Coroutines do not unwind the stack in case of errors

Back to how it was, a coroutine does not unwind its stack in case of
errors (and therefore do not close its to-be-closed variables). This
allows the stack to be examined after the error. The program can
use 'coroutine.kill' to close the variables.

The function created by 'coroutine.wrap', however, closes the
coroutine's variables in case of errors, as it is impossible to examine
the stack any way.

lcorolib.c

@@ -25,6 +25,10 @@ static lua_State *getco (lua_State *L) {
 }
 
 
+/*
+** Resumes a coroutine. Returns the number of results for non-error
+** cases or -1 for errors.
+*/
 static int auxresume (lua_State *L, lua_State *co, int narg) {
   int status, nres;
   if (!lua_checkstack(co, narg)) {
@@ -74,8 +78,14 @@ static int luaB_auxwrap (lua_State *L) {
   lua_State *co = lua_tothread(L, lua_upvalueindex(1));
   int r = auxresume(L, co, lua_gettop(L));
   if (r < 0) {
+    int stat = lua_status(co);
+    if (stat != LUA_OK && stat != LUA_YIELD) {
+      stat = lua_resetthread(co);  /* close variables in case of errors */
+      if (stat != LUA_OK)  /* error closing variables? */
+        lua_xmove(co, L, 1);  /* get new error object */
+    }
     if (lua_type(L, -1) == LUA_TSTRING) {  /* error object is a string? */
-      luaL_where(L, 1);  /* add extra info */
+      luaL_where(L, 1);  /* add extra info, if available */
       lua_insert(L, -2);
       lua_concat(L, 2);
     }

ldo.c

@@ -686,10 +686,8 @@ LUA_API int lua_resume (lua_State *L, lua_State *from, int nargs,
   if (likely(!errorstatus(status)))
     lua_assert(status == L->status);  /* normal end or yield */
   else {  /* unrecoverable error */
-    status = luaF_close(L, L->stack, status);  /* close all upvalues */
     L->status = cast_byte(status);  /* mark thread as 'dead' */
-    luaD_seterrorobj(L, status, L->stack + 1);  /* push error message */
-    L->ci = &L->base_ci;  /* back to the original C level */
+    luaD_seterrorobj(L, status, L->top);  /* push error message */
     L->ci->top = L->top;
   }
   *nresults = (status == LUA_YIELD) ? L->ci->u2.nyield

manual/manual.of

@@ -286,7 +286,7 @@ Lua also offers a system of @emph{warnings} @seeF{warn}.
 Unlike errors, warnings do not interfere
 in any way with program execution.
 They typically only generate a message to the user,
-although this behavior can be adapted from C @see{lua_setwarnf}.
+although this behavior can be adapted from C @seeC{lua_setwarnf}.
 
 }
 
@@ -835,6 +835,9 @@ In case of normal termination,
 plus any values returned by the coroutine main function.
 In case of errors, @Lid{coroutine.resume} returns @false
 plus the error object.
+In this case, the coroutine does not unwind its stack,
+so that it is possible to inspect it after the error
+with the debug API.
 
 A coroutine yields by calling @Lid{coroutine.yield}.
 When a coroutine yields,
@@ -858,8 +861,10 @@ go as extra arguments to @Lid{coroutine.resume}.
 @Lid{coroutine.wrap} returns all the values returned by @Lid{coroutine.resume},
 except the first one (the boolean error code).
 Unlike @Lid{coroutine.resume},
-@Lid{coroutine.wrap} does not catch errors;
-any error is propagated to the caller.
+the function created by @Lid{coroutine.wrap}
+propagates any error to the caller.
+In this case,
+the function also kills the coroutine @seeF{coroutine.kill}.
 
 As an example of how coroutines work,
 consider the following code:
@@ -1534,8 +1539,15 @@ the other pending closing methods will still be called.
 If a coroutine yields inside a block and is never resumed again,
 the variables visible at that block will never go out of scope,
 and therefore they will not be closed.
-(You should use finalizers to handle this case,
-or else call @Lid{coroutine.kill} to close the variables.)
+Similarly, if a coroutine ends with an error,
+it does not unwind its stack,
+so it does not close any variable.
+You should either use finalizers
+or call @Lid{coroutine.kill} to close the variables in these cases.
+However, note that if the coroutine was created
+through @Lid{coroutine.wrap},
+then its corresponding function will close all variables
+in case of errors.
 
