognized, then no mocking will be performed.

MOCKEAGAIN_VERBOSE

You can set the environment MOCKEAGAIN_VERBOSE to 1 if you want more diagnostical outputs of this tool in stderr. But you'll also have to add the following line to your nginx.conf:

env MOCKEAGAIN_VERBOSE;

When setting MOCKEAGAIN_VERBOSE to 1, you'll get messages like the following in your nginx's error.log file:

mockeagain: mocking "writev" on fd 3 to signal EAGAIN.
mockeagain: mocking "writev" on fd 3 to emit 1 of 188 bytes.
mockeagain: mocking "writev" on fd 3 to emit 1 of 187 bytes.
mockeagain: mocking "recv" on fd 5 to read 1 byte of data only

MOCKEAGAIN_WRITE_TIMEOUT_PATTERN

When this environment is set to value "foo bar", then the when the string "foo bar" appears in the output stream (not necessarily in a single write call), then it will trigger an indefinite write timeout.

This feature is very useful in mocking a write timeout in a particular position in the output stream.

Note that this environment also requires that the MOCKEAGAIN variable value contains "w" or "W".

For now, this feature only supports the "writev" call.

Glibc API Mocked

Event API

  • poll

Writing API

  • writev
  • send

Reading API

  • read
  • recv
  • recvfrom

TODO

  • add support for write, sendto, and more writing syscalls.
  • add support for other event interfaces like epoll, select, kqueue, and more.

Success Stories

ngx_echo

The echo_after_body directive used to have a bug that the output filter failed to take into account the NGX_AGAIN constant returned from the downstream output filter chain. Enabling mockeagain's writing mode can trivially capture this issue even on localhost.

To reproduce, update ngx_http_echo_filter.c:165

#if 0

to

#if 1

and run ngx_echo's test file t/echo-after-body.t with mockeagain's writing mode enabled.

ngx_redis

ngx_redis 0.3.5 and earlier had a subtle bug in its redis reply parser as explained here:

https://groups.google.com/group/openresty/browse_thread/thread/b5ddf1b24d3c9677

Even though this bug was first captured by the etcproxy tool, but mockeagain's reading mode can also capture it reliably even on localhost.

To reproduce, just compile ngx_redis 0.3.5 with ngx_srcache, and then run the t/redis.t test file in ngx_srcache's test suite, and with mockeagain reading mode enabled.

libdrizzle

libdrizzle 0.8 and earlier had a subtle bug in its MySQL packet parser and mockeagain's reading mode can capture it reliably even on localhost.

To reproduce, just compile libdrizzle 0.8 with ngx_drizzle, and run the t/sanity.t test file in ngx_drizzle's test suite with mockeagain's reading mode enabled.

ngx_lua

The downstream cosocket API did have a bug in its receive(size) method call that was caught by mockeagain in its reading mode.

To reproduce, check out ngx_lua v0.5.0rc24 and run its t/067-req-socket.t test file with "MOCKEAGAIN=R" and TEST 1 will just hang.

Author

Yichun "agentzh" Zhang (章亦春)

Copyright & License

This module is licenced under the BSD license.

Copyright (C) 2012 by Yichun "agentzh" Zhang (章亦春) agentzh@gmail.com.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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