Double render (phlex-ruby#734)

should resolve phlex-ruby#607
(last pr had some formatting issues)

lib/phlex.rb

@@ -5,6 +5,7 @@
 
 module Phlex
 	autoload :ArgumentError, "phlex/errors/argument_error"
+	autoload :DoubleRenderError, "phlex/errors/double_render_error"
 	autoload :BlackHole, "phlex/black_hole"
 	autoload :CSV, "phlex/csv"
 	autoload :Callable, "phlex/callable"

lib/phlex/errors/double_render_error.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class Phlex::DoubleRenderError < RuntimeError
+	include Phlex::Error
+end

lib/phlex/sgml.rb

@@ -83,6 +83,9 @@ def call(buffer = +"", context: Phlex::Context.new, view_context: nil, parent: n
 			@_view_context = view_context
 			@_parent = parent
 
+			raise Phlex::DoubleRenderError, "You can't render a #{self.class.name} more than once." if @_rendered
+			@_rendered = true
+
 			if fragments
 				@_context.target_fragments(fragments)
 			end

test/phlex/view/call.rb

@@ -64,4 +64,21 @@ def some_method
 			expect(output).to be == "<h1>Hi</h1>"
 		end
 	end
+
+	with "a view that is rendered twice" do
+		view do
+			def view_template
+				plain "Hi"
+			end
+		end
+
+		let(:output) do
+			example.call
+			example.call
+		end
+
+		it "raises an error" do
+			expect { output }.to raise_exception(Phlex::DoubleRenderError)
+		end
+	end
 end