Raise better errors if server fails to start

* When WEBKIT_SERVER_START_TIMEOUT was reached, @port would get set to
  nil. If execution of the SERVER_PROCESS returned immediately, @port
  would get set to 0. This lead to odd cross-platform behavior in
  #connect, which would try to connect to a nonsense port.

lib/capybara/webkit/connection.rb

@@ -55,9 +55,19 @@ module Capybara::Webkit
       @pipe_stdin, @pipe_stdout, @pipe_stderr, @wait_thr = Open3.popen3(SERVER_PATH)
     end
 
+    def parse_port(line)
+      if match = line.to_s.match(/listening on port: (\d+)/)
+        match[1].to_i
+      else
+        raise ConnectionError, "#{SERVER_PATH} failed to start."
+      end
+    end
+
     def discover_port
       if IO.select([@pipe_stdout], nil, nil, WEBKIT_SERVER_START_TIMEOUT)
-        @port = ((@pipe_stdout.first || '').match(/listening on port: (\d+)/) || [])[1].to_i
+        @port = parse_port(@pipe_stdout.first)
+      else
+        raise ConnectionError, "#{SERVER_PATH} failed to start after #{WEBKIT_SERVER_START_TIMEOUT} seconds."
       end
     end
 

lib/capybara/webkit/errors.rb

@@ -17,6 +17,9 @@ module Capybara::Webkit
   class NoSuchWindowError < StandardError
   end
 
+  class ConnectionError < StandardError
+  end
+
   class JsonError
     def initialize(response)
       error = JSON.parse response

spec/connection_spec.rb

@@ -23,6 +23,33 @@ describe Capybara::Webkit::Connection do
     expect { Process.getpgid(webkit_pid) }.to raise_error Errno::ESRCH
   end
 
+  it "raises an error if the server has stopped", skip_on_windows: true do
+    path = 'false'
+    stub_const("Capybara::Webkit::Connection::SERVER_PATH", path)
+
+    expect { Capybara::Webkit::Connection.new }.
+      to raise_error(
+        Capybara::Webkit::ConnectionError,
+        "#{path} failed to start.")
+  end
+
+  it "raises an error if the server is not ready", skip_on_windows: true do
+    server_path = 'sleep 1'
+    stub_const("Capybara::Webkit::Connection::SERVER_PATH", server_path)
+    start_timeout = 0.5
+    stub_const("Capybara::Webkit::Connection::WEBKIT_SERVER_START_TIMEOUT", start_timeout)
+
+    error_string =
+      if defined?(::JRUBY_VERSION)
+        "#{server_path} failed to start."
+      else
+        "#{server_path} failed to start after #{start_timeout} seconds."
+      end
+
+    expect { Capybara::Webkit::Connection.new }.
+      to raise_error(Capybara::Webkit::ConnectionError, error_string)
+  end
+
   it "boots a server to talk to" do
     url = "http://#{@rack_server.host}:#{@rack_server.port}/"
     connection.puts "Visit"