Allow to check for mis-typed attributes and accidental AST changes

Setting the `QUARTO_JOG_CHECK` environment variable will run checks to
identify element attributes that have the wrong type, and will also find
filters that modify the input object, but don't return it. Both of these
can cause issues with jog.

src/resources/filters/ast/runemulation.lua

@@ -47,8 +47,97 @@ local function remove_vault(doc)
   end
 end
 
+--- Create a deep copy of a table.
+local function copy_table (tbl, depth, seen)
+  local tp = type(tbl)
+  if tp == 'table' then
+    local copy = {}
+    -- Iterate 'raw' pairs, i.e., without using metamethods
+    for key, value in next, tbl, nil do
+      if depth == 'shallow' then
+        copy[key] = value
+      else
+        copy[copy_table(key)] = copy_table(value)
+      end
+    end
+    return setmetatable(copy, getmetatable(tbl))
+  elseif tp == 'userdata' then
+    return tbl:clone()
+  else -- number, string, boolean, etc
+    return tbl
+  end
+end
+
+--- Checks if two tables are equal
+function equals(o1, o2)
+  if o1 == o2 then
+    return true
+  end
+  local o1type = type(o1)
+  local o2type = type(o2)
+  if o1type ~= o2type or o1type ~= 'table' then
+    return false
+  end
+
+  local keys = {}
+
+  for key1, value1 in pairs(o1) do
+    local value2 = o2[key1]
+    if value2 == nil or equals(value1, value2) == false then
+      return false
+    end
+    keys[key1] = true
+  end
+
+  for key2 in pairs(o2) do
+    if not keys[key2] then return false end
+  end
+  return true
+end
+
+--- Checks if a filter follows the "nondestructive" property.
+-- The nondestructive property is fulfilled if filter functions returns
+-- an explicit object, or if it returns `nil` while leaving the passed
+-- in object unmodified.
+--
+-- An error is raised if the property is violated.
+--
+-- Only filters with this property can use jog safely, without
+-- unintended consequences.
+local function check_nondestructive_property (namedfilter)
+  for name, fn in pairs(namedfilter.filter) do
+    if type(fn) == 'function' then
+      local copy = function (x)
+        local tp = type(x)
+        return tp ~= 'table' and x:clone() or
+          (pandoc.utils.type(x) == 'Meta' and pandoc.Meta(x) or copy_table(x))
+      end
+      namedfilter.filter[name] = function (obj, context)
+        local orig = copy(obj)
+        local result, descend = fn(obj, context)
+        if result == nil then
+          if type(obj) ~= 'table' and not equals(obj, orig) then
+            warn(
+              "\nFunction '" .. name .. "' in filter '" .. namedfilter.name ..
+              "' returned `nil`, but modified the input."
+            )
+          end
+        -- elseif result.t == obj.t and not rawequal(result, obj) then
+        --   warn(
+        --     "\nFunction '" .. name .. "' in filter '" .. namedfilter.name ..
+        --     "' returned a new object instead of passing the original one through."
+        --   )
+        end
+        return result, descend
+      end
+    end
+  end
+  return namedfilter
+end
+
 local function run_emulated_filter_chain(doc, filters, afterFilterPass, profiling)
   init_trace(doc)
+  local compare_jog_and_walk = os.getenv 'QUARTO_JOG_CHECK'
   for i, v in ipairs(filters) do
     local function callback()
       if v.flags then
@@ -79,7 +168,17 @@ local function run_emulated_filter_chain(doc, filters, afterFilterPass, profilin
         print(pandoc.write(doc, "native"))
       else
         _quarto.ast._current_doc = doc
-        doc = run_emulated_filter(doc, v.filter)
+
+        if compare_jog_and_walk and not v.force_pandoc_walk then
+          v = check_nondestructive_property(v)
+        end
+        doc = run_emulated_filter(doc, v.filter, v.force_pandoc_walk)
+
+        if compare_jog_and_walk and not v.force_pandoc_walk then
+          -- Types of meta values are only check on assignment.
+          doc.meta = doc.meta
+        end
+
         ensure_vault(doc)
 
         add_trace(doc, v.name)
@@ -204,4 +303,4 @@ function run_as_extended_ast(specTable)
   end
 
   return pandocFilterList
-end
+end

src/resources/filters/main.lua

@@ -196,6 +196,10 @@ import("./quarto-init/metainit.lua")
 
 -- [/import]
 
+if os.getenv 'QUARTO_JOG_CHECK' then
+  _quarto.modules.attribcheck.enable_attribute_checks()
+end
+
 initCrossrefIndex()
 
 initShortcodeHandlers()