runtime error: attempt to yield across C-call boundary due to lua-resty-redis connect() invocation

[un-def]

lapis 1.5.0-2
lapis-redis dev-1
openresty 1.9.15.1

nginx.conf:

location / {
  default_type text/html;
  content_by_lua_block {
    require('lapis').serve('app')
  }
}

config.lua

config({'development', 'production'}, {
  redis = {
    host = '127.0.0.1',
    port = 6379,
  }
})

app.lua:

local redis = require('lapis.redis').get_redis()

exception:

[error] 655#0: *8 lua entry thread aborted: runtime error: attempt to yield across C-call boundary
stack traceback:
coroutine 0:
        [C]: in function 'require'
        /usr/local/openresty/luajit/share/lua/5.1/lapis/init.lua:15: in function 'serve'
        content_by_lua(nginx.conf.compiled:30):2: in function <content_by_lua(nginx.conf.compiled:30):1>, client: 127.0.0.1, server: , request: "GET / HTTP/1.1", host: "127.0.0.1:9000"

I found this issue, but I don't understand how this error can be fixed.

[un-def]

Hmm, I've solved this issue with tricky workaround:

redis_wrapper.lua

local redis_wrapper_meta = {
  __index = function(self, method)
    if not rawget(self, 'redis_obj') then
      local redis_obj, err = require('lapis.redis').get_redis()
      if not redis_obj then
        error('lapis.redis error: ' .. err)
      end
      local config = require('lapis.config').get()
      if config.redis.db then
        redis_obj:select(config.redis.db)
      end
      self.redis_obj = redis_obj
    end
    return self.redis_obj[method]
  end
}

return setmetatable({}, redis_wrapper_meta)

app.lua

local redis = require('redis_wrapper')
redis:set('foo', 'bar')

I think I can implement same lazy connect trick in lapis-redis module with optional get_redis argument to prevent connect() call while application module is imported in lapis.init.serve() (https://github.com/leafo/lapis/blob/a1fd7d854bd8d859b02e0af32387b85cb8023e00/lapis/init.moon#L17).
require('lapis.redis').get_redis() — get connected redis object (throw errors immediately)
require('lapis.redis').get_redis(false) — get redis object, but connect only when it's needed (e.g., when get/set method is called)

In addition, It would be great to be able to specify db number in config along with host/port.

What do you think about these ideas?

[leafo]

You have to call get_redis within a request, not on the initialization of an app. The redis connection is managed by nginx's socket pooling so it needs to be handed back to nginx after the request finishes. Lapis will do this automatically if you call get_redis anywhere in the body of a action or before filter.

You can't have a global redis connection per worker because workers can be processing multiple http requests at once when they yield due to asynchronous operations.

[un-def]

Thanks for clarifying! Now I realized that the redis object (as a “singleton”) instantiating mechanism is already implemented in get_redis and multiple get_redis calls don't cause severe overhead.