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assertion hit after attaching dead coroutine #145

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8d6c5fe
cqueues:wrap: Remember to checkstack
daurnimator Sep 6, 2016
b8041a6
src/cqueues: Detect if trying to resume a running or dead coroutine
daurnimator Sep 6, 2016
80aef12
src/cqueues.c: Use != 0 instead of > 0
daurnimator Sep 9, 2016
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20 changes: 20 additions & 0 deletions regress/145-assertion-on-dead.lua
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Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
#!/bin/sh
_=[[
. "${0%%/*}/regress.sh"
exec runlua "$0" "$@"
]]
require"regress".export".*"

local co = coroutine.create(function()
coroutine.yield()
end)
coroutine.resume(co) -- kick off coroutine
coroutine.resume(co) -- resume a yield with no arguments
-- co is now dead

local cq = require"cqueues".new()
cq:attach(co)
check(cq:step())

say"OK"

23 changes: 17 additions & 6 deletions src/cqueues.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1982,10 +1982,22 @@ static cqs_status_t cqueue_resume(lua_State *L, struct cqueue *Q, struct callinf
}
} else {
nargs = lua_gettop(T->L);
if (status != LUA_YIELD) {
if (status == LUA_OK && lua_getstack(T->L, 0, &(lua_Debug){}) > 0) {

@daurnimator daurnimator Sep 6, 2016

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This little bit was adapted from the coroutine.status implementation: http://www.lua.org/source/5.3/lcorolib.c.html#luaB_costatus

@wahern wahern Sep 8, 2016

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I think the test should be status == LUA_OK && lua_getstack(...) <= 0, or even better status == LUA_OK && lua_getstack(...) != 1 IIUC, nargs is decremented because in the initial state we want to exclude the function that will be called from the number of arguments provided to lua_resume.

Alternatively, just remove the assertion and don't decrement nargs if it's 0. If we've been given a dead coroutine or a running coroutine then lua_resume() will return a descriptive error, right?

@daurnimator daurnimator Sep 9, 2016

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I think the test should be status == LUA_OK && lua_getstack(...) <= 0, or even better status == LUA_OK && lua_getstack(...) != 1

Why? does >0 vs <= 0 matter?
==> What I have is the same as what the lua coroutine library has.

This branch is what to do if the coroutine is already running; e.g.:

local cq = cqueues.new()
local co = coroutine.create(function()
    print(cq:step())
end)
cq:attach(co)
coroutine.resume(co)

If we've been given a dead coroutine or a running coroutine then lua_resume() will return a descriptive error, right?

A running coroutine, yes.
However, a dead coroutine is not handled well. Note that lua itself checks for this: http://www.lua.org/source/5.3/lcorolib.c.html#auxresume

@wahern wahern Sep 9, 2016

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Why? does >0 vs <= 0 matter?
==> What I have is the same as what the lua coroutine library has.

The original code was assuming that if status != LUA_YIELD then it must be a newly created thread. That's why it was decrementing nargs--to exclude the function at the bottom of the stack (on the first invocation, lua_resume is like lua_pcall). lua_getstack returns 1 if the thread is running. We want to know whether the thread is in the initial state. If you look at the code in lcorolib.c, the non-yielding, suspended condition is only reached if status == LUA_OK && lua_getstack() <= 0. But using a condition of != 1 better matches the official documentation for lua_getstack.

/* already running */
lua_pushliteral(L, "cannot resume non-suspended coroutine");
I->error.value = lua_gettop(L);
err_setthread(L, I, T);
goto defunct;
} else if ((status != LUA_OK && status != LUA_YIELD) || nargs == 0) {
/* dead coroutine */
lua_pushliteral(L, "cannot resume dead coroutine");
I->error.value = lua_gettop(L);
err_setthread(L, I, T);
goto defunct;
} else if (status == LUA_OK) {
/* initial */
nargs -= 1;
assert(nargs >= 0);
}
} /* else normal yield */
}

timer_del(Q, &T->timer);
Expand Down Expand Up @@ -2294,9 +2306,8 @@ static int cqueue_wrap(lua_State *L) {
luaL_checktype(L, 2, LUA_TFUNCTION);

newL = lua_newthread(L);
for (i = 2; i <= top; i++) {
lua_pushvalue(L, i);
}
lua_insert(L, 2);
luaL_checkstack(newL, top - 1, "too many arguments");
lua_xmove(L, newL, top - 1);

thread_add(L, Q, &I, -1);
Expand Down