[sysprof] libsysprof-capture: add clear/reset API for mapped ring buffer

[Christian Hergert <chergert src gnome org>]

commit 53a351b66eab09fd9cff272300b34d9dc286d6ed
Author: Christian Hergert <chergert redhat com>
Date:   Sun Feb 16 10:17:12 2020 -0700

    libsysprof-capture: add clear/reset API for mapped ring buffer

 src/libsysprof-capture/mapped-ring-buffer.c | 22 ++++++++++++++++++++++
 src/libsysprof-capture/mapped-ring-buffer.h |  2 ++
 2 files changed, 24 insertions(+)
---
diff --git a/src/libsysprof-capture/mapped-ring-buffer.c b/src/libsysprof-capture/mapped-ring-buffer.c
index b051a54..11499b6 100644
--- a/src/libsysprof-capture/mapped-ring-buffer.c
+++ b/src/libsysprof-capture/mapped-ring-buffer.c
@@ -621,3 +621,25 @@ mapped_ring_buffer_create_source (MappedRingBuffer         *self,
 {
   return mapped_ring_buffer_create_source_full (self, source_func, user_data, NULL);
 }
+
+/**
+ * mapped_ring_buffer_clear:
+ * @self: a #MappedRingBuffer
+ *
+ * Resets the head and tail positions back to 0.
+ *
+ * This function is only safe to call when you control both the reader
+ * and writer sides with mapped_ring_buffer_new_readwrite(), or are in
+ * control of when each side reads or writes.
+ */
+void
+mapped_ring_buffer_clear (MappedRingBuffer *self)
+{
+  MappedRingHeader *header;
+
+  g_return_if_fail (self != NULL);
+
+  header = get_header (self);
+  header->head = 0;
+  header->tail = 0;
+}
diff --git a/src/libsysprof-capture/mapped-ring-buffer.h b/src/libsysprof-capture/mapped-ring-buffer.h
index 45387c3..4eee695 100644
--- a/src/libsysprof-capture/mapped-ring-buffer.h
+++ b/src/libsysprof-capture/mapped-ring-buffer.h
@@ -66,6 +66,8 @@ MappedRingBuffer *mapped_ring_buffer_ref                (MappedRingBuffer
 G_GNUC_INTERNAL
 void              mapped_ring_buffer_unref              (MappedRingBuffer         *self);
 G_GNUC_INTERNAL
+void              mapped_ring_buffer_clear              (MappedRingBuffer         *self);
+G_GNUC_INTERNAL
 gpointer          mapped_ring_buffer_allocate           (MappedRingBuffer         *self,
                                                          gsize                     length);
 G_GNUC_INTERNAL