child_process helper: fix stderr blocking for discard case. fix #2609

lib/fluent/plugin_helper/child_process.rb

@@ -257,7 +257,8 @@ def child_process_execute_once(
         readio = writeio = stderrio = wait_thread = nil
         readio_in_use = writeio_in_use = stderrio_in_use = false
 
-        if !mode.include?(:stderr) && !mode.include?(:read_with_stderr) && stderr != :discard # connect
+        if !mode.include?(:stderr) && !mode.include?(:read_with_stderr)
+          spawn_opts[:err] = IO::NULL if stderr == :discard
           writeio, readio, wait_thread = *Open3.popen2(*spawn_args, spawn_opts)
         elsif mode.include?(:read_with_stderr)
           writeio, readio, wait_thread = *Open3.popen2e(*spawn_args, spawn_opts)
@@ -281,7 +282,7 @@ def child_process_execute_once(
           stderrio.set_encoding(external_encoding, internal_encoding, encoding_options)
           stderrio_in_use = true
         else
-          stderrio.reopen(IO::NULL) if stderrio && stderrio == :discard
+          stderrio.reopen(IO::NULL) if stderrio && stderr == :discard
         end
 
         pid = wait_thread.pid # wait_thread => Process::Waiter

test/plugin_helper/test_child_process.rb

@@ -124,6 +124,39 @@ def configure(conf)
     assert_equal expected, ary
   end
 
+  test 'can execute external command at just once, which can handle both of read and write. Ignore stderr message/no block' do
+    m = Mutex.new
+    ary = []
+    Timeout.timeout(TEST_DEADLOCK_TIMEOUT) do
+      ran = false
+      # lots of stderr message should not be blocked and message should not be printed in test.
+      cmd = "ruby -e 'while !STDIN.eof? && line = STDIN.readline; STDERR.puts line.chomp * 1000; STDOUT.puts line.chomp; STDOUT.flush rescue nil; end'"
+      @d.child_process_execute(:t2_and_ignore_stderr, cmd, mode: [:write, :read]) do |writeio, readio|
+        m.lock
+        ran = true
+
+        [[1,2],[3,4],[5,6]].each do |i,j|
+          writeio.write "my data#{i}\n"
+          writeio.write "my data#{j}\n"
+          writeio.flush
+        end
+        writeio.close
+
+        while line = readio.readline
+          ary << line
+        end
+        m.unlock
+      end
+      sleep TEST_WAIT_INTERVAL_FOR_BLOCK_RUNNING until m.locked? || ran
+      m.lock
+      m.unlock
+    end
+
+    assert_equal [], @d.log.out.logs
+    expected = (1..6).map{|i| "my data#{i}\n" }
+    assert_equal expected, ary
+  end
+
   test 'can execute external command at just once, which can handle all of read, write and stderr' do
     m = Mutex.new
     ary1 = []