redesign cross-thread registration/signalling system

This new design will work even when threads that need to receive
messages from RT threads are created *after* the RT threads. The
existing design would fail because the RT thread(s) would never
be known the later created threads, and so signals emitted by the
RT thread and causing call_slot() in the receiver would end up
being enqueued using a lock-protected list. The new design ensures
that communication always uses a lock-free FIFO instead

1 files changed, 18 insertions, 10 deletions

diff --git a/libs/surfaces/control_protocol/control_protocol/control_protocol.h b/libs/surfaces/control_protocol/control_protocol/control_protocol.h
index 8edb3f39a7..71e875419a 100644
--- a/libs/surfaces/control_protocol/control_protocol/control_protocol.h
+++ b/libs/surfaces/control_protocol/control_protocol/control_protocol.h
@@ -154,16 +154,24 @@ class LIBCONTROLCP_API ControlProtocol : public PBD::Stateful, public PBD::Scope
 extern "C" {
 	class ControlProtocolDescriptor {
 	public:
-	    const char* name;      /* descriptive */
-	    const char* id;        /* unique and version-specific */
-	    void*       ptr;       /* protocol can store a value here */
-	    void*       module;    /* not for public access */
-	    int         mandatory; /* if non-zero, always load and do not make optional */
-	    bool        supports_feedback; /* if true, protocol has toggleable feedback mechanism */
-	    bool             (*probe)(ControlProtocolDescriptor*);
-	    ControlProtocol* (*initialize)(ControlProtocolDescriptor*,Session*);
-	    void             (*destroy)(ControlProtocolDescriptor*,ControlProtocol*);
-
+		const char* name;      /* descriptive */
+		const char* id;        /* unique and version-specific */
+		void*       ptr;       /* protocol can store a value here */
+		void*       module;    /* not for public access */
+		int         mandatory; /* if non-zero, always load and do not make optional */
+		bool        supports_feedback; /* if true, protocol has toggleable feedback mechanism */
+		bool             (*probe)(ControlProtocolDescriptor*);
+		ControlProtocol* (*initialize)(ControlProtocolDescriptor*,Session*);
+		void             (*destroy)(ControlProtocolDescriptor*,ControlProtocol*);
+		/* this is required if the control protocol connects to signals
+		   from libardour. they all do. It should allocate a
+		   type-specific request buffer for the calling thread, and
+		   store it in a thread-local location that will be used to
+		   find it when sending the event loop a message
+		   (e.g. call_slot()). It should also return the allocated
+		   buffer as a void*.
+		*/
+		void*            (*request_buffer_factory)(uint32_t);
 	};
 }