Rather more robust nr_elements_in_seq.

analyze_structure.lua

@@ -39,10 +39,10 @@ local LEAF_VALUE_TYPES = {
     ['boolean']  = true,
 }
 
-local SHORT_STRING_MAX_LEN = 7
-local MINIMUM_NUMBER_OF_SET_ELEMENTS = 2
-
-local RECURSIVE_TOSTRING_TIMEOUT = 10
+local SHORT_STRING_MAX_LEN          	=  7	-- Range: [0, ∞[
+local MINIMUM_NUMBER_OF_SET_ELEMENTS	=  2	-- Range: [1, ∞[
+local ALLOWED_HOLE_SIZE_IN_SEQUENCE 	=  1	-- Range: [0, ∞[. Set to 0, to completely disallow holes in sequences.
+local RECURSIVE_TOSTRING_TIMEOUT    	= 10	-- Range: [1, ∞[. High values may result in crashes on specially-crafted input.
 
 --------------------------------------------------------------------------------
 
@@ -98,6 +98,8 @@ local function largest_number_index (t)
 end
 
 local function nr_elements_in_table (t)
+    -- Determines the total number of elements in the table.
+
     assert(type(t) == 'table')
     --
     local k, count = nil, -1
@@ -108,16 +110,30 @@ local function nr_elements_in_table (t)
 end
 
 local function nr_elements_in_seq (t)
+    -- Determines the number of elements in the sequence part of the table.
+    -- Allows holes of size `ALLOWED_HOLE_SIZE_IN_SEQUENCE`, before stopping.
+    -- This function works even when the given table's metamethods throws errors.
+    --
+    -- Returns:
+    --  * Number: number of elements
+    --  * Boolean: whether the table has holes.
+
     assert(type(t) == 'table')
 
-    if getmetatable(t) and getmetatable(t).__index then
-        return 0, false -- FIXME: Temporary stopgap for when __index throws an
-        -- error. I think we need to clone the numbers part of the table, to
-        -- fix this.
+    -- We don't want to crash if the __index metamethod throws an error, so we
+    -- copy the pairs with number keys into a fresh table, which we then operate
+    -- on lower down.
+    if debug.getmetatable(t) and debug.getmetatable(t).__index then
+        local t_prime = {}
+        for k, v in pairs(t) do
+            if type(k) == 'number' then  t_prime[k] = v  end
+        end
+        t = t_prime
     end
 
+    -- Now we run though the table, from 1 and up.
     local i, last_elem_i, nr_elems, has_holes  =  0, 0, 0, false
-    while i <= last_elem_i + 2 do
+    while i <= last_elem_i + 1 + ALLOWED_HOLE_SIZE_IN_SEQUENCE do
         i  =  i + 1
         if t[i] ~= nil then
             last_elem_i, nr_elems  =  i, nr_elems + 1

test/test_analyze_structure.lua

@@ -229,6 +229,13 @@ SUITE:addTest('Can count elements, even though metatable.__index throws errors',
     assert_equal(2, info[input].seq_elems)
 end)
 
+SUITE:addTest('Can count elements, even though metatable.__index throws errors and __metatable returns bogus', function ()
+    local input = setmetatable({ 'hi', 'hello' }, {__index = function (_, k) error('Undefined access on key: '..k) end, __metatable = {}})
+    local info  = analyze_structure(input, math.huge)
+
+    assert_equal(2, info[input].seq_elems)
+end)
+
 --------------------------------------------------------------------------------
 -- Uniform structure