Signal handler

[scossu]

Hello,

I am trying to catch a SIGTERM or SIGINT signal to perform some clean-up tasks before shutting down the server.

Using one of your examples:

-- requires luaposix to be installed

local pegasus = require 'pegasus'
local signal = require "posix.signal"

local pidfile

local function shutdown()
  io.write("Shutting down server...\n")
  -- delete PID file here
  os.exit()
end

signal.signal(signal.SIGINT, shutdown)
signal.signal(signal.SIGTERM, shutdown)

local server = pegasus:new({
  port = '9090',
})

local printTable = function(table)
  for k, v in pairs(table) do
    print(k, '=', v)
  end
end

-- create PID file here 

server:start(function (req, res)
  printTable(req['querystring'])
  res:addHeader('Content-Type', 'text/html'):write('hello pegasus world!')

  -- return a truthy value to indicate the request was handled, no further handling needed
  return res:close()
end)

If I hit CTRL-C or send SIGTERM to the process, nothing happens. Removing the signal handlers, Pegasus shuts down as expected (hitting CTRL-C twice).

Is there a way to catch termination signals and add a handler that runs just before shutdown (ideally also capable of aborting shutdown if instructed to do so)?

If there isn't any, and it seems a valuable addition to the library, I'd be glad to send a PR if I can get some hints about the best place to add it. Thanks.

[scossu]

Never mind. I had an error in my shutdown function that is not reported, so the function would exit quietly. Without the error, the signal handler works.

[scossu]

Disregard my previous comment — this is still an issue, the shutdown function is never reached.

[Tieske]

https://luaposix.github.io/luaposix/modules/posix.signal.html states:

Also note that installed signal handlers are not called immediatly upon occurrence of a signal. Instead, in order to keep the interperter state clean, they are executed in the context of a debug hook which is called as soon as the interpreter enters a new function, returns from the currently executing function, or after the execution of the current instruction has ended.

so I think what is happening is that your server is waiting in a LuaSocket call for an incoming connection. Whilst it does that (on the C-side of things), no Lua code runs and no debug-hook will execute.

You should be able to test, by hitting ctrl-c, and then ensuring to hit the server by requesting a page.

[scossu]

I tested and it works as you describe it. However it's kind of a nonstandard way to terminate a program (a delay may be expected for SIGTERM, but a specific user input is not).

How does the current shutdown procedure work, and is there a straightforward way to hook into that for additional pre-termination tasks?

[Tieske]

depends on what loop you use. The default Pegasus one; https://github.com/EvandroLG/pegasus.lua/blob/master/src/pegasus/init.lua#L113-L122 does a blocking wait on server:accept. If you set a timeout of 0.1 second, and catch the timeout error and let it retry, then that should be enough.
The Lua engine will execute Lua code every 0.1 seconds (the failure+retry), which should then allow the debug hook set by posix.signal to run.

[scossu]

catch the timeout error and let it retry

Can you give me more details about that? I assume I'd be using a pcall, but from the luasocket documentation it's not clear where I would implement that and what a timeout error looks like. Thanks.

[Tieske]

untested, but you get the idea;

  server:settimeout(0.1)  -- configure server socket timeout
  while 1 do
    local client, errmsg = server:accept()

    if client then
      client:settimeout(self.timeout, 'b')
      handler:processRequest(self.port, client, server)
    elseif errmsg ~= "timeout" then  -- timeouts are expected, anything else not
      handler.log:error('Failed to accept connection: %s', errmsg)
    end
  end

(no need for a pcall, since it returns "timeout" as the error message)