 }
 
@@ -6406,11 +6418,12 @@ or if it has stopped with an error.
 Creates a new coroutine, with body @id{f};
 @id{f} must be a function.
 Returns a function that resumes the coroutine each time it is called.
-Any arguments passed to the function behave as the
+Any arguments passed to this function behave as the
 extra arguments to @id{resume}.
-Returns the same values returned by @id{resume},
+The function returns the same values returned by @id{resume},
 except the first boolean.
-In case of error, propagates the error.
+In case of error,
+the function kills the coroutine and propagates the error.
 
 }
 
@@ -6668,6 +6681,10 @@ the file path where the module was found,
 as returned by @Lid{package.searchpath}.
 The first searcher always returns the string @St{:preload:}.
 
+Searchers should raise no errors and have no side effects in Lua.
+(They may have side effects in C,
+for instance by linking the application with a library.)
+
 }
 
 @LibEntry{package.searchpath (name, path [, sep [, rep]])|

testes/coroutine.lua

@@ -346,9 +346,13 @@ do
   local st, res = coroutine.resume(B)
   assert(st == true and res == false)
 
-  A = coroutine.wrap(function() return pcall(A, 1) end)
+  local X = false
+  A = coroutine.wrap(function()
+    local *toclose _ = setmetatable({}, {__close = function () X = true end})
+    return pcall(A, 1)
+  end)
   st, res = A()
-  assert(not st and string.find(res, "non%-suspended"))
+  assert(not st and string.find(res, "non%-suspended") and X == true)
 end
 
 

testes/db.lua

@@ -734,18 +734,24 @@ a, b = coroutine.resume(co, 100)
 assert(a and b == 30)
 
 
--- check traceback of suspended coroutines
+-- check traceback of suspended (or dead with error) coroutines
+
+function f(i)
+  if i == 0 then error(i)
+  else coroutine.yield(); f(i-1)
+  end
+end
 
-function f(i) coroutine.yield(i == 0); f(i - 1) end
 
 co = coroutine.create(function (x) f(x) end)
 a, b = coroutine.resume(co, 3)
 t = {"'coroutine.yield'", "'f'", "in function <"}
-repeat
+while coroutine.status(co) == "suspended" do
   checktraceback(co, t)
   a, b = coroutine.resume(co)
   table.insert(t, 2, "'f'")   -- one more recursive call to 'f'
-until b
+end
+t[1] = "'error'"
 checktraceback(co, t)
 
 

testes/locals.lua

@@ -417,12 +417,13 @@ if rawget(_G, "T") then
 end
 
 
--- to-be-closed variables in coroutines
+print "to-be-closed variables in coroutines"
+
 do
-  -- an error in a coroutine closes variables
+  -- an error in a wrapped coroutine closes variables
   local x = false
   local y = false
-  local co = coroutine.create(function ()
+  local co = coroutine.wrap(function ()
     local *toclose xv = func2close(function () x = true end)
     do
       local *toclose yv = func2close(function () y = true end)
@@ -432,14 +433,31 @@ do
     error(23)              -- error does
   end)
 
-  local a, b = coroutine.resume(co)
-  assert(a and b == 100 and not x and not y)
-  a, b = coroutine.resume(co)
-  assert(a and b == 200 and not x and y)
-  a, b = coroutine.resume(co)
+  local b = co()
+  assert(b == 100 and not x and not y)
+  b = co()
+  assert(b == 200 and not x and y)
+  local a, b = pcall(co)
   assert(not a and b == 23 and x and y)
 end
 
+
+do
+  -- error in a wrapped coroutine raising errors when closing a variable
+  local x = false
+  local co = coroutine.wrap(function ()
+    local *toclose xv = func2close(function () error("XXX") end)
+      coroutine.yield(100)
+      error(200)
+  end)
+  assert(co() == 100)
+  local st, msg = pcall(co)
+print(msg)
+  -- should get last error raised
+  assert(not st and string.find(msg, "%w+%.%w+:%d+: XXX"))
+end
+
+
 -- a suspended coroutine should not close its variables when collected
 local co
 co = coroutine.wrap(function()
@@ -449,6 +467,7 @@ co = coroutine.wrap(function()
 end)
 co()                 -- start coroutine
 assert(co == nil)    -- eventually it will be collected
+collectgarbage()
 
 
 -- to-be-closed variables in generic for loops