Luasocket + nginx error - lua entry thread aborted: runtime error: attempt to yield across C-call boundary

Question

When I use the following script:

local smtp = require("socket.smtp")
local from = "from@host"
local rcpt = "rcpt@host"
local msg = {
  headers = {
    to = rcpt,
    subject = "Hi"
  },
  body = "Hello"
}
smtp.send{from = from,rcpt = rcpt,source = smtp.message(msg)}

I'm getting an error message: lua entry thread aborted: runtime error: attempt to yield across C-call boundary.

I'm using the newest luasocket installed from luarocks with Lua 5.1 using nginx compiled with LuaJIT 2.1. What is causing this error message and how do I fix it?

Answer 1

smtp.send uses LuaSocket's socket.protect function for handling internal errors. This function is implemented in C and doesn't allow yielding in the current releases (the version in git HEAD now allows yielding on Lua 5.2+, see discussion here). Apparently someone tries to yield from within it. In etc/dispatch.lua in the LuaSocket package (better use the git HEAD version) there is a replacement function for socket.protect that should allow yielding on all Lua versions (at the cost of an extra temporary coroutine). You can try replacing the C function with that Lua function like so:

local socket = require("socket")
local base = _G
-- paste modified socket.protect function here

-- continue with your own code:
local smtp = require("socket.smtp")
-- ...

Answer 2

This is caused by the combined use of LuaJIT and socket.smtp, which spins up a coroutine. From https://github.com/openresty/lua-nginx-module/issues/376:

@AterCattus This is a known limitation in LuaJIT (and the standard Lua 5.1 interpreter) that the require() builtin is currently implemented as a C builtin across which you cannot initiate a yield.

It looks like the best workaround may be to use this implementation of require.lua: https://github.com/pygy/require.lua. It's written in pure Lua, instead of C, to get around this problem with LuaJIT.

Answer 3

I've seen this message in a somewhat similar situation; in my case it was related to the design of ngx_lua, which implements its own coroutine scheduler. This means that sock:receive doesn't block, but instead does implicit yield to the scheduler and, as the result, if the call to sock:receive is made with a C function being on the stack, you are likely to get the error you see.

In my case I was making sock:receive call from a debug hook and was getting this error until I switched to using socket methods from my own version of luasocket that doesn't yield. I'd check if socket.smtp is using the "normal" version of luasocket.