restore environment to original state before restarting

- prevents apps from forever poisoning the ENV by doing bad things (something overrides PATH/HOME etc)
 - allows use of Dotenv to load new ENV settings from disk (they do not override)

Test:

```
puts "THIS SHOULD CHANGE #{ENV["TEST"] ||= rand.to_s}"
```

then restart the server

lib/puma/launcher.rb

@@ -163,6 +163,8 @@ module Puma
 
     # Run the server. This blocks until the server is stopped
     def run
+      env = ENV.to_h
+
       @config.clamp
 
       @config.plugins.fire_starts self
@@ -178,6 +180,7 @@ module Puma
         graceful_stop
       when :restart
         log "* Restarting..."
+        ENV.replace(env)
         @runner.before_restart
         restart!
       when :exit

test/hello-env.ru

@@ -0,0 +1,2 @@
+ENV["RAND"] ||= rand.to_s
+run lambda { |env| [200, {"Content-Type" => "text/plain"}, ["Hello RAND #{ENV["RAND"]}"]] }

test/test_integration.rb

@@ -56,11 +56,7 @@ class TestIntegration < Minitest::Test
 
     @server = IO.popen("sh #{tf.path}", "r")
 
-    while (@server.gets) !~ /Ctrl-C/
-      # nothing
-    end
-
-    sleep 1
+    wait_for_server_to_boot
 
     @server
   end
@@ -73,6 +69,35 @@ class TestIntegration < Minitest::Test
     @wait.sysread 1
   end
 
+  # reuses an existing connection to make sure that works
+  def restart_server(connection)
+    signal :USR2
+
+    sleep 1
+
+    connection.write "GET / HTTP/1.1\r\n\r\n" # trigger it to start by sending a new request
+
+    assert_raises Errno::ECONNRESET do
+      Timeout.timeout(2) do
+        raise Errno::ECONNRESET unless connection.read(2)
+      end
+    end
+
+    wait_for_server_to_boot
+  end
+
+  def connect
+    s = TCPSocket.new "localhost", @tcp_port
+    s << "GET / HTTP/1.1\r\n\r\n"
+    true until s.gets == "\r\n"
+    s
+  end
+
+  def wait_for_server_to_boot
+    true while @server.gets =~ /Ctrl-C/
+    sleep(Puma.jruby? ? 5 : 1) # TODO: not sure why that is needed ... something deterministic would be better
+  end
+
   def test_stop_via_pumactl
     if Puma.jruby? || Puma.windows?
       assert true
@@ -186,28 +211,9 @@ class TestIntegration < Minitest::Test
   def test_restart_closes_keepalive_sockets
     server("-q test/hello.ru")
 
-    s = TCPSocket.new "localhost", @tcp_port
-    s << "GET / HTTP/1.1\r\n\r\n"
-    true until s.gets == "\r\n"
-
-    s.readpartial(20)
-    signal :USR2
-
-    sleep 1
-
-    s.write "GET / HTTP/1.1\r\n\r\n"
-
-    assert_raises Errno::ECONNRESET do
-      Timeout.timeout(2) do
-        raise Errno::ECONNRESET unless s.read(2)
-      end
-    end
-
-    while (@server.gets) !~ /Ctrl-C/
-      # nothing
-    end
-
-    sleep 5 if Puma.jruby?
+    s = connect
+    s.read
+    restart_server(s)
 
     s = TCPSocket.new "127.0.0.1", @tcp_port
     s << "GET / HTTP/1.0\r\n\r\n"
@@ -222,26 +228,29 @@ class TestIntegration < Minitest::Test
 
     server("-q -w 2 test/hello.ru")
 
-    s = TCPSocket.new "localhost", @tcp_port
-    s << "GET / HTTP/1.1\r\n\r\n"
-    true until s.gets == "\r\n"
-
-    s.readpartial(20)
-    signal :USR2
-
-    true while @server.gets =~ /Ctrl-C/
-    sleep 1
-
-    s.write "GET / HTTP/1.1\r\n\r\n"
-
-    assert_raises Errno::ECONNRESET do
-      Timeout.timeout(2) do
-        raise Errno::ECONNRESET unless s.read(2)
-      end
-    end
+    s = connect
+    s.read
+    restart_server s
 
     s = TCPSocket.new "localhost", @tcp_port
     s << "GET / HTTP/1.0\r\n\r\n"
     assert_equal "Hello World", s.read.split("\r\n").last
   end
+
+  # It does not share environments between multiple generations, which would break Dotenv
+  def test_restart_restores_environment
+    server("-q test/hello-env.ru")
+
+    s = connect
+    initial_reply = s.read
+    restart_server(s)
+
+    s = TCPSocket.new "localhost", @tcp_port
+    s << "GET / HTTP/1.0\r\n\r\n"
+    new_reply = s.read.split("\r\n").last
+
+    assert_includes initial_reply, "Hello RAND"
+    assert_includes new_reply, "Hello RAND"
+    refute_equal initial_reply, new_reply
+  end
 end