Fix automation alignment for latent plugins

This also solves bi-stable automation for plugins where latency
can change due to automation. e.g.

cycle 1: run (t): automation (t) = on: -> increase latency
cycle 2: run (t-latency): automation (t-latency) = off -> decrease latency
repeat.

1 files changed, 18 insertions, 10 deletions

diff --git a/libs/ardour/route.cc b/libs/ardour/route.cc
index a06d4fd3d6..3bc66f1a30 100644
--- a/libs/ardour/route.cc
+++ b/libs/ardour/route.cc
@@ -513,24 +513,32 @@ Route::process_output_buffers (BufferSet& bufs,
 			pspeed = 0;
 		}
 
-		if (speed < 0) {
-			(*i)->run (bufs, start_sample + latency, end_sample + latency, pspeed, nframes, *i != _processors.back());
-		} else {
-			(*i)->run (bufs, start_sample - latency, end_sample - latency, pspeed, nframes, *i != _processors.back());
-		}
-
-		bufs.set_count ((*i)->output_streams());
-
-		/* Note: plugin latency may change. While the plugin does inform the session via
+		/* Note: plugin latency may change. The plugin does inform the session via
 		 * processor_latency_changed(). But the session may not yet have gotten around to
 		 * update the actual worste-case and update this track's _signal_latency.
-		 *
 		 * So there can be cases where adding up all latencies may not equal _signal_latency.
+		 *
+		 * Also running a plugin may change the plugin's latency, so we need to
+		 * add the latency first. Otherwise this can lead to bistable case
+		 * in case of automation playback. e.g.
+		 *
+		 * cycle 1: run (t): automation (t) = on: -> increase latency
+		 * cycle 2: run (t-latency): automation (t-latency) = off -> decrease latency
+		 * reapeat.
 		 */
+
 		if ((*i)->active ()) {
 			latency += (*i)->effective_latency ();
 		}
 
+		if (speed < 0) {
+			(*i)->run (bufs, start_sample + latency, end_sample + latency, pspeed, nframes, *i != _processors.back());
+		} else {
+			(*i)->run (bufs, start_sample - latency, end_sample - latency, pspeed, nframes, *i != _processors.back());
+		}
+
+		bufs.set_count ((*i)->output_streams());
+
 		if (re_inject_oob_data) {
 			write_out_of_band_data (bufs, nframes);
 		}