cogl/journal: Don't sometimes hold a ref on the framebuffer

d42f1873fc introduced a semi circular
reference between the CoglFramebuffer, and CoglJournal, where
CoglJournal would keep a reference on the CoglFramebuffer when there
were any entries in the journal log.

To avoid risking leaking these objects indefinitely, when freeing
objects without doing anything that triggered a flush, CoglFramebuffer
had a "filter" on cogl_object_unref() calls, which knew
about under what conditions CoglJournal had a reference to it. When it
could detect that there were only the journal itself holding such a
reference, it'd flush the journal, effectively releasing the reference
the journal held, thus freeing itself, as well as the journal.

When CoglFramebuffer was ported to be implemented using GObject instead
of CoglObject, this "filter" was missed, causing not only awkward but
infrequent leaks, but also situations where we'd flush journals when
only the journal itself held the last reference to the framebuffer,
meaning the journal would free the framebuffer, thus itself, in the
middle of flushing, causing memory corruption and crashes.

A way to detect this, by asserting on CoglObject reference count during
flush, is by adding the `g_assert()` as described below, which will
assert instead cause memory corruption.

void
_cogl_journal_flush (CoglJournal *journal
{
   ...
   _cogl_journal_discard (journal);
+  g_assert (journal->_parent.ref_count > 0);
   ...
}

Fix this by making CoglFramebuffer the owner of the journal, which it
already was, and remove any circle referencing that was there before, as
it is not needed given that the CoglFramebuffer pointer is guaranteed to
be valid for the lifetime of CoglJournal as the framebuffer is the owner
of the journal.

However, to not miss flushing before tearing down, which is important as
this flushes painting calls to the driver that is important for e.g.
using the result of those journal entries, flush the journal the first
time cogl_framebuffer_dispose() is called, before doing anything else.

This also adds a test case. Without having broken the circular
reference, the test would fail on g_assert_null (offscreen), as it would
have been "leaked" at this point, but the actual memory corruption would
be a result of the `cogl_texture_get_data()` call, which flushes the
framebuffer, and causes the 'mid-flush' destruction of the journal
described above. Note that the texture keeps track of dependent
framebuffers, but it does not hold any references to them.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1474
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1735>

cogl/cogl/cogl-framebuffer-private.h

@@ -223,9 +223,6 @@ _cogl_framebuffer_save_clip_stack (CoglFramebuffer *framebuffer);
 void
 _cogl_framebuffer_restore_clip_stack (CoglFramebuffer *framebuffer);
 
-void
-_cogl_framebuffer_unref (CoglFramebuffer *framebuffer);
-
 /* This can be called directly by the CoglJournal to draw attributes
  * skipping the implicit journal flush, the framebuffer flush and
  * pipeline validation. */

cogl/cogl/cogl-framebuffer.c

@@ -336,6 +336,8 @@ cogl_framebuffer_dispose (GObject *object)
 
   if (priv->journal)
     {
+      _cogl_journal_flush (priv->journal);
+
       g_signal_emit (framebuffer, signals[DESTROY], 0);
 
       _cogl_fence_cancel_fences_for_framebuffer (framebuffer);
@@ -2138,37 +2140,6 @@ cogl_framebuffer_pop_clip (CoglFramebuffer *framebuffer)
     }
 }
 
-void
-_cogl_framebuffer_unref (CoglFramebuffer *framebuffer)
-{
-  CoglFramebufferPrivate *priv =
-    cogl_framebuffer_get_instance_private (framebuffer);
-
-  /* The journal holds a reference to the framebuffer whenever it is
-     non-empty. Therefore if the journal is non-empty and we will have
-     exactly one reference then we know the journal is the only thing
-     keeping the framebuffer alive. In that case we want to flush the
-     journal and let the framebuffer die. It is fine at this point if
-     flushing the journal causes something else to take a reference to
-     it and it comes back to life */
-  if (priv->journal->entries->len > 0)
-    {
-      unsigned int ref_count = ((CoglObject *) framebuffer)->ref_count;
-
-      /* There should be at least two references - the one we are
-         about to drop and the one held by the journal */
-      if (ref_count < 2)
-        g_warning ("Inconsistent ref count on a framebuffer with journal "
-                   "entries.");
-
-      if (ref_count == 2)
-        _cogl_framebuffer_flush_journal (framebuffer);
-    }
-
-  /* Chain-up */
-  _cogl_object_default_unref (framebuffer);
-}
-
 #ifdef COGL_ENABLE_DEBUG
 static int
 get_index (void *indices,

cogl/cogl/cogl-journal.c

@@ -148,14 +148,7 @@ _cogl_journal_new (CoglFramebuffer *framebuffer)
 {
   CoglJournal *journal = g_slice_new0 (CoglJournal);
 
