use zeroed scratch buffers for "silent" plugin runs

Plugins rewrite the buffer data in-place and some plugins
can produce output even when fed with silence.

Hence, during a PluginInsert::silence() run a plugin can
inject data into the "silent" buffers which causes side-effects.

Kudos to Chris 'oofus' Goddard for finding this issue.

1 files changed, 9 insertions, 1 deletions

diff --git a/libs/ardour/process_thread.cc b/libs/ardour/process_thread.cc
index 5d8d6f34fd..d4a3d2f390 100644
--- a/libs/ardour/process_thread.cc
+++ b/libs/ardour/process_thread.cc
@@ -90,7 +90,7 @@ ProcessThread::get_silent_buffers (ChanCount count)
 }
 
 BufferSet&
-ProcessThread::get_scratch_buffers (ChanCount count)
+ProcessThread::get_scratch_buffers (ChanCount count, bool silence)
 {
         ThreadBuffers* tb = _private_thread_buffers.get();
         assert (tb);
@@ -105,6 +105,14 @@ ProcessThread::get_scratch_buffers (ChanCount count)
 		sb->set_count (sb->available());
 	}
 
+	if (silence) {
+		for (DataType::iterator t = DataType::begin(); t != DataType::end(); ++t) {
+			for (uint32_t i = 0; i < sb->count().get(*t); ++i) {
+				sb->get(*t, i).clear();
+			}
+		}
+	}
+
 	return *sb;
 }