Do lazy allocation of shadow task queues

3 files changed, 11 insertions, 7 deletions

diff --git a/libbuild2/scheduler.cxx b/libbuild2/scheduler.cxx
index 8c0ea17..bdd703d 100644
--- a/libbuild2/scheduler.cxx
+++ b/libbuild2/scheduler.cxx
@@ -571,7 +571,8 @@ namespace build2
     // phase helpers. The way we are going to do it is to temporarily (until
     // pop) replace such queues with empty ones. This should be ok since a
     // thread with such a "shadowed" queue won't wake up until we return to
-    // the old phase.
+    // the old phase (but the shadow queue may be used if the thread in
+    // question is also switching to the new phase).
     //
     // Note also that the assumption here is that while we may still have
     // "phase-less" threads milling around (e.g., transitioning from active to
@@ -589,13 +590,10 @@ namespace build2
 
       if (tq.size != 0)
       {
-        queued_task_count_.fetch_sub (tq.size, memory_order_release);
-
-        // @@ TODO: should we make task_queue::data allocation lazy? On the
-        //    other hand, we don't seem to get many non-empty queues here on
-        //    real-world projects.
+        // Note that task_queue::data will be allocated lazily (there is a
+        // good chance this queue is not going to be used in the new phase).
         //
-        j->data.reset (new task_data[task_queue_depth_]);
+        queued_task_count_.fetch_sub (tq.size, memory_order_release);
         tq.swap (*j);
       }
     }
diff --git a/libbuild2/scheduler.hxx b/libbuild2/scheduler.hxx
index b85d353..da3f0c0 100644
--- a/libbuild2/scheduler.hxx
+++ b/libbuild2/scheduler.hxx
@@ -700,6 +700,9 @@ namespace build2
     // tail, if enabled. If the mark is hit, then it is disabled until the
     // queue becomes empty or it is reset by a push.
     //
+    // Note also that the data array can be NULL (lazy allocation) and one
+    // must make sure it's allocated before calling push().
+    //
     struct task_queue_data
     {
       size_t head = 0;
diff --git a/libbuild2/scheduler.txx b/libbuild2/scheduler.txx
index 9cfc411..5c6b339 100644
--- a/libbuild2/scheduler.txx
+++ b/libbuild2/scheduler.txx
@@ -54,6 +54,9 @@ namespace build2
       if (tq->shutdown)
         throw_generic_error (ECANCELED);
 
+      if (tq->data == nullptr)
+        tq->data.reset (new task_data[task_queue_depth_]);
+
       if (task_data* td = push (*tq))
       {
         // Package the task (under lock).