unicorn: clear environment variables during hot restarts.

Fixes an issue with zero downtime deployments (#4123). Hot restarting
didn't clear out BUNDLER_GEMFILE, which mean that new workers still used
the old Gemfile from previous deployments. This led to mysterious errors
with autoloading libraries (#4138).

config/unicorn/production.rb

@@ -81,3 +81,16 @@ after_fork do |server, worker|
     ActiveRecord::Base.establish_connection
   end
 end
+
+# This runs when we send unicorn a SIGUSR2 to do a hot restart. We need to
+# clear out old BUNDLER_* and GEM_* environment variables, otherwise the new
+# worker will still use the old Gemfile from the previous deployment, which
+# will cause mysterious problems with libraries. BUNDLER_GEMFILE is the main
+# thing we need to clear, but we wipe everything for safety.
+#
+# https://bogomips.org/unicorn/Sandbox.html
+# https://jamielinux.com/blog/zero-downtime-unicorn-restart-when-using-rbenv/
+before_exec do |server|
+  ENV.keep_if { |name, value| name.match?(/\A(RAILS_.*|UNICORN_.*|HOME)\z/) }
+  ENV["PATH"] = "#{ENV["HOME"]}/.rbenv/shims:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+end