Use main thread of current Lua state for callbacks, when known

src/luv.c

@@ -835,6 +835,15 @@ static int loop_gc(lua_State *L) {
 }
 
 LUALIB_API int luaopen_luv (lua_State* L) {
+#ifdef LUA_RIDX_MAINTHREAD
+  // Lua 5.2+ - resolve the main thread of the current Lua state, even if
+  // we were loaded from a different thread (which may become suspended/dead).
+  lua_geti(L, LUA_REGISTRYINDEX, LUA_RIDX_MAINTHREAD);
+  lua_State* ctxL = lua_tothread(L, -1);
+  lua_pop(L, 1);
+#else
+  lua_State* ctxL = L;
+#endif
   luv_ctx_t* ctx = luv_context(L);
 
   luaL_newlib(L, luv_functions);
@@ -862,7 +871,7 @@ LUALIB_API int luaopen_luv (lua_State* L) {
     lua_rawset(L, -3);
 
     ctx->loop = loop;
-    ctx->L = L;
+    ctx->L = ctxL;
     ctx->mode = -1;
 
     ret = uv_loop_init(loop);

tests/test-coroutines-require.lua

@@ -0,0 +1,51 @@
+-- This is a standalone test because it specifically requires luv to
+-- be initially require()d from a thread/coroutine.
+-- Test for issue #503 (PR #734).
+
+local thread = coroutine.create(function ()
+	local uv = require "luv"
+	coroutine.yield();
+end);
+
+-- Resume (start) thread, which will load luv
+-- and then it will yield
+coroutine.resume(thread);
+
+-- thread where luv was initially loaded is now suspended
+
+return require('lib/tap')(function (test)
+
+  if _VERSION == "Lua 5.1" then
+    -- Lua 5.1 and LuaJIT do not provide an API to determine the main
+    -- thread. Therefore it is inherently unsafe to require("luv") in
+    -- a coroutine.
+    test("callback should be in main thread", function(print,p,expect,uv)
+      print("Skipping! This test is expected to fail on Lua 5.1 and LuaJIT.");
+    end);
+  else
+    test("callback should be in main thread", function(print,p,expect,uv)
+        -- Now, in the main thread, load luv and create a timer
+        local t = uv.new_timer();
+        t:start(200, 0, expect(function ()
+          -- If luv calls this callback in the non-main (suspended) thread,
+          -- it violates a requirement specified in the Lua manual to only
+          -- call functions on active threads.
+
+          local our_thread, is_main = coroutine.running();
+
+          -- Basic assertion that we are not running in the suspended thread
+          assert(our_thread ~= thread)
+
+          -- How coroutine.running() reports "main thread" varies between
+          -- different Lua versions. This should handle them all.
+          assert(our_thread == nil or is_main == true)
+
+          -- If we were called in the wrong thread, this may segfault:
+          -- coroutine.resume(thread)
+          t:close();
+        end));
+        uv.run("default");
+    end);
+  end
+end);
+