Restore sync=true on global stdout/stderr streams (#2557)

* Restore sync=true on STDOUT/STDERR streams

* Move mutation of STDOUT and STDERR streams to `redirect_io`

This isn't technically related to redirecting the STDOUT and STDERR
streams, but moving it here keeps all of the STDOUT/STDERR logic
together. It seems like a more natural place to put it.

* Add a test to ensure that STDOUT is flushed by default

History.md

@@ -2,6 +2,7 @@
 
 * Bugfixes
   * Add `#flush` and `#sync` methods to `Puma::NullIO`  ([#2553])
+  * Restore `sync=true` on `STDOUT` and `STDERR` streams #2557
 
 ## 5.2.1 / 2021-02-05
 

lib/puma/configuration.rb

@@ -206,7 +206,8 @@ module Puma
         :first_data_timeout => Const::FIRST_DATA_TIMEOUT,
         :raise_exception_on_sigterm => true,
         :max_fast_inline => Const::MAX_FAST_INLINE,
-        :io_selector_backend => :auto
+        :io_selector_backend => :auto,
+        :mutate_stdout_and_stderr_to_sync_on_write => true,
       }
     end
 

lib/puma/dsl.rb

@@ -888,5 +888,9 @@ module Puma
     def io_selector_backend(backend)
       @options[:io_selector_backend] = backend.to_sym
     end
+
+    def mutate_stdout_and_stderr_to_sync_on_write(enabled=true)
+      @options[:mutate_stdout_and_stderr_to_sync_on_write] = enabled
+    end
   end
 end

lib/puma/runner.rb

@@ -126,6 +126,11 @@ module Puma
         STDERR.puts "=== puma startup: #{Time.now} ==="
         STDERR.flush unless STDERR.sync
       end
+
+      if @options[:mutate_stdout_and_stderr_to_sync_on_write]
+        STDOUT.sync = true
+        STDERR.sync = true
+      end
     end
 
     def load_and_bind

test/rackup/write_to_stdout.ru

@@ -0,0 +1,6 @@
+app = lambda do |env|
+  $stdout.write "hello\n"
+  [200, {"Content-Type" => "text/plain"}, ["Hello World"]]
+end
+
+run app

test/test_integration_single.rb

@@ -163,4 +163,11 @@ class TestIntegrationSingle < TestIntegration
     assert(!File.file?("t2-pid"))
     assert_equal("Puma is started\n", out)
   end
+
+  def test_application_logs_are_flushed_on_write
+    cli_server 'test/rackup/write_to_stdout.ru'
+    read_body connect
+    log_line = @server.gets
+    assert_equal "hello\n", log_line
+  end
 end