From aecc1c65ad57df07859f62a6a11de2db31bf9dcc Mon Sep 17 00:00:00 2001
From: Michal Cichra <michal@cichra.cz>
Date: Tue, 20 Feb 2018 09:17:31 +0100
Subject: [PATCH] handle empty values with __tostring metatable

---
 lib/liquid.lua  | 15 ++++++++++-----
 t/interpreter.t | 25 ++++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/lib/liquid.lua b/lib/liquid.lua
index 5b590ee..69b3852 100644
--- a/lib/liquid.lua
+++ b/lib/liquid.lua
@@ -2018,9 +2018,11 @@ function Interpreter:visit_BinOp( node )
                     return false
                 end
             end
+
             if left_name == "Empty" then
                 local right_value = self:visit(node.right)
-                if right_value == nil or right_value == '' then
+                local str = self:obj2str(right_value)
+                if not str or str == '' then
                     if op == EQ then
                         return true
                     else
@@ -2028,7 +2030,7 @@ function Interpreter:visit_BinOp( node )
                     end
                 end
                 if type(right_value) == "table" then
-                    if next(right_value) then
+                    if next(right_value) or (str and str ~= '') then
                         if op == NE then
                             return true
                         else
@@ -2045,15 +2047,18 @@ function Interpreter:visit_BinOp( node )
                 self:raise_error("Invalid empty comparision", node, 'op')
             else
                 local left_value = self:visit(node.left)
-                if left_value == nil or left_value == '' then
+                local str = self:obj2str(left_value)
+
+                if not str or str == '' then
                     if op == EQ then
                         return true
                     else
                         return false
                     end
                 end
+
                 if type(left_value) == "table" then
-                    if next(left_value) then
+                    if next(left_value) or (str and str ~= '') then
                         if op == NE then
                             return true
                         else
@@ -2723,7 +2728,7 @@ do
         elseif obj_type == "Boolean" then
             return tostring(obj)
         elseif type(mt__tostring(obj)) == 'function' then
-            return tostring(obj)
+            return tostring(obj) or ''
         end
     end
 
diff --git a/t/interpreter.t b/t/interpreter.t
index 9f9f1f0..71341af 100644
--- a/t/interpreter.t
+++ b/t/interpreter.t
@@ -690,7 +690,7 @@ too many loopcount. limit num:3
     location /t {
         content_by_lua_block {
             local Liquid = require 'liquid'
-            local document = [[str = {{ str }}, arr = {{ arr | join: '+' }}]]
+            local document = "str = {{ str }}, arr = {{ arr | join: '+' }}\n{%- if str == empty %}str is empty{%endif%}"
             local template = Liquid.Template:parse(document)
 
             local str = setmetatable({}, { __tostring = function() return 'val' end })
@@ -704,3 +704,26 @@ GET /t
 str = val, arr = val+val
 --- no_error_log
 [error]
+
+
+=== TEST 27: variable with __tostring metatable but returns nil
+--- http_config eval: $::HttpConfig
+--- config
+    location /t {
+        content_by_lua_block {
+            local Liquid = require 'liquid'
+            local document = "str = {{ str }}, arr = {{ arr | join: '+' }}\n{% if str == empty %}str is empty{%endif%}"
+            local template = Liquid.Template:parse(document)
+
+            local str = setmetatable({}, { __tostring = function()  end })
+            local context = Liquid.InterpreterContext:new({ str = str, arr = { str, str } })
+            ngx.say(assert(template:render(context)))
+        }
+    }
+--- request
+GET /t
+--- response_body
+str = , arr = +
+str is empty
+--- no_error_log
+[error]