worker process suffers CPU 100% usage

Hello!

The worker process CPU usage could be 100% if we use the following configuration pattern:

stream {
    server {
        listen 3105 reuseport;

        lua_check_client_abort on; # must be enabled

        content_by_lua_block {
            local sock, err = ngx.req.socket(true)

            sock:settimeout(1800 * 1000)

            local readline = sock:receiveuntil("\n")

            while not ngx.worker.exiting() do
                local line, err = readline(1)
                ngx.log(ngx.ERR, "line: ", line)
                if not line then
                    if err then
                        ngx.log(ngx.ERR, err)
                        break
                    end
                else
                    ngx.sleep(5000) -- other yieldable API is also applicable.
                end
            end
        }
    }
 }

Building a connection through nc and sending some characters then closing it, in the meanwhile, observing the CPU usage by top.

$ nc 127.0.0.1 3105
asjdkl
zxckj
^C

Now you can find the CPU usage reaches 100%.

It's irrelevant with the version of ngx_stream_lua, you can reproduce this problem even with the newest OpenResty.

After investigating the internal of stream-lua-nginx-module, I found it was caused by the indefinitely trigger of ngx_stram_lua_check_broken_connection, which doesn't the check ev->pending_eof flag.

BTW, this issue was found by @tianchaijz.

Start at ngx_stram_lua_check_broken_connection and trace the worker event handling involved in the provided stream configuration. Reproduce the issue with the nginx configuration and nc commands, then verify that closing the connection no longer causes 100% worker CPU usage.