Improve error message of Liquid::MemoryLimit.

The existing error message is too generic since there are three types of limits in place.
It is useful to know which limit was reached to make it easier to debug the error.

History.md

@@ -4,6 +4,7 @@
 
 * Split Strainer class as a factory and a template (#1208) [Thierry Joyal]
 * Remove handling of a nil context in the Strainer class (#1218) [Thierry Joyal]
+* Change `Liquid::MemoryError` message to be more specific about which limit was reached. (#1206) [Alan Tan]
 
 ## 4.0.3 / 2019-03-12
 

lib/liquid/block_body.rb

@@ -192,8 +192,18 @@ module Liquid
 
     def raise_if_resource_limits_reached(context, length)
       context.resource_limits.render_length += length
-      return unless context.resource_limits.reached?
-      raise MemoryError, "Memory limits exceeded"
+
+      error_message =
+        if context.resource_limits.render_length_reached?
+          MemoryError::RENDER_LENGTH_ERROR_MESSAGE
+        elsif context.resource_limits.render_score_reached?
+          MemoryError::RENDER_SCORE_ERROR_MESSAGE
+        elsif context.resource_limits.assign_score_reached?
+          MemoryError::ASSIGN_SCORE_ERROR_MESSAGE
+        end
+
+      return unless error_message
+      raise MemoryError, error_message
     end
 
     def create_variable(token, parse_context)

lib/liquid/errors.rb

@@ -23,11 +23,14 @@ module Liquid
 
     def message_prefix
       str = +""
-      str << if is_a?(SyntaxError)
-        "Liquid syntax error"
-      else
-        "Liquid error"
-      end
+      str <<
+        if is_a?(SyntaxError)
+          "Liquid syntax error"
+        elsif is_a?(MemoryError)
+          "Liquid memory limit error"
+        else
+          "Liquid error"
+        end
 
       if line_number
         str << " ("
@@ -40,6 +43,12 @@ module Liquid
     end
   end
 
+  class MemoryError < Error
+    RENDER_LENGTH_ERROR_MESSAGE = 'Too many bytes rendered.'
+    RENDER_SCORE_ERROR_MESSAGE = 'Too many tags rendered.'
+    ASSIGN_SCORE_ERROR_MESSAGE = 'Too many bytes assigned to variables.'
+  end
+
   ArgumentError       = Class.new(Error)
   ContextError        = Class.new(Error)
   FileSystemError     = Class.new(Error)
@@ -47,7 +56,6 @@ module Liquid
   SyntaxError         = Class.new(Error)
   StackLevelError     = Class.new(Error)
   TaintedError        = Class.new(Error)
-  MemoryError         = Class.new(Error)
   ZeroDivisionError   = Class.new(Error)
   FloatDomainError    = Class.new(Error)
   UndefinedVariable   = Class.new(Error)

lib/liquid/resource_limits.rb

@@ -12,10 +12,16 @@ module Liquid
       reset
     end
 
-    def reached?
-      (@render_length_limit && @render_length > @render_length_limit) ||
-        (@render_score_limit && @render_score > @render_score_limit) ||
-        (@assign_score_limit && @assign_score > @assign_score_limit)
+    def render_length_reached?
+      @render_length_limit && @render_length > @render_length_limit
+    end
+
+    def render_score_reached?
+      @render_score_limit && @render_score > @render_score_limit
+    end
+
+    def assign_score_reached?
+      @assign_score_limit && @assign_score > @assign_score_limit
     end
 
     def reset

test/integration/template_test.rb

@@ -112,8 +112,8 @@ class TemplateTest < Minitest::Test
   def test_resource_limits_render_length
     t = Template.parse("0123456789")
     t.resource_limits.render_length_limit = 5
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.render_length_reached?)
 
     t.resource_limits.render_length_limit = 10
     assert_equal("0123456789", t.render!)
@@ -123,13 +123,13 @@ class TemplateTest < Minitest::Test
   def test_resource_limits_render_score
     t = Template.parse("{% for a in (1..10) %} {% for a in (1..10) %} foo {% endfor %} {% endfor %}")
     t.resource_limits.render_score_limit = 50
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_SCORE_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.render_score_reached?)
 
     t = Template.parse("{% for a in (1..100) %} foo {% endfor %}")
     t.resource_limits.render_score_limit = 50
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_SCORE_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.render_score_reached?)
 
     t.resource_limits.render_score_limit = 200
     assert_equal((" foo " * 100), t.render!)
@@ -139,8 +139,8 @@ class TemplateTest < Minitest::Test
   def test_resource_limits_assign_score
     t = Template.parse("{% assign foo = 42 %}{% assign bar = 23 %}")
     t.resource_limits.assign_score_limit = 1
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::ASSIGN_SCORE_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.assign_score_reached?)
 
     t.resource_limits.assign_score_limit = 2
     assert_equal("", t.render!)
@@ -161,8 +161,8 @@ class TemplateTest < Minitest::Test
     t = Template.parse("{% assign foo = 'aaaa' | reverse %}")
 
     t.resource_limits.assign_score_limit = 3
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::ASSIGN_SCORE_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.assign_score_reached?)
 
     t.resource_limits.assign_score_limit = 5
     assert_equal("", t.render!)
@@ -171,8 +171,8 @@ class TemplateTest < Minitest::Test
   def test_resource_limits_aborts_rendering_after_first_error
     t = Template.parse("{% for a in (1..100) %} foo1 {% endfor %} bar {% for a in (1..100) %} foo2 {% endfor %}")
     t.resource_limits.render_score_limit = 50
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
-    assert(t.resource_limits.reached?)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_SCORE_ERROR_MESSAGE}", t.render)
+    assert(t.resource_limits.render_score_reached?)
   end
 
   def test_resource_limits_hash_in_template_gets_updated_even_if_no_limits_are_set
@@ -186,21 +186,21 @@ class TemplateTest < Minitest::Test
   def test_render_length_persists_between_blocks
     t = Template.parse("{% if true %}aaaa{% endif %}")
     t.resource_limits.render_length_limit = 7
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
     t.resource_limits.render_length_limit = 8
     assert_equal("aaaa", t.render)
 
     t = Template.parse("{% if true %}aaaa{% endif %}{% if true %}bbb{% endif %}")
     t.resource_limits.render_length_limit = 13
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
     t.resource_limits.render_length_limit = 14
     assert_equal("aaaabbb", t.render)
 
     t = Template.parse("{% if true %}a{% endif %}{% if true %}b{% endif %}{% if true %}a{% endif %}{% if true %}b{% endif %}{% if true %}a{% endif %}{% if true %}b{% endif %}")
     t.resource_limits.render_length_limit = 5
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
     t.resource_limits.render_length_limit = 11
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
     t.resource_limits.render_length_limit = 12
     assert_equal("ababab", t.render)
   end
@@ -208,7 +208,7 @@ class TemplateTest < Minitest::Test
   def test_render_length_uses_number_of_bytes_not_characters
     t = Template.parse("{% if true %}すごい{% endif %}")
     t.resource_limits.render_length_limit = 10
-    assert_equal("Liquid error: Memory limits exceeded", t.render)
+    assert_equal("Liquid memory limit error: #{Liquid::MemoryError::RENDER_LENGTH_ERROR_MESSAGE}", t.render)
     t.resource_limits.render_length_limit = 18
     assert_equal("すごい", t.render)
   end