Bug: Yielding in a hook stops in the wrong instruction

Yielding in a hook must decrease the program counter, because it already
counted an instruction that, in the end, was not executed. However,
that decrement should be done only when about to restart the thread.
Otherwise, inspecting the thread with the debug library shows it one
instruction behind of where it really is.

ldebug.c

@@ -925,12 +925,12 @@ int luaG_traceexec (lua_State *L, const Instruction *pc) {
   }
   pc++;  /* reference is always next instruction */
   ci->u.l.savedpc = pc;  /* save 'pc' */
-  counthook = (--L->hookcount == 0 && (mask & LUA_MASKCOUNT));
+  counthook = (mask & LUA_MASKCOUNT) && (--L->hookcount == 0);
   if (counthook)
     resethookcount(L);  /* reset count */
   else if (!(mask & LUA_MASKLINE))
     return 1;  /* no line hook and count != 0; nothing to be done now */
-  if (ci->callstatus & CIST_HOOKYIELD) {  /* called hook last time? */
+  if (ci->callstatus & CIST_HOOKYIELD) {  /* hook yielded last time? */
     ci->callstatus &= ~CIST_HOOKYIELD;  /* erase mark */
     return 1;  /* do not call hook again (VM yielded, so it did not move) */
   }
@@ -952,7 +952,6 @@ int luaG_traceexec (lua_State *L, const Instruction *pc) {
   if (L->status == LUA_YIELD) {  /* did hook yield? */
     if (counthook)
       L->hookcount = 1;  /* undo decrement to zero */
-    ci->u.l.savedpc--;  /* undo increment (resume will increment it again) */
     ci->callstatus |= CIST_HOOKYIELD;  /* mark that it yielded */
     luaD_throw(L, LUA_YIELD);
   }

ldo.c

@@ -792,6 +792,10 @@ static void resume (lua_State *L, void *ud) {
     lua_assert(L->status == LUA_YIELD);
     L->status = LUA_OK;  /* mark that it is running (again) */
     if (isLua(ci)) {  /* yielded inside a hook? */
+      /* undo increment made by 'luaG_traceexec': instruction was not
+         executed yet */
+      lua_assert(ci->callstatus & CIST_HOOKYIELD);
+      ci->u.l.savedpc--;
       L->top.p = firstArg;  /* discard arguments */
       luaV_execute(L, ci);  /* just continue running Lua code */
     }

testes/coroutine.lua

@@ -610,18 +610,20 @@ else
     -- (bug in 5.2/5.3)
     c = coroutine.create(function (a, ...)
       T.sethook("yield 0", "l")   -- will yield on next two lines
-      assert(a == 10)
+      local b = a
       return ...
     end)
 
     assert(coroutine.resume(c, 1, 2, 3))   -- start coroutine
     local n,v = debug.getlocal(c, 0, 1)    -- check its local
-    assert(n == "a" and v == 1)
+    assert(n == "a" and v == 1 and debug.getlocal(c, 0, 2) ~= "b")
     assert(debug.setlocal(c, 0, 1, 10))     -- test 'setlocal'
     local t = debug.getinfo(c, 0)        -- test 'getinfo'
-    assert(t.currentline == t.linedefined + 1)
+    assert(t.currentline == t.linedefined + 2)
     assert(not debug.getinfo(c, 1))      -- no other level
     assert(coroutine.resume(c))          -- run next line
+    local n,v = debug.getlocal(c, 0, 2)    -- check next local
+    assert(n == "b" and v == 10)
     v = {coroutine.resume(c)}         -- finish coroutine
     assert(v[1] == true and v[2] == 2 and v[3] == 3 and v[4] == undef)
     assert(not coroutine.resume(c))