-  /* The journal keeps a pointer back to the framebuffer because there
-     is effectively a 1:1 mapping between journals and framebuffers.
-     However, to avoid a circular reference the journal doesn't take a
-     reference unless it is non-empty. The framebuffer has a special
-     unref implementation to ensure that the journal is flushed when
-     the journal is the only thing keeping it alive */
   journal->framebuffer = framebuffer;
-
   journal->entries = g_array_new (FALSE, FALSE, sizeof (CoglJournalEntry));
   journal->vertices = g_array_new (FALSE, FALSE, sizeof (float));
 
@@ -1272,10 +1265,6 @@ _cogl_journal_discard (CoglJournal *journal)
   g_array_set_size (journal->vertices, 0);
   journal->needed_vbo_len = 0;
   journal->fast_read_pixel_count = 0;
-
-  /* The journal only holds a reference to the framebuffer while the
-     journal is not empty */
-  g_object_unref (journal->framebuffer);
 }
 
 /* Note: A return value of FALSE doesn't mean 'no' it means
@@ -1533,12 +1522,6 @@ _cogl_journal_log_quad (CoglJournal  *journal,
 
   COGL_TIMER_START (_cogl_uprof_context, log_timer);
 
-  /* If the framebuffer was previously empty then we'll take a
-     reference to the current framebuffer. This reference will be
-     removed when the journal is flushed */
-  if (journal->vertices->len == 0)
-    g_object_ref (framebuffer);
-
   /* The vertex data is logged into a separate array. The data needs
      to be copied into a vertex array before it's given to GL so we
      only store two vertices per quad and expand it to four while

cogl/tests/conform/meson.build

@@ -16,6 +16,7 @@ cogl_test_conformance_sources = [
   'test-sub-texture.c',
   'test-custom-attributes.c',
   'test-offscreen.c',
+  'test-journal.c',
   'test-primitive.c',
   'test-sparse-pipeline.c',
   'test-read-texture-formats.c',

cogl/tests/conform/test-conform-main.c

@@ -98,6 +98,7 @@ main (int argc, char **argv)
   ADD_TEST (test_custom_attributes, TEST_REQUIREMENT_GLSL, 0);
 
   ADD_TEST (test_offscreen, 0, 0);
+  ADD_TEST (test_journal_unref_flush, 0, 0);
   ADD_TEST (test_framebuffer_get_bits,
             TEST_REQUIREMENT_OFFSCREEN | TEST_REQUIREMENT_GL,
             0);

cogl/tests/conform/test-declarations.h

@@ -28,6 +28,7 @@ void test_pipeline_uniforms (void);
 void test_snippets (void);
 void test_custom_attributes (void);
 void test_offscreen (void);
+void test_journal_unref_flush (void);
 void test_framebuffer_get_bits (void);
 void test_point_size (void);
 void test_point_size_attribute (void);

cogl/tests/conform/test-journal.c

@@ -0,0 +1,46 @@
+#include <cogl/cogl.h>
+
+#include <stdio.h>
+#include <string.h>
+
+#include "test-declarations.h"
+#include "test-utils.h"
+
+void
+test_journal_unref_flush (void)
+{
+  CoglTexture2D *texture;
+  CoglOffscreen *offscreen;
+  CoglPipeline *pipeline;
+  const int width = 1;
+  const int height = 1;
+  const int stride = width * 4;
+  uint8_t reference_data[] = {
+    0x33, 0x33, 0x33, 0x33,
+  };
+  uint8_t data[G_N_ELEMENTS (reference_data)];
+
+  G_STATIC_ASSERT (sizeof data == sizeof reference_data);
+
+  texture = cogl_texture_2d_new_with_size (test_ctx, width, height);
+  offscreen = cogl_offscreen_new_with_texture (COGL_TEXTURE (texture));
+  g_object_add_weak_pointer (G_OBJECT (offscreen), (gpointer *) &offscreen);
+
+  pipeline = cogl_pipeline_new (test_ctx);
+  cogl_pipeline_set_color4ub (pipeline, 0x33, 0x33, 0x33, 0x33);
+  cogl_framebuffer_draw_rectangle (COGL_FRAMEBUFFER (offscreen),
+                                   pipeline,
+                                   -1, -1, 1, 1);
+  cogl_object_unref (pipeline);
+
+  g_object_unref (offscreen);
+  g_assert_null (offscreen);
+
+  cogl_texture_get_data (COGL_TEXTURE (texture),
+                         COGL_PIXEL_FORMAT_RGBA_8888_PRE,
+                         stride, data);
+  g_assert_cmpmem (data, sizeof (data),
+                   reference_data, sizeof (reference_data));
+
+  cogl_object_unref (texture);
+}