Generate correct epoch timestamp even after switching Daylight Saving Time

fluentd.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency("sigdump", ["~> 0.2.2"])
   gem.add_runtime_dependency("tzinfo", [">= 1.0", "< 3.0"])
   gem.add_runtime_dependency("tzinfo-data", ["~> 1.0"])
-  gem.add_runtime_dependency("strptime", [">= 0.2.2", "< 1.0.0"])
+  gem.add_runtime_dependency("strptime", [">= 0.2.4", "< 1.0.0"])
   gem.add_runtime_dependency("webrick", [">= 1.4.2", "< 1.8.0"])
 
   # build gem for a certain platform. see also Rakefile

lib/fluent/time.rb

@@ -226,19 +226,16 @@ def initialize(format = nil, localtime = true, timezone = nil)
 
       format_with_timezone = format && (format.include?("%z") || format.include?("%Z"))
 
-      # unixtime_in_expected_tz = unixtime_in_localtime + offset_diff
-      offset_diff = case
-                    when format_with_timezone then nil
-                    when timezone  then
-                      offset = Fluent::Timezone.utc_offset(timezone)
-                      if offset.respond_to?(:call)
-                        ->(t) { Time.now.localtime.utc_offset - offset.call(t) }
-                      else
-                        Time.now.localtime.utc_offset - offset
-                      end
-                    when localtime then 0
-                    else Time.now.localtime.utc_offset # utc
-                    end
+      utc_offset = case
+                   when format_with_timezone then
+                     nil
+                   when timezone then
+                     Fluent::Timezone.utc_offset(timezone)
+                   when localtime then
+                     nil
+                   else
+                     0 # utc
+                   end
 
       strptime = format && (Strptime.new(format) rescue nil)
 
@@ -247,16 +244,20 @@ def initialize(format = nil, localtime = true, timezone = nil)
                when format_with_timezone             then ->(v){ Fluent::EventTime.from_time(Time.strptime(v, format)) }
                when format == '%iso8601'             then ->(v){ Fluent::EventTime.from_time(Time.iso8601(v)) }
                when strptime then
-                 if offset_diff.respond_to?(:call)
-                   ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + offset_diff.call(t), t.nsec) }
+                 if utc_offset.nil?
+                   ->(v){ t = strptime.exec(v); Fluent::EventTime.new(t.to_i, t.nsec) }
+                 elsif utc_offset.respond_to?(:call)
+                   ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + t.utc_offset - utc_offset.call(t), t.nsec) }
                  else
-                   ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + offset_diff, t.nsec) }
+                   ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + t.utc_offset - utc_offset, t.nsec) }
                  end
-               when format   then
-                 if offset_diff.respond_to?(:call)
-                   ->(v){ t = Time.strptime(v, format); Fluent::EventTime.new(t.to_i + offset_diff.call(t), t.nsec) }
+               when format then
+                 if utc_offset.nil?
+                   ->(v){ t = Time.strptime(v, format); Fluent::EventTime.new(t.to_i, t.nsec) }
+                 elsif utc_offset.respond_to?(:call)
+                   ->(v){ t = Time.strptime(v, format); Fluent::EventTime.new(t.to_i + t.utc_offset - utc_offset.call(t), t.nsec) }
                  else
-                   ->(v){ t = Time.strptime(v, format); Fluent::EventTime.new(t.to_i + offset_diff, t.nsec) }
+                   ->(v){ t = Time.strptime(v, format); Fluent::EventTime.new(t.to_i + t.utc_offset - utc_offset, t.nsec) }
                  end
                else ->(v){ Fluent::EventTime.parse(v) }
                end

test/test_time_parser.rb

@@ -337,4 +337,26 @@ class DummyForTimeParser
       assert_equal_event_time(time, parser.parse('2021-01-01T12:00:00+0900'))
     end
   end
+
+  # https://github.com/fluent/fluentd/issues/3195
+  test 'change timezone without zone specifier in a format' do
+    expected = 1607457600 # 2020-12-08T20:00:00Z
+    time1 = time2 = nil
+
+    with_timezone("UTC-05") do # EST
+      i = DummyForTimeParser.new
+      i.configure(config_element('parse', '', {'time_type' => 'string',
+                                               'time_format' => '%Y-%m-%dT%H:%M:%SZ',
+                                               'utc' => true}))
+      parser = i.time_parser_create
+
+      time1 = parser.parse('2020-12-08T20:00:00Z').to_i
+      time2 = with_timezone("UTC-04") do # EDT
+        # to avoid using cache, increment 1 sec
+        parser.parse('2020-12-08T20:00:01Z').to_i
+      end
+    end
+
+    assert_equal([expected, expected + 1], [time1, time2])
+  end
 end