Sequel datetime class consistency (#151)

* timezones: absent instruction otherwise, interpret times as local

these tests SUCCEED when run on their own, but FAIL when run after any
instance of a plugin has been initialized with `jdbc_default_timezone`,
since doing so globally configures Sequel to use DateTime instead of
Time:

To validate, modify the `.ci/run.sh` execution line to be:
> ~~~
> bundle exec rspec spec --format documentation --example "without jdbc_default_timezone"
> ~~~

* timezones: consistently pre-load Sequel, force datetime_class=Time

By default, when a plugin is configured _without_ `jdbc_default_timezone`
timestamps are assumed to be in the same timezone as the Logstash host
machine, because they are parsed with Ruby's `Time#parse` which uses
local context.

However, when any one plugin declares `jdbc_default_timezone`, we load
Sequel's `named_timezones` extension, which has a side-effect of globally
changing `Sequel.datetime_class` to ruby's `DateTime`. The plugins that
are configured with `jdbc_default_timezone` have enough information to
apply the separately-provided offset, but the plugins that do _not_ have
a `jdbc_default_timezone` directive become broken and effectively
fail to parse the timestamps as they did when run on their own.

Sequel's `named_timezones` extension supports being used with `Time`
objects, and is noted to override `Sequel#datetime_class` on load only
for historic reasons.

We force Sequel to use ruby's `Time` class globally (enforcing its
default and preventing it from being changed). This is done inside
of a separately-required bootstrap, which allows us to ensure it is
loaded exacltly once.

* bump version & add changelog entry

CHANGELOG.md

@@ -1,4 +1,5 @@
 ## 5.4.11
+- Fixes an issue in which any one instance of a JDBC input plugin using `jdbc_default_timezone` changes the behaviour of plugin instances that do _not_ use `jdbc_default_timezone`, ensuring that timezone offsets remain consistent for each instance of the plugin _as configured_ [#151](https://github.com/logstash-plugins/logstash-integration-jdbc/pull/151)
 - Fixes an exception that could occur while reloading `jdbc_static` databases when the underlying connection to the remote has been broken [#165](https://github.com/logstash-plugins/logstash-integration-jdbc/pull/165)
 
 ## 5.4.10

lib/logstash/filters/jdbc/basic_database.rb

@@ -1,6 +1,6 @@
 # encoding: utf-8
 require "fileutils"
-require "sequel"
+require "logstash/plugin_mixins/jdbc/sequel_bootstrap"
 require "sequel/adapters/jdbc"
 require "java"
 require "logstash/util/loggable"

lib/logstash/plugin_mixins/jdbc/common.rb

@@ -24,7 +24,8 @@ def load_driver
       return @driver_impl if @driver_impl ||= nil
 
       require "java"
-      require "sequel"
+
+      require_relative "sequel_bootstrap"
       require "sequel/adapters/jdbc"
 
       # execute all the driver loading related duties in a serial fashion to avoid

lib/logstash/plugin_mixins/jdbc/sequel_bootstrap.rb

@@ -0,0 +1,21 @@
+# encoding: utf-8
+
+require "sequel"
+
+# prevent Sequel's datetime_class from being modified,
+# and ensure behaviour is restored to the library's default
+# if something else in the Ruby VM has already changed it.
+Sequel.synchronize do
+  def Sequel.datetime_class=(klass)
+    # noop
+  end
+  def Sequel.datetime_class
+    ::Time
+  end
+end
+
+# load the named_timezones extension, which will attempt to
+# override the global Sequel::datetime_class; for safety,
+# we reset it once more.
+Sequel.extension(:named_timezones)
+Sequel.datetime_class = ::Time

spec/inputs/jdbc_spec.rb

@@ -601,6 +601,33 @@
       # With no timezone set, no change should occur
       expect(event.get("custom_time").time).to eq(Time.iso8601("2015-01-01T12:00:00Z"))
     end
+
+    %w(
+        Etc/UTC
+        America/Los_Angeles
+        Europe/Berlin
+        Asia/Tokyo
+      ).each do |local_timezone|
+      context "when host machine has timezone `#{local_timezone}`" do
+        around(:each) do |example|
+          begin
+            previous_tz = ENV['TZ']
+            ENV['TZ'] = local_timezone
+            example.call
+          ensure
+            ENV['TZ'] = previous_tz
+          end
+        end
+
+        let(:tz) { TZInfo::Timezone.get(local_timezone) }
+
+        it "converts the time using the machine's local timezone" do
+          plugin.run(queue)
+          event = queue.pop
+          expect(event.get("custom_time").time).to eq(Time.new(2015,1,1,12,0,0,tz))
+        end
+      end
+    end
   end
 
   context "when iteratively running plugin#run" do