Stop server on SIGTERM, SIGINT by default (#406)

* Stop server on SIGTERM by default

Fix #174

* Fix variable name

* Handle SIGTERM only inside Dream server

Don't install the signal handler at the module scope, to avoid polluting
the user's application with potentially unused handlers.

* Handle SIGTERM and SIGINT

* Update docs

---------

Co-authored-by: Sebastian Willenbrink <sebastian.willenbrink@tngtech.com>

Author: yawaramin

example/z-playground/server/playground.ml

@@ -479,12 +479,6 @@ let rec gc ?(initial = true) () =
 let () =
   Dream.log "Starting playground";
 
-  (* Stop when systemd sends SIGTERM. *)
-  let stop, signal_stop = Lwt.wait () in
-  Lwt_unix.on_signal Sys.sigterm (fun _signal ->
-    Lwt.wakeup_later signal_stop ())
-  |> ignore;
-
   (* Build the base image. *)
   Lwt_main.run begin
     Lwt_io.(with_file ~mode:Output "Dockerfile" (fun channel ->
@@ -520,7 +514,7 @@ let () =
     Dream.html (Client.html example)
   in
 
-  Dream.run ~interface:"0.0.0.0" ~port:80 ~stop ~adjust_terminal:false
+  Dream.run ~interface:"0.0.0.0" ~port:80 ~adjust_terminal:false
   @@ Dream.logger
   @@ Dream.router [
 

src/dream.mli

@@ -2130,7 +2130,7 @@ val run :
     - [~stop] is a promise that causes the server to stop accepting new
       requests, and {!Dream.run} to return. Requests that have already entered
       the Web application continue to be processed. The default value is a
-      promise that never resolves.
+      promise that resolves when [SIGTERM] or [SIGINT] is received.
     - [~error_handler] handles all errors, both from the application, and
       low-level errors. See {!section-errors} and example
       {{:https://github.com/camlworks/dream/tree/master/example/9-error#folders-and-files}

src/http/http.ml

@@ -673,7 +673,19 @@ let serve_with_maybe_https
 
 let default_interface = "localhost"
 let default_port = 8080
-let never = fst (Lwt.wait ())
+
+(* Lazy option default to avoid side effects. *)
+let option_or opt default = match opt with
+  | Some v -> v
+  | None -> default ()
+
+(* Lazy signal handler to avoid side effects. *)
+let on_termination_signal () =
+  let term_promise, term_resolve = Lwt.wait () in
+  let int_promise, int_resolve = Lwt.wait () in
+  ignore (Lwt_unix.on_signal Sys.sigterm (fun _ -> Lwt.wakeup_later term_resolve ()));
+  ignore (Lwt_unix.on_signal Sys.sigint (fun _ -> Lwt.wakeup_later int_resolve ()));
+  Lwt.pick [term_promise; int_promise]
 
 let network ~port ~socket_path =
   match socket_path with
@@ -684,7 +696,7 @@ let serve
     ?(interface = default_interface)
     ?(port = default_port)
     ?socket_path
-    ?(stop = never)
+    ?stop
     ?(error_handler = Error_handler.default)
     ?(tls = false)
     ?certificate_file
@@ -696,7 +708,7 @@ let serve
     "serve"
     ~interface
     ~network:(network ~port ~socket_path)
-    ~stop
+    ~stop:(option_or stop on_termination_signal)
     ~error_handler
     ~tls:(if tls then `OpenSSL else `No)
     ?certificate_file
@@ -712,7 +724,7 @@ let run
     ?(interface = default_interface)
     ?(port = default_port)
     ?socket_path
-    ?(stop = never)
+    ?stop
     ?(error_handler = Error_handler.default)
     ?(tls = false)
     ?certificate_file
@@ -759,7 +771,7 @@ let run
       "run"
       ~interface
       ~network:(network ~port ~socket_path)
-      ~stop
+      ~stop:(option_or stop on_termination_signal)
       ~error_handler
       ~tls:(if tls then `OpenSSL else `No)
       ?certificate_file ?key_file