Reuse cosocket after close

resty-cosocket-reuse.lua

local pl_pretty = require("pl.pretty").write

-- create cosocket
local sock = ngx.socket.tcp()

sock:settimeouts(1000, 15000, 15000)

ngx.say("sock 1 = ", pl_pretty(sock))

-- create kernel socket
local ok, err = sock:connect("8.8.8.8", 53)
if not ok then
   ngx.say("connection err: ", err)
end

ngx.say("sock 2 = ", pl_pretty(sock))

-- create a new kernel socket
ok, err = sock:connect("1.1.1.1", 53)
if not ok then
   ngx.say("connection err: ", err)
end

ngx.say("sock 3 = ", pl_pretty(sock))

-- ss -npeat dst "8.8.8.8" or dst "1.1.1.1"
-- ll /proc/<pid>/fd
ngx.sleep(30)

-- only discard kernel socket
ngx.say("discard kernel sockets")
sock:close()

-- cosocket remains
ngx.say("sock 4 = ", pl_pretty(sock))

-- ss -npeat dst "8.8.8.8" or dst "1.1.1.1"
ngx.sleep(60)

1. close() only discard kernel socket (TCP connection) but preserve cosocket itself.
2. setkeepalive preserve kernel socket (TCP connection) as well.