Handle BlockBody#blank? at parse time (#1287)

lib/liquid/block_body.rb

@@ -130,6 +130,26 @@ module Liquid
       @blank
     end
 
+    # Remove blank strings in the block body for a control flow tag (e.g. `if`, `for`, `case`, `unless`)
+    # with a blank body.
+    #
+    # For example, in a conditional assignment like the following
+    #
+    # ```
+    # {% if size > max_size %}
+    #   {% assign size = max_size %}
+    # {% endif %}
+    # ```
+    #
+    # we assume the intention wasn't to output the blank spaces in the `if` tag's block body, so this method
+    # will remove them to reduce the render output size.
+    #
+    # Note that it is now preferred to use the `liquid` tag for this use case.
+    def remove_blank_strings
+      raise "remove_blank_strings only support being called on a blank block body" unless @blank
+      @nodelist.reject! { |node| node.instance_of?(String) }
+    end
+
     def render(context)
       render_to_output_buffer(context, +'')
     end
@@ -143,10 +163,8 @@ module Liquid
 
         if node.instance_of?(String)
           output << node
-        elsif node.instance_of?(Variable)
-          render_node(context, output, node)
         else
-          render_node(context, node.blank? ? +'' : output, node)
+          render_node(context, output, node)
           # If we get an Interrupt that means the block must stop processing. An
           # Interrupt is any command that stops block execution such as {% break %}
           # or {% continue %}. These tags may also occur through Block or Include tags.

lib/liquid/tags/capture.rb

@@ -25,10 +25,9 @@ module Liquid
     end
 
     def render_to_output_buffer(context, output)
-      previous_output_size = output.bytesize
+      capture_output = render(context)
-      super
+      context.scopes.last[@to] = capture_output
-      context.scopes.last[@to] = output
+      context.resource_limits.assign_score += capture_output.bytesize
-      context.resource_limits.assign_score += (output.bytesize - previous_output_size)
       output
     end
 

lib/liquid/tags/case.rb

@@ -21,6 +21,9 @@ module Liquid
     def parse(tokens)
       body = BlockBody.new
       body = @blocks.last.attachment while parse_body(body, tokens)
+      if blank?
+        @blocks.each { |condition| condition.attachment.remove_blank_strings }
+      end
     end
 
     def nodelist

lib/liquid/tags/for.rb

@@ -59,8 +59,13 @@ module Liquid
     end
 
     def parse(tokens)
-      return unless parse_body(@for_block, tokens)
+      if parse_body(@for_block, tokens)
-      parse_body(@else_block, tokens)
+        parse_body(@else_block, tokens)
+      end
+      if blank?
+        @for_block.remove_blank_strings
+        @else_block&.remove_blank_strings
+      end
     end
 
     def nodelist

lib/liquid/tags/if.rb

@@ -31,6 +31,9 @@ module Liquid
     def parse(tokens)
       while parse_body(@blocks.last.attachment, tokens)
       end
+      if blank?
+        @blocks.each { |condition| condition.attachment.remove_blank_strings }
+      end
     end
 
     def unknown_tag(tag, markup, tokens)

test/integration/template_test.rb

@@ -176,7 +176,7 @@ class TemplateTest < Minitest::Test
   end
 
   def test_resource_limits_hash_in_template_gets_updated_even_if_no_limits_are_set
-    t = Template.parse("{% for a in (1..100) %} {% assign foo = 1 %} {% endfor %}")
+    t = Template.parse("{% for a in (1..100) %}x{% assign foo = 1 %} {% endfor %}")
     t.render!
     assert(t.resource_limits.assign_score > 0)
     assert(t.resource_limits.render_score > 0)
@@ -215,7 +215,7 @@ class TemplateTest < Minitest::Test
 
   def test_default_resource_limits_unaffected_by_render_with_context
     context = Context.new
-    t = Template.parse("{% for a in (1..100) %} {% assign foo = 1 %} {% endfor %}")
+    t = Template.parse("{% for a in (1..100) %}x{% assign foo = 1 %} {% endfor %}")
     t.render!(context)
     assert(context.resource_limits.assign_score > 0)
     assert(context.resource_limits.render_score > 0)