subrequest cycle problem

[rubyist1982]

nginx version: openresty/1.19.9.1
ngx_lua-0.10.20

-- my nginx.conf
worker_processes 1;
user root;
pcre_jit on;

events {
worker_connections 1024;
}

http {
lua_package_path '$prefix/?.lua;$prefix/resty_modules/lualib/?.lua;;';
lua_code_cache off;
default_type text/plain;
keepalive_timeout 65;

server {
    listen 8080;
    location / {
        content_by_lua_file lua/not_found.lua;
    }

    location = /main {
        content_by_lua_file lua/main.lua;
    }

    location = /sub {
        content_by_lua_file lua/sub.lua;
    }

}

}

-- main.lua
local res = ngx.location.capture("/sub")
ngx.say(res.body)
ngx.say("main")

-- sub.lua
ngx.say("sub")

-- question
with lua_code_cache off;
when first issue curl /sub then curl /main
just got subrequest cycle error as follow

stack traceback:
coroutine 0:
[C]: in function 'capture'
/root/study/test/lua/main.lua:1: in main chunk, client: 127.0.0.1, server: , request: "GET /main HTTP/1.1", subrequest: "/sub", host: "localhost:8080"
2022/04/19 01:21:07 [error] 25279#25279: *10 lua subrequests cycle while processing "/sub", client: 127.0.0.1, server: , request: "GET /main HTTP/1.1", subrequest: "/sub", host: "localhost:8080"
2022/04/19 01:21:07 [error] 25279#25279: *10 lua entry thread aborted: runtime error: /root/study/test/lua/main.lua:1: failed to issue subrequest: -1
stack traceback:
coroutine 0:
[C]: in function 'capture'
/root/study/test/lua/main.lua:1: in main chunk, client: 127.0.0.1, server: , request: "GET /main HTTP/1.1", subrequest: "/sub", host: "localhost:8080"

and curl /main just print 51 line "mian" without any "sub"
is this intended ?
and i couldn't figure it out

[zhuizhuhaomeng]

-- sub.lua
local res = ngx.location.capture("/sub")
ngx.say(res.body)
ngx.say("main")

It it a typo? ngx.say("main") should be ngx.say("sub")

[rubyist1982]

-- sub.lua
local res = ngx.location.capture("/sub")
ngx.say(res.body)
ngx.say("main")

It it a typo? ngx.say("main") should be ngx.say("sub")

sorry for paste error. sub.lua just one line code as follow,
-- sub.lua
ngx.say("sub")

[zhuizhuhaomeng]

Do you still have doubts?
you also remove the ngx.capture from sub.lua, is that expected?

-- sub.lua
ngx.say("sub")

[rubyist1982]

Do you still have doubts? you also remove the ngx.capture from sub.lua, is that expected?

-- sub.lua
ngx.say("sub")

yes it is just that simple. and when lua_code_cache off; it do produce subrquests cycle problem.

[gavin0616]

It seems that this issue is caused by the same value of content_src_ref being shared between the main request and the subrequest.
The /sub location retrieves an incorrect code chunk (the main request's code chunk) due to the same content_src_ref.

In this case:

1. Request /sub

• Before:
  /sub content_src_ref: LUA_REFNIL
• After:
  /sub content_src_ref: 1

2. Request /main

• Before:
  /main content_src_ref: LUA_REFNIL
/sub content_src_ref: 1
• Execute main.lua:
  1. Push the /main code chunk to the top of the Lua stack
  2. Set /main content_src_ref: 1
  3. Call /sub via ngx.location.capture("/sub")
     This attempts to use /sub's content_src_ref to retrieve its code chunk. However, since /sub's content_src_ref is the same as /main's content_src_ref, and /sub is a subrequest issued by /main(sharing the same Lua VM with /main), the lookup mistakenly retrieves /main's code chunk instead of actually reading the /sub.lua file. This causes /main's code chunk to be executed recursively until it eventually results in an error.

The ngx.location.capture function (which creates a subrequest via Lua) can be executed during the rewrite_by_lua*, access_by_lua*, and content_by_lua* phases. Note that the access_by_lua* phase is skipped for subrequests. Therefore, this issue may currently occur in both the rewrite_by_lua_file and content_by_lua_file phases. However, for code consistency and future maintainability, I believe that when code_cache is disabled, it would be better to set *_src_ref to LUA_REFNIL before calling the ngx_http_lua_cache_load_code function.

[gavin0616]

@zhuizhuhaomeng, could you please provide some feedback when you have time, at your convenience?