[PATCH v4 10/11] tools/ocaml/xenstored: validate config file before live update

[Edwin Török <edvin.torok@xxxxxxxxxx>]

The configuration file can contain typos or various errors that could prevent
live update from succeeding (e.g. a flag only valid on a different version).
Unknown entries in the config file would be ignored on startup normally,
add a strict --config-test that live-update can use to check that the config 
file
is valid *for the new binary*.

For compatibility with running old code during live update recognize
--live --help as an equivalent to --config-test.

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---
Changes since v2:
* repost of lost patch from 2021: 
https://patchwork.kernel.org/project/xen-devel/patch/a53934dfa8ef984bffa858cc573cc7a6445bbdc0.1620755942.git.edvin.torok@xxxxxxxxxx/
---
 tools/ocaml/xenstored/parse_arg.ml | 26 ++++++++++++++++++++++++++
 tools/ocaml/xenstored/xenstored.ml | 11 +++++++++--
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/tools/ocaml/xenstored/parse_arg.ml 
b/tools/ocaml/xenstored/parse_arg.ml
index 1a85b14ef5..b159b91f00 100644
--- a/tools/ocaml/xenstored/parse_arg.ml
+++ b/tools/ocaml/xenstored/parse_arg.ml
@@ -26,8 +26,14 @@ type config =
     restart: bool;
     live_reload: bool;
     disable_socket: bool;
+    config_test: bool;
   }
 
+let get_config_filename config_file =
+  match config_file with
+  | Some name -> name
+  | None      -> Define.default_config_dir ^ "/oxenstored.conf"
+
 let do_argv =
   let pidfile = ref "" and tracefile = ref "" (* old xenstored compatibility *)
   and domain_init = ref true
@@ -38,6 +44,8 @@ let do_argv =
   and restart = ref false
   and live_reload = ref false
   and disable_socket = ref false
+  and config_test = ref false
+  and help = ref false
   in
 
   let speclist =
@@ -55,10 +63,27 @@ let do_argv =
       ("-T", Arg.Set_string tracefile, ""); (* for compatibility *)
       ("--restart", Arg.Set restart, "Read database on starting");
       ("--live", Arg.Set live_reload, "Read live dump on startup");
+      ("--config-test", Arg.Set config_test, "Test validity of config file");
       ("--disable-socket", Arg.Unit (fun () -> disable_socket := true), 
"Disable socket");
+      ("--help", Arg.Set help, "Display this list of options")
     ] in
   let usage_msg = "usage : xenstored [--config-file <filename>] 
[--no-domain-init] [--help] [--no-fork] [--reraise-top-level] [--restart] 
[--disable-socket]" in
   Arg.parse speclist (fun _ -> ()) usage_msg;
+  let () =
+    if !help then begin
+      if !live_reload then
+        (*
+          Transform --live --help into --config-test for backward compat with
+          running code during live update.
+          Caller will validate config and exit
+        *)
+        config_test := true
+      else begin
+        Arg.usage_string speclist usage_msg |> print_endline;
+        exit 0
+      end
+    end
+  in
   {
     domain_init = !domain_init;
     activate_access_log = !activate_access_log;
@@ -70,4 +95,5 @@ let do_argv =
     restart = !restart;
     live_reload = !live_reload;
     disable_socket = !disable_socket;
+    config_test = !config_test;
   }
diff --git a/tools/ocaml/xenstored/xenstored.ml 
b/tools/ocaml/xenstored/xenstored.ml
index 366437b396..1aaa3e995e 100644
--- a/tools/ocaml/xenstored/xenstored.ml
+++ b/tools/ocaml/xenstored/xenstored.ml
@@ -88,7 +88,7 @@ let default_pidfile = Paths.xen_run_dir ^ "/xenstored.pid"
 
 let ring_scan_interval = ref 20
 
-let parse_config filename =
+let parse_config ?(strict=false) filename =
   let pidfile = ref default_pidfile in
   let options = [
     ("merge-activate", Config.Set_bool Transaction.do_coalesce);
@@ -129,11 +129,12 @@ let parse_config filename =
     ("xenstored-port", Config.Set_string Domains.xenstored_port); ] in
   begin try Config.read filename options (fun _ _ -> raise Not_found)
     with
-    | Config.Error err -> List.iter (fun (k, e) ->
+    | Config.Error err as e -> List.iter (fun (k, e) ->
         match e with
         | "unknown key" -> eprintf "config: unknown key %s\n" k
         | _             -> eprintf "config: %s: %s\n" k e
       ) err;
+      if strict then raise e
     | Sys_error m -> eprintf "error: config: %s\n" m;
   end;
   !pidfile
@@ -358,6 +359,12 @@ let tweak_gc () =
 let () =
   Printexc.set_uncaught_exception_handler Logging.fallback_exception_handler;
   let cf = do_argv in
+  if cf.config_test then begin
+    let path = config_filename cf in
+    let _pidfile:string = parse_config ~strict:true path in
+    Printf.printf "Configuration valid at %s\n%!" path;
+    exit 0
+  end;
   let pidfile =
     if Sys.file_exists (config_filename cf) then
       parse_config (config_filename cf)
-- 
2.34.1