Manually compact GC before fork (#2093)

Rub 2.7.0 introduced `GC.compact` which allows manual compaction of Ruby slots. A good time to do this is before forking so that memory fragmentation can be reduced.

- https://www.ruby-lang.org/en/news/2019/12/17/ruby-2-7-0-rc1-released/
- https://www.youtube.com/watch?v=1F3gXYhQsAY

One issue with memory fragmentation when forking is that while a page in memory might have only one free slot, as that slot is written to after fork, the entire page is written so the benefit of CoW optimizations are greatly reduced. Manually compacting GC reduces the number of pages with empty slots.

This PR manually compacts memory right before Puma forks and after other `before_fork` hooks are called.

.travis.yml

@@ -52,6 +52,7 @@ matrix:
     - rvm: ruby-head
       env: jit=yes
     - rvm: truffleruby
+    - rvm: 2.7.0-preview3
 
   allow_failures:
     - rvm: jruby-9.2.9.0

History.md

@@ -4,6 +4,7 @@
   * Add pumactl `thread-backtraces` command to print thread backtraces (#2053)
   * Configuration: `environment` is read from `RAILS_ENV`, if `RACK_ENV` can't be found (#2022)
   * `Puma.stats` now returns a Hash instead of a JSON string (#2086)
+  * `GC.compact` is called before fork if available (#2093)
 
 * Bugfixes
   * Your bugfix goes here (#Github Number)

lib/puma/cluster.rb

@@ -486,6 +486,7 @@ module Puma
       @master_read, @worker_write = read, @wakeup
 
       @launcher.config.run_hooks :before_fork, nil
+      GC.compact if GC.respond_to?(:compact)
 
       spawn_workers