From b6aefaf100cfaaf15297a8921cca51a575c5e78e Mon Sep 17 00:00:00 2001
From: Paul Davis <paul@linuxaudiosystems.com>
Date: Mon, 24 Sep 2018 17:34:25 -0400
Subject: initial hacks towards a truly thread-safe SafeTime object, using
 boost::atomic

---
 libs/ardour/ardour/transport_master.h | 34 ++++++++++++++++++++++++++++++----
 libs/ardour/mtc_slave.cc              | 32 ++++++--------------------------
 2 files changed, 36 insertions(+), 30 deletions(-)

diff --git a/libs/ardour/ardour/transport_master.h b/libs/ardour/ardour/transport_master.h
index b41d107015..da30abfae9 100644
--- a/libs/ardour/ardour/transport_master.h
+++ b/libs/ardour/ardour/transport_master.h
@@ -23,6 +23,7 @@
 #include <vector>
 
 #include <boost/optional.hpp>
+#include <boost/atomic.hpp>
 
 #include <glibmm/threads.h>
 
@@ -282,18 +283,43 @@ class LIBARDOUR_API TransportMaster : public PBD::Stateful {
 };
 
 struct LIBARDOUR_API SafeTime {
-	volatile int guard1;
+	boost::atomic<int> guard1;
 	samplepos_t   position;
 	samplepos_t   timestamp;
 	double       speed;
-	volatile int guard2;
+	boost::atomic<int> guard2;
 
 	SafeTime() {
-		guard1 = 0;
+		guard1.store (0);
 		position = 0;
 		timestamp = 0;
 		speed = 0;
-		guard2 = 0;
+		guard2.store (0);
+	}
+
+	SafeTime (SafeTime const & other)
+		: guard1 (other.guard1.load (boost::memory_order_acquire))
+		, position (other.position)
+		, timestamp (other.timestamp)
+		, speed (other.speed)
+		, guard2 (other.guard2.load (boost::memory_order_acquire))
+	{}
+
+	SafeTime& operator= (SafeTime const & other) {
+		guard1 = other.guard1.load (boost::memory_order_acquire);
+		position = other.position;
+		timestamp = other.timestamp;
+		speed = other.speed;
+		guard2 = other.guard2.load (boost::memory_order_acquire);
+		return *this;
+	}
+
+	void update (samplepos_t p, samplepos_t t, double s) {
+		guard1.fetch_add (1, boost::memory_order_acquire);
+		position = p;
+		timestamp = t;
+		speed = s;
+		guard2.fetch_add (1, boost::memory_order_acquire);
 	}
 };
 
diff --git a/libs/ardour/mtc_slave.cc b/libs/ardour/mtc_slave.cc
index a2b73ba2f4..7a6d6a6ed6 100644
--- a/libs/ardour/mtc_slave.cc
+++ b/libs/ardour/mtc_slave.cc
@@ -273,7 +273,7 @@ MTC_TransportMaster::read_current (SafeTime *st) const
 		*st = current;
 		tries++;
 
-	} while (st->guard1 != st->guard2);
+	} while (st->guard1.load (boost::memory_order_acquire) != st->guard2.load (boost::memory_order_acquire));
 }
 
 void
@@ -312,11 +312,7 @@ MTC_TransportMaster::update_mtc_qtr (Parser& p, int which_qtr, samplepos_t now)
 		mtc_speed = (t1 - t0) / qtr_d;
 		DEBUG_TRACE (DEBUG::MTC, string_compose ("qtr sample DLL t0:%1 t1:%2 err:%3 spd:%4 ddt:%5\n", t0, t1, e, mtc_speed, e2 - qtr_d));
 
-		current.guard1++;
-		current.position = mtc_frame;
-		current.timestamp = now;
-		current.speed = mtc_speed;
-		current.guard2++;
+		current.update (mtc_frame, now, mtc_speed);
 
 		last_inbound_frame = now;
 	}
@@ -499,10 +495,7 @@ MTC_TransportMaster::update_mtc_time (const MIDI::byte *msg, bool was_full, samp
 				init_mtc_dll(mtc_frame, qtr);
 				mtc_frame_dll = mtc_frame;
 			}
-			current.guard1++;
-			current.position = mtc_frame;
-			current.timestamp = now;
-			current.guard2++;
+			current.update (mtc_frame, now, current.speed);
 			reset_window (mtc_frame);
 		}
 	}
@@ -525,28 +518,15 @@ MTC_TransportMaster::update_mtc_status (MIDI::MTC_Status status)
 
 	switch (status) {
 	case MTC_Stopped:
-		current.guard1++;
-		current.position = mtc_frame;
-		current.timestamp = 0;
-		current.speed = 0;
-		current.guard2++;
-
+		current.update (mtc_frame, 0, 0);
 		break;
 
 	case MTC_Forward:
-		current.guard1++;
-		current.position = mtc_frame;
-		current.timestamp = 0;
-		current.speed = 0;
-		current.guard2++;
+		current.update (mtc_frame, 0, 0);
 		break;
 
 	case MTC_Backward:
-		current.guard1++;
-		current.position = mtc_frame;
-		current.timestamp = 0;
-		current.speed = 0;
-		current.guard2++;
+		current.update (mtc_frame, 0, 0);
 		break;
 	}
 	busy_guard2++;
-- 
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