Update docs for low_latency option and fix parsing issue (#2631)

* Treat low_latency=false as false instead of true

Previously the binder.parse code was just checking that the low_latency
key existed, but the docs (and other options) support passing options in
the format `option=false`. This commit now treats `low_latency=false` as
false, while still treating `low_latency` with no `=` as true

* Update docs to reflect code behaviour

* Skip getsockopt tests in JRuby

JRuby (at least this version) doesn't support basic Socket constants or
methods

Ref: jruby/jruby#3438

lib/puma/binder.rb

@@ -163,7 +163,7 @@ def parse(binds, logger, log_msg = 'Listening')
             ios_len = @ios.length
             params = Util.parse_query uri.query
 
-            opt = params.key?('low_latency')
+            opt = params.key?('low_latency') && params['low_latency'] != 'false'
             bak = params.fetch('backlog', 1024).to_i
 
             io = add_tcp_listener uri.host, uri.port, opt, bak

lib/puma/dsl.rb

@@ -201,7 +201,7 @@ def load(file)
     # * Set the socket backlog depth with +backlog+, default is 1024.
     # * Set up an SSL certificate with +key+ & +cert+.
     # * Set whether to optimize for low latency instead of throughput with
-    #   +low_latency+, default is to optimize for low latency. This is done
+    #   +low_latency+, default is to not optimize for low latency. This is done
     #   via +Socket::TCP_NODELAY+.
     # * Set socket permissions with +umask+.
     #

test/test_binder.rb

@@ -192,6 +192,33 @@ def test_pre_existing_unix
     end
   end
 
+  def test_binder_parses_nil_low_latency
+    skip_if :jruby
+    @binder.parse ["tcp://0.0.0.0:0?low_latency"], @events
+
+    socket = @binder.listeners.first.last
+
+    assert socket.getsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY).bool
+  end
+
+  def test_binder_parses_true_low_latency
+    skip_if :jruby
+    @binder.parse ["tcp://0.0.0.0:0?low_latency=true"], @events
+
+    socket = @binder.listeners.first.last
+
+    assert socket.getsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY).bool
+  end
+
+  def test_binder_parses_false_low_latency
+    skip_if :jruby
+    @binder.parse ["tcp://0.0.0.0:0?low_latency=false"], @events
+
+    socket = @binder.listeners.first.last
+
+    refute socket.getsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY).bool
+  end
+
   def test_binder_parses_tlsv1_disabled
     skip_unless :ssl
     @binder.parse ["ssl://0.0.0.0:0?#{ssl_query}&no_tlsv1=true"], @events