From 40c05cc7e3ffe60e8c717f465c12c05a20f8a5b1 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Wed, 24 Mar 2021 18:44:41 -0400
Subject: [PATCH] wip: feat(pool): keep one idle thread

see #360; in combination with max_size=1 it means the pool contains
exactly one thread.
---
 src/threads/CCPool.ml | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/src/threads/CCPool.ml b/src/threads/CCPool.ml
index 59dffa92..2f2ca6f3 100644
--- a/src/threads/CCPool.ml
+++ b/src/threads/CCPool.ml
@@ -79,17 +79,24 @@ module Make(P : PARAM) = struct
   (* thread: seek what to do next (including dying).
      Assumes the pool is locked. *)
   let get_next_ pool =
-    (*Printf.printf "get_next (cur=%d, idle=%d, stop=%B)\n%!" pool.cur_size pool.cur_idle pool.stop;*)
-    if pool.stop || (Queue.is_empty pool.jobs && pool.cur_size > 0) then (
-      (* die: the thread would be idle otherwise  *)
-      (*Printf.printf "time… to die (cur=%d, idle=%d, stop=%B)\n%!" pool.cur_size pool.cur_idle pool.stop;*)
+    Printf.printf "get_next (cur=%d, idle=%d, stop=%B)\n%!" pool.cur_size pool.cur_idle pool.stop;
+    if pool.stop then (
       decr_size_ pool;
       Die
-    ) else if Queue.is_empty pool.jobs then (
-      Wait
     ) else (
-      let job = Queue.pop pool.jobs in
-      Process job
+      match Queue.take pool.jobs with
+      | exception Queue.Empty ->
+        if pool.cur_idle > 0 then (
+          (* die: there's already at least one idle thread *)
+          Printf.printf "DIE (idle>0)\n%!";
+          decr_size_ pool;
+          Die
+        ) else (
+          incr_idle_ pool;
+          Wait
+        )
+      | job ->
+        Process job
     )
 
   (* Thread: entry point. They seek jobs in the queue *)
@@ -103,9 +110,9 @@ module Make(P : PARAM) = struct
   and run_cmd = function
     | Die -> ()
     | Wait ->
+      Printf.printf "WAIT\n%!";
       with_lock_ pool
         (fun p ->
-           incr_idle_ pool;
            Condition.wait p.cond p.mutex;
            decr_idle_ pool);
       serve pool
@@ -167,10 +174,13 @@ module Make(P : PARAM) = struct
 
   (* kill threads in the pool *)
   let stop () =
+    Printf.printf "STOP\n%!";
     with_lock_ pool
       (fun p ->
          p.stop <- true;
-         Queue.clear p.jobs)
+         Queue.clear p.jobs;
+         Condition.broadcast p.cond; (* wait up idlers *)
+      )
 
   (* stop threads if pool is GC'd *)
   let () = Gc.finalise (fun _ -> stop ()) pool