[Libguestfs] [PATCH common 4/4] mltools: Allow waiting for killed PIDs

[Richard W.M. Jones]

Add a new, optional [?wait] parameter to On_exit.kill, allowing
programs to wait for a number of seconds for the subprocess to exit.
---
 mltools/on_exit.ml  | 30 +++++++++++++++++++++---------
 mltools/on_exit.mli | 14 ++++++++++++--
 2 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/mltools/on_exit.ml b/mltools/on_exit.ml
index e8353df..cdaa83c 100644
--- a/mltools/on_exit.ml
+++ b/mltools/on_exit.ml
@@ -24,10 +24,10 @@ open Unix
 open Printf
 
 type action =
-  | Unlink of string     (* filename *)
-  | Rm_rf of string      (* directory *)
-  | Kill of int * int    (* signal, pid *)
-  | Fn of (unit -> unit) (* generic function *)
+  | Unlink of string        (* filename *)
+  | Rm_rf of string         (* directory *)
+  | Kill of int * int * int (* wait, signal, pid *)
+  | Fn of (unit -> unit)    (* generic function *)
 
 (* List of (priority, action). *)
 let actions = ref []
@@ -35,18 +35,30 @@ let actions = ref []
 (* Perform a single exit action, printing any exception but
  * otherwise ignoring failures.
  *)
-let do_action action =
+let rec do_action action =
   try
     match action with
     | Unlink file -> Unix.unlink file
     | Rm_rf dir ->
        let cmd = sprintf "rm -rf %s" (Filename.quote dir) in
        ignore (Tools_utils.shell_command cmd)
-    | Kill (signal, pid) ->
-       kill pid signal
+    | Kill (wait, signal, pid) ->
+       do_kill ~wait ~signal ~pid
     | Fn f -> f ()
   with exn -> debug "%s" (Printexc.to_string exn)
 
+and do_kill ~wait ~signal ~pid =
+  kill pid signal;
+
+  let rec loop i =
+    if i > 0 then (
+      let pid', _ = waitpid [ WNOHANG ] pid in
+      if pid' = 0 then
+        loop (i-1)
+    )
+  in
+  loop wait
+
 (* Make sure the actions are performed only once. *)
 let done_actions = ref false
 
@@ -106,6 +118,6 @@ let rm_rf ?(prio = 1000) dir =
   register ();
   List.push_front (prio, Rm_rf dir) actions
 
-let kill ?(prio = 1000) ?(signal = Sys.sigterm) pid =
+let kill ?(prio = 1000) ?(wait = 0) ?(signal = Sys.sigterm) pid =
   register ();
-  List.push_front (prio, Kill (signal, pid)) actions
+  List.push_front (prio, Kill (wait, signal, pid)) actions
diff --git a/mltools/on_exit.mli b/mltools/on_exit.mli
index 910783e..dd35101 100644
--- a/mltools/on_exit.mli
+++ b/mltools/on_exit.mli
@@ -58,12 +58,22 @@ val unlink : ?prio:int -> string -> unit
 val rm_rf : ?prio:int -> string -> unit
 (** Recursively remove a temporary directory on exit (using [rm -rf]). *)
 
-val kill : ?prio:int -> ?signal:int -> int -> unit
+val kill : ?prio:int -> ?wait:int -> ?signal:int -> int -> unit
 (** Kill [PID] on exit.  The signal sent defaults to [Sys.sigterm].
 
     Use this with care since you can end up unintentionally killing
     another process if [PID] goes away or doesn't exist before the
-    program exits. *)
+    program exits.
+
+    The optional [?wait] flag attempts to wait for a specified
+    number of seconds for the subprocess to go away.  For example
+    using [~wait:5] will wait for up to 5 seconds.   Since this
+    runs when virt-v2v is exiting, it is best to keep waiting times
+    as short as possible.  Also there is no way to report errors
+    in the subprocess.  If reliable cleanup of a subprocess is
+    required then this is not the correct place to do it.
+
+    [?wait] defaults to [0] which means we do not try to wait. *)
 
 val register : unit -> unit
 (** Force this module to register its at_exit function and signal
-- 
2.37.0.rc2