New implementation for to-be-closed variables

To-be-closed variables are linked in their own list, embedded into the
stack elements. (Due to alignment, this information does not change
the size of the stack elements in most architectures.)  This new list
does not produce garbage and avoids memory errors when creating tbc
variables.

testes/locals.lua

@@ -529,6 +529,40 @@ local function checktable (t1, t2)
 end
 
 
+do    -- test for tbc variable high in the stack
+
+   -- function to force a stack overflow
+  local function overflow (n)
+    overflow(n + 1)
+  end
+
+  -- error handler will create tbc variable handling a stack overflow,
+  -- high in the stack
+  local function errorh (m)
+    assert(string.find(m, "stack overflow"))
+    local x <close> = func2close(function (o) o[1] = 10 end)
+    return x
+  end
+
+  local flag
+  local st, obj
+  -- run test in a coroutine so as not to swell the main stack
+  local co = coroutine.wrap(function ()
+    -- tbc variable down the stack
+    local y <close> = func2close(function (obj, msg)
+      assert(msg == nil)
+      obj[1] = 100
+      flag = obj
+    end)
+    collectgarbage("stop")
+    st, obj = xpcall(overflow, errorh, 0)
+    collectgarbage("restart")
+  end)
+  co()
+  assert(not st and obj[1] == 10 and flag[1] == 100)
+end
+
+
 if rawget(_G, "T") then
 
   -- memory error inside closing function
@@ -563,13 +597,13 @@ if rawget(_G, "T") then
 
   local function test ()
     local x <close> = enter(0)   -- set a memory limit
-    -- creation of previous upvalue will raise a memory error
-    assert(false)    -- should not run
+    local y = {}    -- raise a memory error
   end
 
   local _, msg = pcall(test)
   assert(msg == "not enough memory" and closemsg == "not enough memory")
 
+
   -- repeat test with extra closing upvalues
   local function test ()
     local xxx <close> = func2close(function (self, msg)
@@ -580,8 +614,7 @@ if rawget(_G, "T") then
       assert(msg == "not enough memory");
     end)
     local x <close> = enter(0)   -- set a memory limit
-    -- creation of previous upvalue will raise a memory error
-    os.exit(false)    -- should not run
+    local y = {}   -- raise a memory error
   end
 
   local _, msg = pcall(test)
@@ -607,7 +640,7 @@ if rawget(_G, "T") then
     -- concat this table needs two buffer resizes (one for each 's')
     local a = {s, s}
 
-    collectgarbage()
+    collectgarbage(); collectgarbage()
 
     m = T.totalmem()
     collectgarbage("stop")
@@ -630,7 +663,7 @@ if rawget(_G, "T") then
     -- second buffer was released by 'toclose'
     assert(T.totalmem() - m <= extra)
 
-    -- userdata, upvalue, buffer, buffer, final string
+    -- userdata, buffer, buffer, final string
     T.totalmem(m + 4*lim + extra)
     assert(#table.concat(a) == 2*lim)
 
@@ -753,8 +786,8 @@ do
   checktable({co()}, {true, 10, 20, 30})
   checktable(trace, {"nowX", "z1", "z2", "nowY", "y1", "y2", "x1", "x2"})
 
-end 
- 
+end
+
 
 do
   -- yielding inside closing metamethods after an error