lavu: add a public refcounting API

The AVBuffer API is currently often abused as a generic thread-safe
refcount in cases it is not appropriate for.

This API is intended to replace those uses.

3 files changed, 140 insertions, 0 deletions

diff --git a/libavutil/Makefile b/libavutil/Makefile
index 9435a0bfb0..e411b45ae7 100644
--- a/libavutil/Makefile
+++ b/libavutil/Makefile
@@ -70,6 +70,7 @@ HEADERS = adler32.h                                                     \
           random_seed.h                                                 \
           rc4.h                                                         \
           rational.h                                                    \
+          refcount.h                                                    \
           replaygain.h                                                  \
           ripemd.h                                                      \
           samplefmt.h                                                   \
@@ -155,6 +156,7 @@ OBJS = adler32.o                                                        \
        pixelutils.o                                                     \
        random_seed.o                                                    \
        rational.o                                                       \
+       refcount.o                                                       \
        reverse.o                                                        \
        rc4.o                                                            \
        ripemd.o                                                         \
diff --git a/libavutil/refcount.c b/libavutil/refcount.c
new file mode 100644
index 0000000000..27c0718116
--- /dev/null
+++ b/libavutil/refcount.c
@@ -0,0 +1,61 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <stdatomic.h>
+
+#include "refcount.h"
+
+typedef struct AVRefcountImpl {
+    void (*free)(void *opaque);
+    void *opaque;
+    atomic_uint refcount;
+} AVRefcountImpl;
+
+
+void av_refcount_init(AVRefcount *rc, void (*free)(void *opaque),
+                      void *opaque)
+{
+    AVRefcountImpl *rci = (AVRefcountImpl*)rc;
+    rci->free   = free;
+    rci->opaque = opaque;
+    atomic_init(&rci->refcount, 1);
+}
+
+void av_refcount_inc(AVRefcount *rc)
+{
+    AVRefcountImpl *rci = (AVRefcountImpl*)rc;
+    atomic_fetch_add_explicit(&rci->refcount, 1, memory_order_relaxed);
+}
+
+unsigned int av_refcount_dec(AVRefcount *rc)
+{
+    AVRefcountImpl *rci = (AVRefcountImpl*)rc;
+    unsigned int    ret = atomic_fetch_sub_explicit(&rci->refcount, 1,
+                                                    memory_order_acq_rel) - 1;
+
+    if (!ret && rci->free)
+        rci->free(rci->opaque);
+
+    return ret;
+}
+
+int av_refcount_exclusive(AVRefcount *rc)
+{
+    AVRefcountImpl *rci = (AVRefcountImpl*)rc;
+    return atomic_load(&rci->refcount) == 1;
+}
diff --git a/libavutil/refcount.h b/libavutil/refcount.h
new file mode 100644
index 0000000000..92039ec020
--- /dev/null
+++ b/libavutil/refcount.h
@@ -0,0 +1,77 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_REFCOUNT_H
+#define AVUTIL_REFCOUNT_H
+
+#include <stdint.h>
+
+/**
+ * A thread-safe reference counter.
+ *
+ * Unlike most libav* structs, sizeof(AVRefcount) IS a part of the public ABI
+ * and so it can be embedded directly in other structs.
+ *
+ * None of its fields should be accessed directly by the caller, always use the
+ * av_refcount_*() functions.
+ */
+typedef struct AVRefcount {
+    void   (*_internal0)(void *);
+    void    *_internal1;
+    intmax_t _internal2;
+} AVRefcount;
+
+/**
+ * Initialize an AVRefcount to a reference count of 1. This function MUST be
+ * called on an AVRefcount before doing anything else with it.
+ *
+ * @param free The callback to be invoked when the counter reaches zero. May be
+ *             NULL, then nothing is called.
+ * @param opaque arbitrary user data that will be passed to the free() callback.
+ */
+void av_refcount_init(AVRefcount *rc, void (*free)(void *opaque),
+                      void *opaque);
+
+/**
+ * Increase the reference count by 1.
+ */
+void av_refcount_inc(AVRefcount *rc);
+
+/**
+ * Decrease the reference count by 1.
+ *
+ * @return
+ * - 0 when the new reference count is 0. If a free() callback was passed to
+ *   av_refcount_init(), it has been invoked.
+ * - a positive value when the new reference count is non-zero
+ *
+ * @note the return value is the new reference count, which may be useful for
+ * debugging, but this is not guaranteed by the API and the callers MUST NOT
+ * rely on this
+ */
+unsigned int av_refcount_dec(AVRefcount *rc);
+
+/**
+ * Check whether the caller is the exclusive owner of the reference.
+ * @return
+ * - a positive number if the reference count is exactly 1
+ * - zero otherwise
+ */
+int av_refcount_exclusive(AVRefcount *rc);
+
+#endif /* AVUTIL_REFCOUNT_H */