[sysprof] libsysprof-capture: remove use of GAtomicRCBox

[Christian Hergert <chergert src gnome org>]

commit be6b979714f15229a67bab8d93e882412d75eda3
Author: Christian Hergert <chergert redhat com>
Date:   Thu Feb 20 10:39:33 2020 -0800

    libsysprof-capture: remove use of GAtomicRCBox
    
    If we backport to an older GLib release, we won't have the newer atomic
    helpers available. It's really not too much burden to do that manually if
    it means we can run on older systems.

 src/libsysprof-capture/mapped-ring-buffer.c | 30 ++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)
---
diff --git a/src/libsysprof-capture/mapped-ring-buffer.c b/src/libsysprof-capture/mapped-ring-buffer.c
index 9b68274..656845c 100644
--- a/src/libsysprof-capture/mapped-ring-buffer.c
+++ b/src/libsysprof-capture/mapped-ring-buffer.c
@@ -61,11 +61,12 @@ G_STATIC_ASSERT (sizeof (MappedRingHeader) == 16);
  */
 struct _MappedRingBuffer
 {
-  int    mode;
-  int    fd;
-  void  *map;
-  gsize  body_size;
-  gsize  page_size;
+  volatile gint ref_count;
+  int           mode;
+  int           fd;
+  void         *map;
+  gsize         body_size;
+  gsize         page_size;
 };
 
 static inline MappedRingHeader *
@@ -199,7 +200,8 @@ mapped_ring_buffer_new_reader (gsize buffer_size)
   header->offset = page_size;
   header->size = buffer_size - page_size;
 
-  self = g_atomic_rc_box_new0 (MappedRingBuffer);
+  self = g_slice_new0 (MappedRingBuffer);
+  self->ref_count = 1;
   self->mode = MODE_READER;
   self->body_size = buffer_size - page_size;
   self->fd = fd;
@@ -296,7 +298,8 @@ mapped_ring_buffer_new_writer (gint fd)
       return NULL;
     }
 
-  self = g_atomic_rc_box_new0 (MappedRingBuffer);
+  self = g_slice_new0 (MappedRingBuffer);
+  self->ref_count = 1;
   self->mode = MODE_WRITER;
   self->fd = fd;
   self->body_size = buffer_size - page_size;
@@ -325,13 +328,22 @@ mapped_ring_buffer_finalize (MappedRingBuffer *self)
 void
 mapped_ring_buffer_unref (MappedRingBuffer *self)
 {
-  g_atomic_rc_box_release_full (self, (GDestroyNotify)mapped_ring_buffer_finalize);
+  g_return_if_fail (self != NULL);
+  g_return_if_fail (self->ref_count > 0);
+
+  if (g_atomic_int_dec_and_test (&self->ref_count))
+    mapped_ring_buffer_finalize (self);
 }
 
 MappedRingBuffer *
 mapped_ring_buffer_ref (MappedRingBuffer *self)
 {
-  return g_atomic_rc_box_acquire (self);
+  g_return_val_if_fail (self != NULL, NULL);
+  g_return_val_if_fail (self->ref_count > 0, NULL);
+
+  g_atomic_int_inc (&self->ref_count);
+
+  return self;
 }
 
 gint