[ruby-core:59728] Ruby 2.1.0 in Production: known bugs and patches

Last week, we upgraded the github.com rails app to ruby 2.1.0 in production.
While testing the new build for rollout, we ran into a number of bugs. Most
of
these have been fixed on trunk already, but I've documented them below to
help
anyone else who might be testing ruby 2.1 in production.

@naruse I think we should backport these patches to the ruby_2_1 branch and
release 2.1.1 sooner rather than later, as some of the bugs are quite
critical.
I'm happy to offer any assistance I can to expedite this process.

- Struct#send(:setter=, rhs) does not return rhs
  #9353 (r44501)
  breaks the faraday gem

- Array#uniq behavior change
  #9340 (r44512)
  affects the sass gem

- Timeout behavior change
  #9354 (r44517, r44518, r44519)
  #9380 (r44523)
  breaks the faraday gem

- [BUG] assertion failure in socket.c
  #9352 (r44490, r44496)
  breaks the excon gem

- segfault in method() usage
  #9334, #9313, #9310, #9326, #9337
  #9315 (r44455, r44458, r44510, r44527, r44552, r44553)
  segv inside activesupport4 and other gems

- Hash lookup with #hash and #eql broken
  #9381 (r44525, r44534, r44537)

- bigdecimal division issue
  #9316
  affects some payment processing gems
  workaround patch by @phasis:
    https://github.com/github/ruby/commit/1e778b3b20

- SizedQueue not working
  #9302, #9343, #9396
  patches by @normalperson:
    https://bugs.ruby-lang.org/attachments/download/4113
    https://bugs.ruby-lang.org/attachments/download/4140

In addition to the bugfix patches above, we're running the following
performance patches in production. These have proven quite stable in our
environment, so I encourage you to try them if you're running a large ruby
app.

- funny-falcon's st.c density patch
  https://github.com/github/ruby/commit/1e025cbd2f

- funny-falcon's st.c pool allocator
  https://github.com/github/ruby/commit/3d37e2b5a3

- funny-falcon's method cache patch
  https://github.com/funny-falcon/ruby/compare/trunk...class_local_cache
  proposed for trunk in #9262

- optimized Hash#[] and Hash#[]= with string literals
  https://github.com/github/ruby/commit/86946e72d7
  merged to trunk in #9382 (r44551)

Aside from the bugs above, our upgrade to 2.1 was very smooth. I'm happy to
report that github.com is now running faster (and with less GC overhead)
than
ever before.

  Aman