Freeze block body after parsing completes

lib/liquid/block.rb

@@ -13,6 +13,7 @@ module Liquid
       @body = new_body
       while parse_body(@body, tokens)
       end
+      @body.freeze(parse_context)
     end
 
     # For backwards compatibility

lib/liquid/block_body.rb

@@ -16,9 +16,12 @@ module Liquid
     def initialize
       @nodelist = []
       @blank    = true
+      @frozen   = false
     end
 
     def parse(tokenizer, parse_context, &block)
+      raise FrozenError, "can't modify frozen Liquid::BlockBody" if @frozen
+
       parse_context.line_number = tokenizer.line_number
 
       if tokenizer.for_liquid_tag
@@ -28,6 +31,10 @@ module Liquid
       end
     end
 
+    def freeze(_context)
+      @frozen = true
+    end
+
     private def parse_for_liquid_tag(tokenizer, parse_context)
       while (token = tokenizer.shift)
         unless token.empty? || token =~ WhitespaceOrNothing
@@ -192,6 +199,8 @@ module Liquid
     end
 
     def render_to_output_buffer(context, output)
+      raise "Can only render when frozen" unless @frozen
+
       context.resource_limits.increment_render_score(@nodelist.length)
 
       idx = 0

lib/liquid/document.rb

@@ -22,6 +22,7 @@ module Liquid
     def parse(tokenizer, parse_context)
       while parse_body(tokenizer)
       end
+      @body.freeze(parse_context)
     rescue SyntaxError => e
       e.line_number ||= parse_context.line_number
       raise

lib/liquid/tags/case.rb

@@ -20,7 +20,11 @@ module Liquid
 
     def parse(tokens)
       body = new_body
-      body = @blocks.last.attachment while parse_body(body, tokens)
+      while parse_body(body, tokens)
+        body.freeze(parse_context)
+        body = @blocks.last.attachment
+      end
+      body.freeze(parse_context)
       if blank?
         @blocks.each { |condition| condition.attachment.remove_blank_strings }
       end

lib/liquid/tags/for.rb

@@ -61,7 +61,9 @@ module Liquid
     def parse(tokens)
       if parse_body(@for_block, tokens)
         parse_body(@else_block, tokens)
+        @else_block.freeze(parse_context)
       end
+      @for_block.freeze(parse_context)
       if blank?
         @for_block.remove_blank_strings
         @else_block&.remove_blank_strings

lib/liquid/tags/if.rb

@@ -31,6 +31,7 @@ module Liquid
     def parse(tokens)
       while parse_body(@blocks.last.attachment, tokens)
       end
+      @blocks.each { |block| block.attachment.freeze(parse_context) }
       if blank?
         @blocks.each { |condition| condition.attachment.remove_blank_strings }
       end