From bf10593a3a912cd3cac69569c7474e687c0d0cd8 Mon Sep 17 00:00:00 2001
From: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Mon, 29 Mar 2021 12:57:32 -0300
Subject: Allow yields inside '__pairs'

---
 lbaselib.c         |  7 ++++++-
 testes/nextvar.lua | 21 +++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/lbaselib.c b/lbaselib.c
index 83ad306d..fd6687e6 100644
--- a/lbaselib.c
+++ b/lbaselib.c
@@ -261,6 +261,11 @@ static int luaB_next (lua_State *L) {
 }
 
 
+static int pairscont (lua_State *L, int status, lua_KContext k) {
+  (void)L; (void)status; (void)k;  /* unused */
+  return 3;
+}
+
 static int luaB_pairs (lua_State *L) {
   luaL_checkany(L, 1);
   if (luaL_getmetafield(L, 1, "__pairs") == LUA_TNIL) {  /* no metamethod? */
@@ -270,7 +275,7 @@ static int luaB_pairs (lua_State *L) {
   }
   else {
     lua_pushvalue(L, 1);  /* argument 'self' to metamethod */
-    lua_call(L, 1, 3);  /* get 3 values from metamethod */
+    lua_callk(L, 1, 3, 0, pairscont);  /* get 3 values from metamethod */
   }
   return 3;
 }
diff --git a/testes/nextvar.lua b/testes/nextvar.lua
index 29cb05d5..076f6361 100644
--- a/testes/nextvar.lua
+++ b/testes/nextvar.lua
@@ -764,4 +764,25 @@ for k,v in ipairs(a) do
 end
 assert(i == a.n)
 
+
+-- testing yield inside __pairs
+do
+  local t = setmetatable({10, 20, 30}, {__pairs = function (t)
+    local inc = coroutine.yield()
+    return function (t, i)
+             if i > 1 then return i - inc, t[i - inc]  else return nil end
+           end, t, #t + 1
+  end})
+
+  local res = {}
+  local co = coroutine.wrap(function ()
+    for i,p in pairs(t) do res[#res + 1] = p end
+  end)
+
+  co()     -- start coroutine
+  co(1)    -- continue after yield
+  assert(res[1] == 30 and res[2] == 20 and res[3] == 10 and #res == 3)
+  
+end
+
 print"OK"
-- 
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