Implement LV2 worker extension.

This is done by way of a generic Worker object/thread, which currently just
applies to one LV2 plugin, but the idea is to share one thread and set of
buffers among many plugins.  The same pattern may also be useful elsewhere in
Ardour.  The responding part gets a bit tricker when sharing a worker between
plugins, it's not a blocker, and I'm lazy, sooo here's this.

This commit also adds a new portable in-process semaphore to PBD.  The existing
one is pretty weird and uses a named semaphore on OSX for reasons unknown to
me.  Perhaps as a quick fix to avoid POSIX semaphores being utterly broken on
OSX?  It would probably be a good idea to replace that with this new one, which
uses Mach kernel semaphores on OSX which work well, though I am not sure how
pedantically real-time safe they are to signal.


git-svn-id: svn://localhost/ardour2/branches/3.0@11790 d708f5d6-7413-0410-9779-e7cbd77b26cf

1 files changed, 198 insertions, 0 deletions

diff --git a/libs/pbd/pbd/semaphore.h b/libs/pbd/pbd/semaphore.h
new file mode 100644
index 0000000000..f1b07ea4f5
--- /dev/null
+++ b/libs/pbd/pbd/semaphore.h
@@ -0,0 +1,198 @@
+/*
+  Copyright (C) 2012 Paul Davis
+  Author: David Robillard
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program; if not, write to the Free Software
+  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+*/
+
+#ifndef __pbd_semaphore_h__
+#define __pbd_semaphore_h__
+
+#ifdef __APPLE__
+#    include <mach/mach.h>
+#elif defined(_WIN32)
+#    include <windows.h>
+#else
+#    include <semaphore.h>
+#    include <errno.h>
+#endif
+
+#include "pbd/failed_constructor.h"
+
+namespace PBD {
+
+/**
+   Unnamed (process local) counting semaphore.
+
+   The civilized person's synchronisation primitive.  A counting semaphore is
+   an integer which is always non-negative, so, an attempted decrement (or
+   "wait") will block if the value is 0, until another thread does an increment
+   (or "post").
+
+   At least on Lignux, the main advantage of this is that it is fast and the
+   only safe way to reliably signal from a real-time audio thread.  The
+   counting semantics also complement ringbuffers of events nicely.
+*/
+class Semaphore
+{
+public:
+	/**
+	   Create a new semaphore.
+
+	   Chances are you want 1 wait() per 1 post(), an initial value of 0.
+	*/
+	inline Semaphore(unsigned initial);
+
+	inline ~Semaphore();
+
+	/** Post/Increment/Signal */
+	inline void post();
+
+	/** Wait/Decrement.  Returns false on error. */
+	inline bool wait();
+
+	/** Attempt Wait/Decrement.  Returns true iff a decrement occurred. */
+	inline bool try_wait();
+
+private:
+#if defined(__APPLE__)
+	semaphore_t _sem;  // sem_t is a worthless broken mess on OSX
+#elif defined(_WIN32)
+	HANDLE _sem;  // types are overrated anyway
+#else
+	sem_t _sem;
+#endif
+};
+
+#ifdef __APPLE__
+
+inline
+Semaphore::Semaphore(unsigned initial)
+{
+	if (semaphore_create(mach_task_self(), &sem->sem, SYNC_POLICY_FIFO, 0)) {
+		throw failed_constructor();
+	}
+}
+
+inline
+Semaphore::~Semaphore()
+{
+	semaphore_destroy(mach_task_self(), _sem);
+}
+
+inline void
+Semaphore::post()
+{
+	semaphore_signal(_sem);
+}
+
+inline bool
+Semaphore::wait()
+{
+	if (semaphore_wait(_sem) != KERN_SUCCESS) {
+		return false;
+	}
+	return true;
+}
+
+inline bool
+Semaphore::try_wait()
+{
+	const mach_timespec_t zero = { 0, 0 };
+	return semaphore_timedwait(_sem, zero) == KERN_SUCCESS;
+}
+
+#elif defined(_WIN32)
+
+inline
+Semaphore::Semaphore(unsigned initial)
+{
+	if (!(_sem = CreateSemaphore(NULL, initial, LONG_MAX, NULL))) {
+		throw failed_constructor();
+	}
+}
+
+inline
+Semaphore::~Semaphore()
+{
+	CloseHandle(_sem);
+}
+
+inline void
+Semaphore::post()
+{
+	ReleaseSemaphore(_sem, 1, NULL);
+}
+
+inline bool
+Semaphore::wait()
+{
+	if (WaitForSingleObject(_sem, INFINITE) != WAIT_OBJECT_0) {
+		return false;
+	}
+	return true;
+}
+
+inline bool
+Semaphore::try_wait()
+{
+	return WaitForSingleObject(sem->sem, 0) == WAIT_OBJECT_0;
+}
+
+#else  /* !defined(__APPLE__) && !defined(_WIN32) */
+
+Semaphore::Semaphore(unsigned initial)
+{
+	if (sem_init(&_sem, 0, initial)) {
+		throw failed_constructor();
+	}
+}
+
+inline
+Semaphore::~Semaphore()
+{
+	sem_destroy(&_sem);
+}
+
+inline void
+Semaphore::post()
+{
+	sem_post(&_sem);
+}
+
+inline bool
+Semaphore::wait()
+{
+	while (sem_wait(&_sem)) {
+		if (errno != EINTR) {
+			return false;  // We are all doomed
+		}
+		/* Otherwise, interrupted (rare/weird), so try again. */
+	}
+
+	return true;
+}
+
+inline bool
+Semaphore::try_wait()
+{
+	return (sem_trywait(&_sem) == 0);
+}
+
+#endif
+
+}  // namespace PBD
+
+#endif  /* __pbd_semaphore_h__ */