libsysprof-capture: remove framing data from MappedRingBuffer

This removes the 8 bytes of framing data from the MappedRingBuffer which
means we can write more data without racing. But also this means that we
can eventually use the mapped ring buffer as our normal buffer for
capture writing (to be done later).

src/tests/test-mapped-ring-buffer.c

@@ -5,25 +5,27 @@
 static gsize real_count;
 
 static gboolean
-drain_nth_cb (gconstpointer data,
-              gsize         len,
-              gpointer      user_data)
+drain_nth_cb (gconstpointer  data,
+              gsize         *len,
+              gpointer       user_data)
 {
   const gint64 *v64 = data;
-  g_assert_cmpint (len, ==, 8);
+  g_assert_cmpint (*len, >=, 8);
   g_assert_cmpint (*v64, ==, GPOINTER_TO_SIZE (user_data));
+  *len = sizeof *v64;
   return G_SOURCE_CONTINUE;
 }
 
 static gboolean
-drain_count_cb (gconstpointer data,
-                gsize         len,
-                gpointer      user_data)
+drain_count_cb (gconstpointer  data,
+                gsize         *len,
+                gpointer       user_data)
 {
   const gint64 *v64 = data;
-  g_assert_cmpint (len, ==, 8);
+  g_assert_cmpint (*len, >=, 8);
   ++real_count;
   g_assert_cmpint (real_count, ==, *v64);
+  *len = sizeof *v64;
   return G_SOURCE_CONTINUE;
 }
 
@@ -91,13 +93,14 @@ typedef struct
 } ThreadedMessage;
 
 static gboolean
-handle_msg (gconstpointer data,
-            gsize         length,
-            gpointer      user_data)
+handle_msg (gconstpointer  data,
+            gsize         *length,
+            gpointer       user_data)
 {
   const ThreadedMessage *msg = data;
   gboolean *done = user_data;
   *done = msg->done;
+  *length = sizeof *msg;
   return G_SOURCE_CONTINUE;
 }