From 600da2b97e937b9c96791c291cb5e08cd8526bdd Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 7 Aug 2020 08:57:01 +0200
Subject: Add ability to allocate additional active threads to current thread

---
 libbuild2/scheduler.cxx | 34 +++++++++++++++++++++--
 libbuild2/scheduler.hxx | 71 ++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 102 insertions(+), 3 deletions(-)

diff --git a/libbuild2/scheduler.cxx b/libbuild2/scheduler.cxx
index c660bba..03842ec 100644
--- a/libbuild2/scheduler.cxx
+++ b/libbuild2/scheduler.cxx
@@ -144,8 +144,8 @@ namespace build2
     if (collision)
       stat_wait_collisions_++;
 
-    // If we have spare active threads, then become active. Otherwise it
-    // enters the ready queue.
+    // If we have spare active threads, then become active. Otherwise we enter
+    // the ready queue.
     //
     if (external)
       external_--;
@@ -186,6 +186,36 @@ namespace build2
   }
 
   size_t scheduler::
+  allocate (size_t n)
+  {
+    if (max_active_ == 1) // Serial execution.
+      return 0;
+
+    lock l (mutex_);
+
+    if (active_ < max_active_)
+    {
+      size_t d (max_active_ - active_);
+      if (n == 0 || d < n)
+        n = d;
+      active_ -= n;
+      return n;
+    }
+    else
+      return 0;
+  }
+
+  void scheduler::
+  deallocate (size_t n)
+  {
+    if (max_active_ == 1) // Serial execution.
+      return;
+
+    lock l (mutex_);
+    active_ += n;
+  }
+
+  size_t scheduler::
   suspend (size_t start_count, const atomic_count& task_count)
   {
     wait_slot& s (
diff --git a/libbuild2/scheduler.hxx b/libbuild2/scheduler.hxx
index b7bc3c5..7e052cd 100644
--- a/libbuild2/scheduler.hxx
+++ b/libbuild2/scheduler.hxx
@@ -159,6 +159,72 @@ namespace build2
     static void
     active_sleep (const duration&);
 
+    // Allocate additional active thread count to the current active thread,
+    // for example, to be "passed" to an external program:
+    //
+    // scheduler::alloc_guard ag (ctx.sched, ctx.sched.max_active () / 2);
+    // args.push_back ("-flto=" + to_string (1 + ag.n));
+    // run (args);
+    // ag.deallocate ();
+    //
+    // The allocate() function reserves up to the specified number of
+    // additional threads returning the number actually allocated (which can
+    // be less than requested, including 0). If 0 is specified, then it
+    // allocates all the currently available threads.
+    //
+    // The deallocate() function returns the specified number of previously
+    // allocated threads back to the active thread pool.
+    //
+    // Note that when the thread is deactivated (directly or indirectly via
+    // wait, phase switching, etc), the additionally allocated threads are
+    // considered to be still active (this semantics could be changed if we
+    // have a plausible scenario for where waiting, etc., with allocated
+    // threads is useful).
+    //
+    size_t
+    allocate (size_t);
+
+    void
+    deallocate (size_t);
+
+    struct alloc_guard
+    {
+      size_t n;
+
+      alloc_guard (): n (0), s_ (nullptr) {}
+      alloc_guard (scheduler& s, size_t m): n (s.allocate (m)), s_ (&s) {}
+      alloc_guard (alloc_guard&& x): n (x.n), s_ (x.s_) {x.s_ = nullptr;}
+      alloc_guard& operator= (alloc_guard&& x)
+      {
+        if (&x != this)
+        {
+          n = x.n;
+          s_ = x.s_;
+          x.s_ = nullptr;
+        }
+        return *this;
+      }
+
+      ~alloc_guard ()
+      {
+        if (s_ != nullptr && n != 0)
+          s_->deallocate (n);
+      }
+
+      void
+      deallocate ()
+      {
+        if (n != 0)
+        {
+          s_->deallocate (n);
+          n = 0;
+        }
+      }
+
+    private:
+      scheduler* s_;
+    };
+
     // Startup and shutdown.
     //
   public:
@@ -248,13 +314,16 @@ namespace build2
       size_t o_;
     };
 
-    // Return true if the scheduler is configured to run tasks serially.
+    // Return scheduler configuration.
     //
     // Note: can only be called from threads that have observed startup.
     //
     bool
     serial () const {return max_active_ == 1;}
 
+    size_t
+    max_active () const {return max_active_;}
+
     // Wait for all the helper threads to terminate. Throw system_error on
     // failure. Note that the initially active threads are not waited for.
     // Return scheduling statistics.
-- 
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